Download Regulation of coronary artery tone in relation to the

1167
lA CC Vol. 9. NO. 5
May 1987:1167- 75
BASIC CONCEPTS IN CARDIOLOGY
Arnold M. Katz, MD, FACC, Gu est Editor
Regulation of Coronary Artery Tone in Relation to the Activation of
Signal Transductors That Regulate Calcium Homeostasis
TOSHIYUKI SASAGURI, MD, TAKEO ITOH, PHD ,* MASATO HIRATA, DO,
KENJI KITAMURA, DO, HIROSI KURIYAMA, MD
Fukuoka, Japan
Muscle tone of coronary arteries is regulated by free
calcium concentration in the myoplasm. Various agonists, autacoids and putative peptides modify the calcium
concentration directl y or through actions of second messengers (signal transductors), such as cyclic guanosine
monophosphate (GMP), cyclic adenosine monophosphate (AMP), inositol 1,4,5·trisphosphate, diacylglycerol or calmodulin. For example , acetylcholine (in the
presence of intact endothelium cells), alpha-human natriuretic peptide or nitrate compounds increase the amount
of cyclic GMP and isoproterenol, prostacyclin (prostaglandin 12 ) or vasoactive intestinal polypeptide increases
cyclic AMP. Both cyclic nucleotides reduce free calcium
Calcium Homeostasis in Vascular Smooth
Muscles in Relation to the ContractionRelaxation Cycle
The contraction-relaxation cycle in vasc ular smooth muscle is regulated by changes in free myoplasmic ca lcium
concentration . Increa sing calciu m concentration up to 0 . 1
J-tM activa tes the contrac tile mach inary whereas lower ing
conce ntration relaxes the tissue ( 1,2). Th e exact level of
myoplasm ic calcium needed to activa te vasc ular smoo th
muscle depends on the calci um sensitivity of the co ntrac tile
protein s (3) .
M ech anisms of increased intracellular calcium. Free
myoplasm ic ca lcium con cent ration ca n be increased by calcium influx across the plasma membrane or by release of
Fro m the Depart ment of Pharmacology, Faculty of Medici ne and the
*Departmenl of Bioche mistry , Facu lty of Dent istry, Kyushu University .
Fuku oka. Japan .
Manuscript received August 4. 1986 : revised manusc ript rece ived Septembe r 30 . 1986. accepted October 7 . 1986.
Address for reprints: Hiro si Kuriyama . MD. Departme nt of Pharmaco logy . Facu lty of Medicine. Kyushu Univers ity . Fukuoka 812. Jap an .
© 1987 by the American College of Cardiology
concentration. On the other hand , acetylcholine (in the
presence or absence of endothelium cells), norepinephrine or thromboxane A2 increases inositol 1,4,5-tris phosphate and diacylglycerol, thus causing the increase
in the free calcium concentration, whereas vasoactive
intestinal peptide and alpha-human natriuretic peptid e
reduce them. Calmodulin acts as an internal calcium
receptor for regulation of the contractile machinary.
Regulation of calcium homeostasis in relation to the muscle tone in the coronary arteries including other vascular
tissues is discussed together with the role of second messengers.
(J Am Coil Cardiol1987,'9:Jl67-75)
calcium fro m intracellular storage sites, mainly the sarcoplasmic reticulum (4), or both . At present, there appear to
be four basic mechanisms by which intracellular calcium
concentration can be increased :
1) Passive calcium influx . which involves simple diffusion of calci um across the plasma membrane throu gh a
conce ntratio n gradient; the extracellular ca lcium concentration of I to 2 mM is much higher than the intracellul ar
calcium co ncentration, which is in the micromolar range .
2) Calcium influx through voltage-dependent plasma
membrane channels that open in respon se to membrane
This article is part of a series of inf ormal teaching reviews devoted to subjects in basic cardiology that are of
particular interest because oftheir high potential f or clinical
application . The series is edited by Arnold M . Katz. MD.
