Download Long posterior ciliary arterial blood flow and systemic blood

Investigative Ophthalmology & Visual Science, Vol. 31, No. 5, May 1990
Copyright © Association for Research in Vision and Ophthalmology
Long Posterior Ciliary Arterial Blood Flow
and Systemic Blood Pressure
Hirofumi Okubo, Tseggoi Gherezghiher, and Michael C. Koss
The purpose of the current study was to investigate the relationship between systemic blood pressure
(BP) and long posterior ciliary arterial (LPCA) blood flow (BF) in response to autonomic drugs, and to
determine the types of receptors involved. LPCA BF was measured in anesthetized rabbits at the
retrobulbar region using laser Doppler flowmetry. The observation that most of the short posterior
ciliary arteries diverge from two LPCAs in rabbits was confirmed using a vascular casting technique.
Therefore, the LPCA BF at the retrobulbar region should be representative of total uveal BF. Norepinephrine (IV) increased systemic BP and decreased LPCA BF. These responses were suppressed only
slightly by yohimbine, but were inhibited markedly by prazosin and by the combination of yohimbine
and prazosin. B-HT 920 (IV), a selective alpha-2 adrenoceptor agonist, increased systemic BP and
decreased LPCA BF. B-HT 920 (IA) also decreased LPCA BF, with all effects of B-HT 920 antagonized by yohimbine. Methacholine (IV) decreased systemic BP and increased LPCA BF whether
administered IV or IA. These effects were blocked uniformly by atropine. The current results suggest
that LPCA BF is controlled both by systemic BP and local ocular vascular tone. There are vasoconstrictive alpha-1 and alpha-2 adrenoceptors and vasodilative muscarinic receptors in the rabbit ocular
vascular tree. Invest Ophthalmol Vis Sci 31:819-826,1990
these experiments, ocular BF was measured along the
retrobulbar lateral long posterior ciliary artery
(LPCA) with laser Doppler flowmetry.8'9 Effects of
norepinephrine (NE), B-HT 920, and methacholine
(MC) were investigated.
Earlier studies have shown that topical epinephrine
decreases blood flow in the anterior segment1 and
that intravenous administration of epinephrine produces vasoconstriction of choroidal blood vessels and
of long posterior ciliary arteries.2 In contrast, systemic administration of epinephrine3 and norepinephrine4"6 have been reported to increase choroidal
blood flow (BF) accompanied by a drug-induced increase in systemic arterial blood pressure (BP). Thus,
ocular BF may be related both to systemic BP levels
as well as to local ocular vascular tone. In a similar
fashion, systemic administration of acetylcholine has
been shown to increase choroidal BF of cats5'7 and
rabbits,7 but when administered subcutaneously
to cats, methacholine decreases choroidal blood
volume.4
The purpose of the current study was to investigate
further the relationship between systemic BP and ocular BF changes in response to autonomic drugs. In
Materials and Methods
Adult New Zealand White rabbits (Mel Wishart
Rabbit Ranch, Sand Springs, OK) of either sex were
sedated with ketamine (35 mg/kg, IM) and Xylazine
(5 mg/kg, IM), and anesthetized with pentobarbital
(15-30 mg/kg, IV) through a marginal ear vein. A
femoral vein and artery were cannulated for drug administration and monitoring of systemic BP, respectively. After tracheotomy, the animals were paralyzed
with gallamine triethiodide (2-5 mg/kg, IV) and artificially ventilated with room air. In some experiments, an external maxillary or an ascending pharyngeal artery was cannulated for close intraarterial drug
administration. Additional doses of pentobarbital
were given intravenously as required.
A 360° peritomy was performed, and four rectus
and a portion of the retractor muscles were dissected
carefully so as not to damage the vortex veins. The
superior and laterial orbital margins sometimes were
removed in order to more easily expose the LPCA in
the retrobulbar region. Connective tissue around the
LPCA was carefully removed.
For BF measurement, we used a laser Doppler
flowmeter (Model Pf2; Perimed, Sweden), which de-
From the Dean A. McGee Eye Institute and Departments of
Ophthalmology and of Pharmacology, University of Oklahoma,
Health Sciences Center, Oklahoma City, Oklahoma.
Supported in part by grants from the National Science Foundation (OK 8710676) and Research to Prevent Blindness, Inc.
Presented in part at the 1989 meeting of the Association for
Research in Vision and Ophthalmology, Sarasota, Florida.
Submitted for publication: April 27, 1989; accepted September
29, 1989.
Reprint requests: Hirofumi Okubo, MD, Dean A. McGee Eye
Institute, 608 Stanton L. Young Boulevard, Oklahoma City, OK
73104.