FACC. a leading proponent of the view that basic science
can be presented in a clear and stimulating fa shion. The
intent of the series is to help the clinician keep abreast of
important advances in our understanding ofthe basic mechanisms underlying normal and abnormal cardiac function.
0735-109 7/87/$3.50
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lACC Vol. 9. No.5
SASAGURI ET AL.
CORONARY TONE AND SIGNALTRANSDUCTORS
depolarization or generation of an action potential. In some
vascular tissues (for example, the myogenic and resistance
vessels), action potentials resulting from calcium influx can
be evoked by nerve stimulationunder physiologic conditions
(5,6).
3) Calcium influx through receptor-operated plasma
membrane channels that open in response to the interaction
of agonists with specific receptors. Such interactions may
also increase membrane sodium permeability, therebycausing membrane depolarization, which initiates a voltage-dependent calcium influx.
4) Intracellular calcium release. Agonist-induced receptor activation may also trigger calcium release from the
sarcoplasmic reticulum. In skeletal and cardiac muscles,
calcium release from the sarcoplasmic reticulum is thought
to be initiated by activation of voltage-dependent calcium
release and calcium-induced calcium release mechanisms
(7,8). A voltage-dependent calcium release in smooth muscle has not been clearly shown, but receptor-operated and
calcium-induced calcium release mechanisms seem to play
a definite role (4,9).
Reduction in free myoplasmic calcium. Smooth muscle relaxation is brought about by a reduction in free myoplasmic calcium concentrationcaused largely by an active,
adenosine triphosphate (ATP)-dependent,calciumextrusion
mechanism (calcium pump) associated with the activation
of a calcium-adenosine triphosphatase (ATPase) (10). This
is in contrast to the sodium-calcium exchange diffusion
mechanism thought to be more important than the calcium
pump for reducing free myoplasmic calcium concentration
in cardiac muscle. The sodium-calcium exchanger utilizes
the potential energy produced by sodium influx down its
concentration gradient rather than energy produced by ATP
hydrolysis (11,12). In cardiac and smooth muscles, myoplasmic calcium is also reduced by the reuptakeof calcium
intocellular stores, mainly the sarcoplasmic reticulum, through
an ATP-dependent calcium transport mechanism (13).
Drugs that influence calcium movements. A wide variety of drugs influence the calcium movements that regulate
the contraction-relaxation cycle in smooth muscle. For example, voltage-dependent calcium influx can be increased
by the calcium "agonists" Bay K-8644, YC-170 or CGP
28392 and can be inhibited by calcium channel blockers
like nifedipine, verapamil and diltiazem (14,15). Receptoroperated calcium influx can also be activated by agonists
that bind to several receptors, such as adrenergic agonists,
and inhibited by the corresponding antagonists. The release
of calcium from smooth muscle sarcoplasmic reticulum is
accelerated by a newly discovered intracellular messenger,
inositol I ,4,5-trisphosphate (16) as well as by caffeine, and
can be inhibited by procaine. A number of signal transductors (second messengers) are involved in calcium homeostasis, including calcium, cyclic nucleotides and inositol
trisphosphate, which mediate the actions of coronary
May 1987:1167-75
vasoconstrictors and vasodilators through actions of, for
example, alpha- and beta-adrenoceptor agonists and nitrates.
This article describes the regulation of coronary artery tone
through the actions of signal transductors that regulate calcium homeostasis.
Regulation of Coronary Artery Tone
What triggers contraction in vascular smooth muscle?
(Fig. 1). Activation of the contractile proteins in vascular
smooth muscle is thought to begin when calcium binds to
the regulator protein, calmodulin. The binding of four calcium ions to calmodulin induces a conformational change
in the calmodulin molecule that allows the calcium-calmodulin complex to interact with the inactive apoenzyme
of an enzyme called myosin light chain kinase. The active
ternary complex: calcium.-calmodulin-myosin light chain
kinase formed in this reaction then catalyzes the phosphorylation of the smooth muscle myosin light chain. Phosphorylation of myosin light chain enables the myosin magnesium-A'TPase enzyme to interact with actin, resulting in
cross-bridge formation and contraction. Reduction in myoplasmic free calcium concentration by any of the mechanisms described here dissociates the calcium-calmodulin
complex, which in turn dissociates calmodulinfrom myosin
light chain kinase, thereby regenerating the inactive apoenzyme of the latter enzyme. As a result, myosin phosphorylation ceases, myosin light chain is dephosphorylated by
a myosin light chain phosphatase and the muscle relaxes
(3,17-19).