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INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / May 1990
100
50
10
20
40
SO
60
70
B0
90
IOP (mmHg)
Fig. 1. Relationship between percent of the BF in the LPCA and
IOP. IOP was artificially increased by intracameral infusion of
normal saline at a rate of 50 jtl/min during continuous measurement of LPCA blood flow. Note that there was an inverse relationship between LPCA blood flow and an increase in IOP. n = 5-6.
tects the velocity and the number of moving red
blood cells by using the Doppler phenomenon.8-9
That is, the frequency and the magnitude of the
shifted Doppler signal are related to the velocity and
the number of red blood cells, respectively. The blood
flow is calculated internally by multiplying the veloc-
Vol. 31
ity by number. The instrument measures the BF in a
tissue hemisphere with a radius of approximately 1
mm.8-9 A black vinyl membrane was inserted beneath
the LPCA at the posterior aspect of the eyeball close
to the optic nerve in order to prevent the detection of
flow from surrounding tissues. The reliability of the
instrument to detect LPCA BF was tested by measuring LPCA BF against a graded increase of intraocular
pressure (IOP) (Fig. 1).
Drugs used were as follows: NE hydrochloride
(Sigma, St. Louis, MO), MC bromide (Sigma), B-HT
920 (C. W. Boehringer Sohn, Ingelheim, Rhein, West
Germany),10 prazosin hydrochloride (Pfizer, Groton,
CT), yohimbine hydrochloride (Aldrich, Milwaukee,
WI), atropine sulfate (Sigma). Drugs were administered either intravenously or by close intraarterial injection.
The data was presented as means ± SEM. Statistical differences between the means were determined
by the student t-test for paired observations.
In some experiments, ocular vascular structures
were examined with a casting technique.'' After anesthesia with pentobarbital, a common carotid artery
was cannulated and all blood was washed out with
normal saline. Subsequent to injection of 5 ml of 2%
glutaraldehyde, 20 ml of base resin (Mercox CL-2R;
Dainippon Ink and Chemicals, Tokyo, Japan) and 1
ml of polymerizer were mixed and injected. The eye
was enucleated after the animal was left at room temperature for 20 min. The eye was placed in the warm
water for 24 hr and subsequently immersed in a 20%
Fig. 2. Lateral aspect of
vascular cast of rabbit eye.
Most of the short posterior
ciliary arteries (arrow heads)
diverge from the LPCA
(arrow).
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OCULAR BLOOD FLOW / OKubo er ol
No. 5
KOH for one week in order to dissolve the ocular
tissues. The vascular resin casts were washed and observed in distilled water using an operation microscope.
All animals used in these studies were treated in
accordance with the ARVO Resolution on the Use of
Animals in Research.
Results
The vascular casts of rabbit eyes showed that most
short posterior ciliary arteries (SPCAs) diverge from
the LPCA (Fig. 2). As SPCAs run close to the LPCA
at the retrobulbar region, a black vinyl membrane
usually was inserted beneath the lateral LPCA and a
portion of one or more SPCAs.
Intravenous administration of NE (1 Mg/kg) produced a marked increase in systemic BP and simultaneously decreased BF measured at the retrobulbar
LPCA, as shown in Figure 3A. These changes were
dose-related from 0.1 to 3 Mg/kg (Fig. 4A). Prazosin
pretreatment (0.03 mg/kg, IV) reduced the increase
of systemic BP to approximately two thirds that of
control. In contrast, the decrease of BF was corn-
mm Hg
1 min.
200
norepinephrine
norepinephrine
yohimbine 0.5 mg/kg
(V)
norepinephrine
prazosin 0.03 mg/kg
10
norepinephrine
Fig. 3. Tracing of typical
responses of mean systemic
BP (upper panel) and the
LPCA BF (lower panel) to
(A) 1 Mg/kg (IV) of NE and
(B) 1 Mg/kg (IV) of MC.
norepinephrine
norepinephrine
A
mmHg
1 min.
200
atropine 0.3 mg/kg
(V)
alropine 1 mg/kg
10
methacholine
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methacholine
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INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Moy 1990
NE(i.v.)(n
Vol. 31
NE(i.v.)(n = 5)
100
+ 10
O
control
•o
o
2
prazosin
0
<
o
Q.
a
Q.
-I
<^
O
0)
o
E
c
prazosin
control
50
- 50
c
o
B
0)
Q.
A
0.1
1
0.1
1
3
Dose of NE (/ug/kg)
3
Dose of NE (jjg/kg)
Fig. 4. Systemic BP changes (A) and LPCA BF changes (B) produced by NE (0.1-3 Mg/kg, IV) in control and following prazosin
pretreatment (0.03 mg/kg, IV). *, P < 0.05, •*, P < 0.01: significantly different from the control response, n = 4-5.
pletely abolished by prazosin (Fig. 4B). In another
group of animals, yohimbine pretreatment (0.5
mg/kg, IV) appeared to moderately reduce the NEinduced increase of systemic BP (Fig. 5A) as well as
the NE-induced decrease of BF (Fig. 5B). In these
same yohimbine-pretreated animals, prazosin (0.03
mg/kg, IV) markedly inhibited both the NE-induced
increase in systemic BP and the decrease of LPCA BF
(Fig. 5).