Although most investigations support the view that myosin
phosporylation is the first step in smooth musclecontraction,
Figure 1. Regulation of contractile proteins in vascular smooth
muscles. ADP = adenosine diphosphate; ATP = adenosine triphosphate; Ca = calcium; MLCK = myosin light chain kinase;
myosin-® = phosphorylated myosin; Pi = inorganic phosphate.
calmodulin
+
!f
Ca2 + 4 -
MLCK
(inactive) -
calmodulin
.0..
MLCK
(active)
{}
ATP
actin
+
ADP
>-<
myosin
myosin-® + actin
P~
if
I
n
actin - myosin- ®
phosphatase
relaxation c- - - - - - - - - - - -
-~
contraction
JACC Vol. 'I. NO.5
May l'Ig7:1I67-75
some unsolved problems remain. For example, when potassium chloride or an agonist is applied to smooth muscle
for long periods of time, sustained contractions are seen
despite marked reduction of myosin light chain phosphorylation. This finding may reflect a second calcium-dependent regulatory mechanism that enables nonphosphorylated
myosin cross bridges to maintain the interactions between
myosin and actin, and so sustain contraction. These nonphosphorylated myosin cross bridges exhibit slow crossbridge cyclings that have been termed" latch bridges" ( 17).
These findings, therefore, suggest the existence of a dual
activation mechanism in which increased free calcium concentration first activates myosin light chain kinase, whieh
phosphorylates myosin and leads to rapid cross-bridge cycling. Myoplasmic calcium concentration then declines to
a value intermediate between the peak and rest levels, at
which myosin light chain kinase is inactivated and myosin
light chain-phosphatase dephosphorylates myosin. However, contraction is maintained, possibly by a second calcium-dependent regulatory mechanism having a higher calcium sensitivity than that of the myosin light chain kinase
system. Recent measurements of myoplasmic free calcium
concentration indicate that calcium levels remain high during a potassium-induced contraction, but decline rapidly
after the appearance of a transient peak in contraction induced by agonists or caffeine (20,21). However, the detailed
relation between free calcium concentration and the force
of contraction remains to be clarified.
How do nitrates relax coronary vascular tissue? Nitroglycerin, sodium nitroprusside and isosorbide dinitrate
are well known to dilate vascular smooth muscle. Nitrates
are more potent in relaxing venous than arterial strips (22);
these observations have been confirmed using blood flow
measurements in vivo (23). Angiographic observations support the view that nitroglycerin also causes vasodilation of
spastic regions in experimental animals and patients (24,25).
Nitrates relax coronary artery strips precontracted by
voltage-dependent (high potassium concentrations) or receptor-operated calcium influx (norepinephrine or acetylcholine) (5,26,27). Because the causes of angina pectoris
are multifactorial, such nonselective relaxation may be of
more value in alleviating anginal pain than that caused by
more selective drugs such as calcium channel blockers that
act principally on the voltage-dependentcalcium influx. The
nitrates apparently do not act on receptors or calcium channels at the surface of the myoplasmic membrane, and they
do not act directly on the sarcoplasmic reticulum of "skinned"
smooth muscles prepared by disrupting the myoplasmic
membrane with detergents (4). Nitrates do not appear to act
directly on the myoplasmic membrane, but instead may
modify the production of substances that regulate myoplasmic calcium homeostasis. One such substance is cyclic
guanosine monophosphate (GMP), whose concentration in
vascular tissues is increased by nitrates. Exogenously ap-
SASAGURI ET AL.