B-HT 920, a selective alpha-2 agonist10 (10-100
Mg/kg, IV), increased systemic BP and decreased
LPCA BF in a dose-dependent manner, as shown in
Figs. 6A and 6B. B-HT 920 (1 and 10 fig, IA) significantly decreased LPCA BF at the 10-/*g dose. This
local ocular action was blocked by yohimbine pretreatment (0.5 mg/kg, IV) as shown in Fig. 6C.
Intravenous administration of MC (0.01-1 /ug/kg)
decreased systemic BP and simultaneously increased
LPCA BF in a dose-dependent fashion. These effects
were dose-dependent in the 0.01-1 /xg/kg range (Figs.
3B, 7A, 7B). Intraarterial injection of MC (0.001-1
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Hg) also increased LPCA BF in a dose-related manner
(Fig. 7C). These changes produced by MC also were
significantly blocked by atropine pretreatment
(Fig. 7).
Discussion
It has been pointed out that most SPCAs diverge
from two LPCAs in rabbits,12 although both SPCAs
and LPCAs diverge from ophthalmic arteries in dogs
and cats.13 We have confirmed this anatomic vascular characteristic in rabbit eyes by means of vascular
casts. In addition, the anterior ciliary artery does not
contribute greatly to the intraocular circulation of
rabbits.14 Thus, the LPCA bloodflowmeasured at the
retrobulbar region would be expected to be proportional to total uveal blood inflow.
Intravenous administration of NE increased systemic BP in a dose-dependent manner. This effect
was partially antagonized by either prazosin and yohimbine given alone and more fully antagonized by
the combination of these two antagonists. These re-
OCULAR DLOOD FLOW / Okubo er ol
No. 5
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NE(i.v.)(n = 6-8)
NE(i.v.)(n = 6-8)
100
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control
TJ
O
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yohimbine
+ prazosin
<
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yohimbine
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yohimbine
c
-50
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O
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o
4)
Q.
c
n
c
0)
o
o
yohimbine
+ prazosin
Q.
-60
B
0.1
1
3
Dose of NE (pg/kg)
*+
A
0.1
1
Dose of NE
Fig. 5. Systemic BP changes (A) and LPCA BF changes (B) produced by NE (0.1-3 Mg/kg, IV) in control, after yohimbine (0.5 mg/kg, IV)
and after subsequent administration of prazosin (0.03 mg/kg, IV). *, P < 0.05, •*, P < 0.01: significantly different from the control response.
+, P < 0.05, ++, P < 0.01: significantly different from the yohimbine-treated response, n = 6-8.
suits indicate that the increase in systemic BP induced by NE (IV) is mediated by both alpha-1 and
alpha-2 adrenoceptors. B-HT 920, a selective alpha-2
adrenoceptor agonist, produced a marked dose-related increase in systemic BP that was almost totally
abolished by yohimbine. This observation further
confirms the role of alpha-2 adrenoceptors in control
of systemic BP in this species. If all other factors remain unchanged, an increase in systemic BP would
be expected to increase ocular BF by elevation of the
pressure head. However, in the face of this, LPCA BF
was decreased in a dose-dependent manner by both
NE and B-HT 920 given intravenously. This indicates that NE and B-HT 920 produce a vasoconstriction of the ocular vessels sufficiently potent to override the effect of the elevated systemic BP. Results of
other investigators support the contention that systemic administration of NE produces vasoconstriction of choroidal vessels as well as of the LPCAs in
rabbits.2
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In contrast to the current study, systemic administration of NE has been reported to increase choroidal
BF in cats5 and also to increase the BF of the iris and
ciliary body, choroid, and retina in pigs.6 Epinephrine
also has been reported to increase the choroidal and
retinal BF of rabbits.3 In some cases, differences between the current findings and those of these earlier
reports may be due to species differences or to differences of technique in flow measurement. However, it
also is likely that regional differences in BF distribution may account for these apparent discrepancies.
For example, the BF measured in this report is believed to be representative of total uveal BF, whereas
previous studies focused on regional BF changes. In
this regard, alteration of flow may be more pronounced in the ciliary processes, where BF has been
shown to be greatly reduced by sympathetic nerve
stimulation15 and by alpha-adrenoceptor agonists.1
All other factors being unchanged, prazosin antagonism of the NE-induced BP rise should result in a
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INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / May 1990
B-HT920(i.v.)(n =
Q.