CORONARY TONE AND SIGNALTRANSDUCTORS
1169
plied dibutyryl cyclic GMP, a permeable derivative of cyclic
GMP, has much the same action on intact muscle tissues
as does nitroglycerin (27). However, nitrates do not modify
the amount of cyclic adenosine monophosphate(AMP) (27),
the metabolism of phosphatidyl inositol 4,5-bisphosphate
or the amounts of diacylglycerol and inositol trisphosphate
in vascular smooth muscles (27-29).
Role of cyclic GMP in relation to smooth muscle relaxation. Increasedcyclic GMP concentrations in such vascular smooth muscles as coronary arteries activates a cyclic
GMP-dependent protein kinase (G-kinase) that catalyzes the
phosphorylation of several proteins. This, in turn, leads to
relaxation of precontracted smooth muscle by the following
mechanisms: I) Cyclic GMP activates a G-kinase that phosphorylates myosin light chain kinase, thereby inhibiting the
phosphorylation of myosin light chain induced by the calcium-calmodulin-myosin light chain kinase complex
(27,30,31). 2) The G-kinase activated by cyclic GMP through
another phosphorylation reaction accelerates the calcium
pump of the smooth muscle plasma membrane, thereby
leading to a reduction in myoplasmic free calcium concentration (32).3) Although G-kinase activated by cyclic GMP
does not appear to modify calcium accumulation into sarcoplasmic reticulum storage sites, it may slightly inhibit
calcium release from them (1,27). These observations indicate that the activation of G-kinase by cyclic GMP produced
in response to the nitrates reduces myoplasmic free calcium
concentration, directly inhibits the contractile proteins and,
perhaps most important, accelerates active calcium extrusion from the myoplasm.
Cyclic GMP levels in smooth muscle can also be increased by atrial natriureticpeptide and endothelium-derived
(dependent) relaxing factor (33-35). Nicorandil (2-nicotineamideethylnitrate), a new antianginal agent that possesses
actions similar to those of nitrates (36,37), also increases
cyclic GMP levels (38). Unlike nitroglycerin and isosorbide
dinitrate, this agent hyperpolarizes the plasma membrane
by opening calcium-independent potassium channels
(36,37,39). Therefore, this drug acts on coronary vascular
smooth muscle in a mannerdifferent from that of the calcium
antagonists and other nitrates.
Why do beta-adrenoceptor agonists have a cardiotonic action on cardiac muscle but a vasodilating action
on the coronary artery? Activation of the betayadrenoceptors of visceral smooth muscles by catecholamines such
as isoproterenol, epinephrine and norepinephrine leads to
relaxation by a response that is thought to be mediated by
cyclic AMP. After treatment with alpha-adrenoceptorblocking agents, the beta--adrenoceptor agonists block the generation of spike potentials and may hyperpolarize the myoplasmic membrane in various visceral muscle cells (5,40);
however, these catecholamines consistently cause smooth
muscle relaxation in the presence of alpha-adrenoceptor
blocking agents. This relaxing effect is accompanied by
SASAGURI ET AL.
CORONARY TONE AND SIGNAL TRANSDUCTORS
1170
JACC Vol. 9. No.5
May 1987:1167-75
increased levels of cyclic AMP with no significant change
in cylic GMP levels (27).
The relaxing effects of cyclic AMP are believed to be
mediated by activation of a cyclic AMP dependent protein
kinase (A-kinase) by the following mechanisms (Fig. 2): I)
Activated A-kinase phosphorylates myosin light chain kinase in a manner similar to that of G-kinase, thereby causing
the muscle to relax by inhibiting the phosphorylation of
myosin (3,17-19,27). 2) Activated A-kinase enhances calcium accumulation by the sarcoplasmic reticulum (27 ,32).
3) Activated A-kinase accelerates calcium extrusion by the
plasma membrane calcium pump (32).
These effects of activated A-kinase are thus similar to
those of activated G-kinase except that these two cyclic
nucleotide-dependent protein kinases differ with regard to
their effects on calcium accumulation into the sarcoplasmic
reticulum. Calcium accumulation is stimulated by A-kinase
but not by G-kinase, so that nitroglycerin reduces the amount
of calcium stored in the sarcoplasmic reticulum but increases
calcium storage in these intracellular membranes (27).
Exogenously applied dibutyryl cyclic AMP, a permeable
derivative of cyclic AMP that has essentially the same effects as isoproterenol (27), and intracellularly applied cyclic
AMP do not modify the amplitude of the inward calcium
current in visceral smooth muscles (41). These effects differ
from the well known effects of beta-adrenocepror activation, intracellularly applied cyclic AMP and extracellularly
applied dibutyryl cyclic AMP to increase voltage-dependent
calcium influx in cardiac muscle.
These findings indicate that the beta-adrenoceptor blockers (which indirectly inhibit the effects of A-kinase de-
scribed) do not prevent angina pectoris by acting directly
as coronary vasodilators; rather, they inhibit vasodilation.
Thus, their antianginal effects arise from their negative inotropic and chronotropic actions, which reduce cardiac oxygen consumption, their ability to reduce renin secretion
and angiotensin II synthesis (42) and their inhibitory actions
on the vasomotor center in the central nervous system (43).
Relation of Inositol Phospholipid to ReceptorOperated Calcium Release From the
Sarcoplasmic Reticulum
Inositol trisphosphate and calcium release (Fig. 2).
Smooth muscle contraction can occur without a significant
change in membrane electrical properties and in the absence
of extracellular calcium. Such contractions are generated by
release of calcium from the sarcoplasmic reticulum by a
process known as pharmacomechanical coupling (1,2,44).
The manner in which the activated receptors trigger calcium
release has recently been explained by the discovery of a
new second messenger, inositol trisphosphate (16,45,46)
(Fig. 2). This substance is derived from inositol phospholipids in the membrane; these include phosphatidylinositol
and its phosphorylated derivatives phosphatidylinositol 4phosphate and phosphatidylinositoI4,5-bisphosphate. In response to receptor activation, phosphatidyl inositol bisphosphate (PI-P z) is rapidly hydrolyzed to form inositol
trisphosphate and diacylglycerol. When a receptor binds to
its specific agonist, a phosphodiesterase (phospholipase C),
is stimulated to hydrolyze phosphatidyl inositol bisphos-
PMA,OAG
extracellular signal
A23187
extracellular space
PI
PIP
OG
PA
plasma membrane
cytoaol
Ca2 + ATPase
sarcoplasmic reticulum
Figure 2. Transmembrane signaling
by way of the turnover of inositol
phospholipids. A23187 = a calcium
ionophore; ATPase = adenosine triphosphatase; Ca = calcium; CaM =
calmodulin; CDP = cytidine diphosphate; C-kinase = cyclic AMP-dependent protein kinase C; CMP = cytidine 5' -monophosphate; DG =
diacylglycerol; GTP = guanosine 5'triphosphate; InsP, InsPz, InsP 3 =
inositol 1- or 4-monophosphate, inositol 4-bisphosphate, inositol 1,4,5trisphosphate, respectively; OAG =
l-oleyl-2-acetyl glycerol; OH = hydroxy residue; ® = energy rich phosphate; PA = phosphatidic acid; PDE
= phosphodiesterase (phospholipase
C); PI, PIP, PIPz = phosphatidylinositol, phosphatidylinositol 4-monophosphate, phosphatidylinositol 4,5bisphosphate, respectively; PMA
phorbol 12-myristate 13-acetate.
SASAGURI ET AL.
CORONARY TONE AND SIGNAL TRANSDUCTORS
lACC Vol. 9, No.5
May 1987:1167-75
phate to form inositol trisphosphate and diacylglycerol (Fig,
2). Subsequently, inositol trisphosphate is convertedto inositol through inositol bisphosphate and inositol phosphate
by stepwise dephosphorylations induced by activation of
individual phosphatases. The breakdown of inositol trisphosphate is accelerated by calcium concentrations up to I
p.,M, the maximal free calcium concentration found in the
myoplasm (47). In a parallel reaction, diacylglycerol kinase
forms phosphatidic acid from diacylglycerol and ATP, and
phosphatidic acid can then be converted to cytidine diphosphate-diacyl glycerate in the presence of cytidine triphosphate. Activation of the enzyme cytidine diphosphatediacylglycerol-inositol phosphatidate transferase allows
phosphatidyl inositol to be synthesized from cytidine diphosphate-diacylglycerol and inositol. Phosphatidyl inositol
is then phosphorylated to produce phosphatidyl inositol
monophsphate and phosphatidyl inositol bisphosphate by
the actions of specific kinase (Fig. 2).