B-HT920(i.v.)(n =
,0 + 1 0
50
a
Vol. 31
o
control
4)
yohimbine
<
o
0.
o
W
0)
O)
c
n
r
o
£
control
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yohimbine
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O
-40
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Q.
10
A
30
100
Dose of B-HT920 (pg/kg)
B
10
30
100
Dose of B-HT920
B-HT920(i.a.)(n =
"2
o +
10
o
<
yohimbine
a
Q.
O
0)
control
c
Fig. 6. Systemic BP changes (A) and LPCA BF changes (B) to
B-HT 920 (10-100 jtg/kg, IV), and LPCA BF changes (C) to intraarterial injection of B-HT 920 (I and 10 /ig)> before and after
yohimbine treatment (0.5 mg/kg, IV). *, P < 0.05, **, P < 0.01:
significantly different from the control response, n = 9, 4.
-30
C
10
Dose of B-HT920(^jg)
potentiated ocular vascular effect (as the opposing increase of the systemic pressure head is reduced). The
observation that prazosin also totally blocked the ocular NE-induced decrease in LPCA BF demonstrates
the importance of alpha-1 adrenoceptors in the ocular vasculature. In a similar manner, the modest suppression of the ocular NE effect produced by yohimbine pretreatment would be expected to be more apparent if the systemic BP increase were not also
suppressed by yohimbine. The interpretation of these
data is that alpha-2 adrenoceptors are also found in
the ocular vasculature; however, they appear to play a
much less significant role than do alpha-1 adrenoceptors in controlling ocular vascular tone. Yohimbine and prazosin given together produce an even
more marked inhibition of the BP rise induced by
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NE. This blockade of the increased pressure head
consequently allowed for the "unmasking" of the residual NE-induced decrease in LPCA BF seen in
the animals pretreated with both blocking agents
(Fig. 5B).
The presence of alpha-2 adrenoceptors in the ocular vascular beds was more clearly revealed by intraarterial injection of the selective alpha-2 adrenoceptor agonist, B-HT 920. The selectivity of this agonist
was underscored by the effectiveness of blockade by
yohimbine, which also is "selective" for alpha-2
adrenoceptors.
A similar argument can be made to explain the
actions of methacholine. Although the systemic BP
was decreased by MC (IV), LPCA BF was increased.
These results indicate that MC produced a sufii-
OCULAR BLOOD FLOW / OKubo er ol
No. 5
u
i0)
MC(i
MC(i.v.)(n = 6)
a.
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+ 10
**
0
>
(A
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1 mg/kg
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-L 0.3 mg/kg
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-40
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atropine
mg/kg
0
0
5 -io
Q.
Dose of MC (ug/kg)
0.01
0.1
1
Dose of MC (fig/kg)
MC(i.a.)(n =
70
o
•o
o
o
<
50
control
o
.J
Fig. 7. Systemic BP changes (A) and LPCA BF changes (B) to
MC (0.01-3 Mg/kg, IV), and LPCA BF changes (C) to intraarterial
injection of MC 0.00l-l ng), before and after 0.3 and I mg/kg of
atropine (IV). *, P < 0.05, **, P < 0.01: significantly different from
the control response, n = 6, 5.
"o
0)
O)
c
10
o
c
0)
atropine
1 mg/kg
o
0)
Q.
0.001
0.01
0.1
Dose of MC
ciently potent ocular vasodilation to overcome the
effect of the systemic BP decrease. This vasodilation
is mediated by muscarinic receptors, as it was inhibited by atropine in a dose-dependent fashion. Experiments with close intraarterial administration of MC
confirm both the potent ocular vasodilation as well as
the muscarinic nature of these responses.
Results from previous studies concerning the ocular vascular effects of muscarinic agonists are conflicting. For example, systemic administration of MC
has been reported to decrease choroidal blood vol-
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ume.4 In contrast, acetylcholine was shown to increase choroidal blood flow in rabbits7 and cats.57 In
the current study, total uveal BF was believed to be
increased by MC. As with NE (see above), species,
methodologic, or regional differences within the eye
may explain the discrepant observations in the literature.
In the current study, the LPCA BF was measured
at the retrobulbar region, and drug-induced alteration of ocular BF seemed to be of sufficient potency
to counteract an opposing change of the systemic BP.
826
INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / May 1990
Key words: ocular circulation, laser Doppler flowmetry,
alpha-adrenoceptors, muscarinic receptors, vascular structures
Acknowledgments
The authors would like to thank Ms. Marlene Richardson for typing the manuscript; Ms. Linda Turner, Ms.
Linda Kuhlman, Hanh Nguyen, and Mr. Robert Adams
for technical assistance; and Ms. Terri Hamby for preparing
the illustrations.
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