Our present knowledge of the effects of agonist-induced
phosphatidyl inositol bisphosphate hydrolysis on calcium
homeostasis in vascular smooth muscles can be summarized
as follows .. I) In the porcine coronary artery and in rabbit
and dog mesenteric arteries, both acetylcholine and norepinephrine reduce phosphatidyl inositol bisphosphate and
increase phosphatidic acid and 3H-inositol-labeled inositol
trisphosphate. These actions of acetylcholine and norepinephrineare inhibitedby atropineand prazosin, respectively
(48). 2) In these vascular smooth muscles, both acetylcholine and norepinephrine can produce a contraction in calcium-free solution (2). 3) Inositol trisphosphate causes calcium release from isolated sarcoplasmic reticulum fraction,
but only a small calcium release from porcine coronary
artery smooth muscle sarcolemma (49).4) In porcine coronary arteries inositol trisphosphate releases calcium from
skinned muscles and dispersed single muscle cells. 5) Application of inositol trisphosphate produces contractions in
skinned porcine coronary and rabbit mesenteric arteries. 6)
Breakdown of inositoltrisphosphate to inositol bisphosphate
by inositol trisphosphatase is stimulated by increasing calcium concentration (47), and inositol trisphosphatase activity is enhanced by the activation of C-kinase in porcine
coronary arteries. These observations indicate that inositol
trisphosphate, hydrolyzed from phosphatidyl inositol bisphosphate by the activation of phosphodiesterase, is a physiologic second messenger for the agonist-receptor-induced
release of calcium from sarcoplasmic reticulum in vascular
smooth muscle.
Diacylglycerol and protein kinase C. The other product
of phosphatidyl inositol, diacylglycerol, activates another
protein kinase, C-kinase, in the presence of both calcium
and phosphatidylserine (50). These effects of diacylglycerol
can also be produced by a class of carcinogens called phorbol esters (12-0-tetradecanoylphorbol-13-acetate (TPA) or
phorbol 12-myristate-13-acetate) (50).
Activation of C-kinase leads to the phosphorylation of
various proteins in vascular tissue (51) including a 20 kDa
protein of myosin light chain. C-kinase is found mainly in
the cytoplasm, but when this enzyme is activated, it is
concentrated in the sarcolemma by a translocation process
(50,52,53); the importance of this translocation in vascular
smooth muscles to inhibit the contraction has not been established. Whereas TPA in high concentrations has inhibitory actions on the contractile proteins (51), acetylcholine
and norepinephrine have excitatory actions on the mechanextracellular
.---
Figure3. Transmembrane signaling by
way of the adenylate cyclase system.
ADP = adenosine 5/-diphosphate; Akinase = cyclic AMP-dependent protein kinase A; AMP = adenosine monophosphate; ATP = adenosine triphosphate; cAMP = cyclic AMP; GOP =
guanosine diphosphate; Gi, Gs = inhibitory, stimulatory GTP binding protein, respectively; GTP = guanosine
triphosphate; lAP = islet activating
protein (pertussis toxin); NAD = nicotinamide adenine dinucleotide; Ns = Gs;
Ni = Gi; Pi = inorganic phosphate.
/
signals
~
.---
_
"'0
~~o
cholera
t-Q'<
toxiny
NAD
1171
+
+
Pi
Pi
AlP
t
cAMPphosphodiesterase
A kinase
I
+
cellular +
response
protein phosphorylation
1172
lACC Vol. 9, No.5
May 1987:1167-75
SASAGURI ET AL.
CORONARY TONE AND SIGNALTRANSDUCTORS
agonist_
cAMP
-I
A kinase
~
agonist __
cellular response
response
ical properties in porcine coronary and rabbit mesenteric
arteries, respectively. Therefore, inositol trisphosphate, rather
than the C-kinase system, seems to playa major role in the
initiation of contraction (54).
Inositol trisphosphate- and calcium-induced calcium
release mechanism in vascular tissues. In vascularsmooth
muscle, depolarization of the plasma membrane directly
increases free myoplasmic calcium concentration by activation of voltage-dependent calcium influx. Although the
amount of calcium influx calculated from measurements of
the calcium inward current is enough to fully activate the
contractile proteins, the calcium that enters the cell by this
mechanism does not directly activate the contractile proteins. Instead, this calcium is sequestered in a storage site
of the sarcoplasmic reticulum from which it can be released
when a calcium-induced calcium release mechanism is triggered by inositol trisphosphate.
The previously described mechanism appears to activate
the contractile proteins (4, JJ ,26,31) for the following rea-
Figure 4. Interaction between main pathways of
signal transductions. A-, C- andG-kinases = cyclic
AMP-dependent protein kinase A, protein kinase
C and cyclic GMP dependent protein kinase G,
respectively; cGMP = cyclic guanosine monophosphate; RA = adenylate cyclase activating receptor; Rc = phosphodiesterase (phospholipase
C) activating receptor; RG = guanylate cyclase
activating receptor; other abbreviations as in Figures 2 and 3. Bidirectional ( - ) and nondirectional
(+ ) control systems.
sons: I) Procaine inhibits both contractions and calcium
releasefrom smoothmusclesarcoplasmic reticulumbut does
not inhibit the action potential. Thus, procaine uncouples
electricalfrom mechanical activity. 2) Procainehas no direct
effect on contractile proteins as estimated from calciuminduced contraction in skinned smooth muscle tissue. 3)
After depletion of calcium stores in depolarized vascular
smoothmuscle tissue, additionof calcium initially increases
calcium accumulation into sarcoplasmic reticulum and only
subsequently causes contraction. 4) In skinned vascular smooth
muscle tissues, calcium accumulation into the sarcoplasmic
reticulum is increased in direct proportion to appliedcalcium
below 1.0 JLM; further increases in calcium concentration
reduce the amount of calcium stored because of calciuminduced calcium release.
Only one-third of the stored calcium is released from
fragmented sarcoplasmic reticulum by high concentrations
of inositol trisphosphate (10 to 20 JLM). However, in intact
vascularsmoothmuscletissues, nearlyall the storedcalcium
Table 1. Effects of Various Agents on Contraction and Relaxation of Visceral Smooth Muscles
in Relation to Second Messengers
Membrane
Potential
Contraction
Relaxation
ACh
Depo., Hyper.. No change
Contraction
NAd(a!l
NAd (a,)
ISOP
a-hANP
Depo., No change
Depo.
Hyper., No change
No change
Contraction
Contraction
Relaxation
Relaxation
VIP
No change
Relaxation
TXA,
EDRF
No change. Depo.
Hyper.. No change
Contraction
Relaxation
Secondary
Messenger
PI-P,
(cGMP
PI-P,
cAMP
cAMP
cGMP
PI-P,
cAMP
PI-P,
PI-P,
cGMP
(pl-P,
+
+)
+
+
+
+
+
+
-)
In the case of acetylcholine, cyclic guanosine monophosphate (GMP) is increased in the presence of
endothelium; in the case of endothelium-derived relaxing factors (EDRF), phosphat idyl inositol bisphosphate
(PI-P 2 ) is reduced after amounts of cyclic GMP are increased. ACh = acetylcholine; cAMP = cyclic adenosine
monophosphate; depo. = depolarization; hANP = human atrial natriuretic peptide; hyper. = hyperpolarization
of myoplasmic membrane; ISOP = isoprenaline; TXA 2 = thromboxane A2 ; VIP = vasoactive intestinal
peptide.
JACC Vol. 9. No.5
May 1987:11 67- 75
seems to be released by high concentrations of agonists
(26.54,55). Thus, the calcium released by inositol trisphosphate may accelerate or trigger a larger calcium-induced calcium release from sarcoplasmic reticulum to produce
a large contraction in vascular smooth muscle.
Do individual signal transductors (second messengers) act independently or do they interact in the regulation of cellular vascular smooth muscle activity? We
have seen how neurotransmitters can have multiple actions
on vascular smooth muscles by promoting the synthesis of
such signal transductors as cyclic AMP, cyclic GMP and
inositol trisphosphate. Acetylcholine applied exogenously
to excised coronary arteries can generate a variety of responses in smooth muscle from different species; in the
guinea pig coronary artery. acetylcholine hyperpolarizes the
myoplasmic membrane and increases calcium-dependent
potassium permeability. In the rabbit coronary artery. acetylcholine depolarizes the myoplasmic membrane and increases sodium permeability (55), but in the porcine coronary artery, it has no effect on membrane potential and
resistance (5,9). However, in all these tissues, acetylcholine
produces a contraction that is blocked by atropine. Therefore, the mechanisms of action of the same agonist differ
in different species (2).
Acetylcholin e accelerates PI-P 2 breakdown and increases production o] inos itol trisphosphate . Stimulation of
the alpha.vadrenoceptor by norepinephrine also accelerates
PI-P1 breakdown with or without depolarization of the myoplasmic membrane, whereas alpha--adrenoceptor agonists
inhibit synthesis of cylic AMP and have no effect on PI-P,
turnover. Previously, the alpha- adrenoceptor was though;
to be distributed on prejunctional nerve terminals and to act
as a negative feedback controller on transmitter release at
nerve terminals. However, the alphayadrenoceptor is also
distributed on the plasma membranes of both arteries and
veins (56). Stimulation of both beta,- and beta-adrenoceptors increases cyclic AMP. Cyclic AMP production is inhibited by cholera toxin and stimulated by pertussis toxin
(Fig. 3) through adenosine diphosphate-ribosylation of
guanosine triphosphate-binding regulatory proteins that are
found in the plasma membranes (57.58).
In general, the A- , G- and Cikinase systems (including
inositol trisphosphate ) play an important role in the tran smembrane control process of calcium homeostasis in man v
types of vasc ular smooth muscle. However, these systems
do not seem to act independently of each other. One system
potentiates the effect of the other in pancreatic islet cells
(monodirectional control system), whereas one inhibits the
other in lymphocytes and neutrophils (bidirectional control
system) (50) (Fig. 4). The A-, G- and C-kinases are distinctly different in their catalytic functions, but may interact
at the level of their target proteins in vascular tissues; for
example, activation of peptide receptors modulate second
messenger systems to modify the contraction-relaxation cycle.
SASAGURI ET AL.
CORONA RY TONE AND SIGN AL TRANSDUCTORS
1173
Clinical Implications
Contraction and relaxation of visceral smooth muscle are
mediated by second messengers that are generated in response to various neurotransmitters and autacoids (Table 1).
The actions of vasodilators. such as nitrates, alpha- and
beta-adrenoceptor agonists and blocking agents. and some
peptide hormones have been briefly described in terms of
the actions of signal transductors like cyclic nucleotides,
inositol trisphosphate and diacylglycerol on calcium mobilization and homeostasis.
Muscle tone in coronary arteries can be regulated by
many factors. so that the causes of vasospastic angina are
probably multifactorial. Neurogenic , myogenic, humoral and
endothelium-derived factors can generate a variety of messengers that may contribute differently from one patient to
another. Advances in these areas have been rapid and remarkable. but even though calcium homeostasis in smooth
muscle is now better understood, most of the results described in this review introduce more complications than
clarifications. More detailed and intense investigations at
cellular or molecular levels are needed to explain the physiologic and pathophysiologic states of the coronary artery
smooth muscle tone.
We thank A. H. Weston. PhD for critical reading of the manuscript.
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