Download A Putative Role for Natriuretic Peptides in Fish Osmoregulation

Other functions
of PKC
in vascular
smooth muscle
PKC activation
and translocation
may also play roles in long-term
responses such as gene expression
and
cell proliferation
(Fig. 1). For instance, PKC has been shown to affect
DNA synthesis
by activating
serum
response
elements
associated
with
immediate
early gene transcription
(11). These effects may be related to
the finding mentioned
above that
PKC is prominently
located to a nuclear area in vascular
smooth muscle.
Several lines of evidence also suggest that PKC modulates
ion conductance by phosphorylating
membrane proteins
such as channels,
pumps, and ion-exchange
proteins,
PKC has been proposed to play a role
in extrusion
of Ca2+ immediately
after its mobilization
into the cytosol; the Ca’+-transport
ATPase is a
possible target of this protein kinase.
The Na+-H+ exchange
protein has
been reported
to be another target
that is activated by phorbol esters or
by permeant
DAG analogues,
and
thereby
PKC may function
to increase cytoplasmic
pH (11) (Fig. 1).
Thus PKC appears to perform
a
variety
of functions
in vascular
smooth muscle. Activated
PKC may
translocate
to various cellular membranes. The catalytic
domain may
remain in the vicinity
of the membranes and phosphorylate
nuclear
proteins
or membrane
pumps;
it
may relocate to the cell interior; or
it may act through third messengers
to phosphorylate
cytoplasmic
substrates
that induce
or enhance
smooth muscle contraction.
Different PKC isoforms may have different
locations, substrates,
and functions.
3.
4.
5.
6.
7.
8.
9.
Emerging evidence indicates that
hormones may play a significant
osmoregulation.
Surprisingly,
the
supports a role in salt rather than
Sci./Am.
Physiol.
12.
13.
14.
15.
atria1 natriuretic
peptide-like
peptide
role in various aspects of fish
bulk of current evidence
volume regulation.
Introduction
In the 10 years since de Bold and
his colleagues first described the natriuresis induced in the rat by injection of atria1 extracts, there has been
intense
interest
in what
is now
known
to be a family of natriuretic
21). H. Evans is in the Dept. of Zoology
at the
University
of Florida,
Gainesville,
FL 2261 I,
USA, and is Director
of The Mount
Desert
Island Biology
Laboratory,
Salsbury
Cove, ME
04672, USA. Y. Takei is in the Dept. of Physiology, Kitasato
University
School of Medicine,
Kanagawa
228, Japan.
1. Adam,
L. P., J. R. Haeberle,
and D. R.
Hathaway.
Phosphorylation
of caldesmon
in arterial
smooth
muscle.
1. Biol. Chem.
264:7698-7703,1989.
2. Brozovich,
F. V., M. P. Walsh,
and K. G.
Physiol.
11.
David H. Evans and Yoshio Takei
References
$2.00 0 1992 Int. Union
10.
and T. Tamaoki.
Calphostin
C (UCN1028C), a novel microbial
compound,
is a
highly potent and specific inhibitor
of protein kinase
C. Biochem.
Biophys.
Res.
Commun.
159: 548-553,
1989.
Marston,
S. B. The regulation
of smooth
muscle
contractile
proteins.
Prog. Biophys. Mol. Biol. 41: l-41, 1982.
Nishizuka,
Y. The family
of protein
kinase C for signal transduction.
J. Am. Med.
Assoc. 262:1826-1833,1989.
Sobue, K., and J. R. Sellers. Caldesmon,
a
novel regulatory
protein
in smooth
muscle and nonmuscle
actomyosin
systems. J.
Biol.Chem.
266:12115-12118,1991.
Stull, J. T., P. J. Gallagher,
B. P. Herring,
and K. E. Kamm. Vascular
smooth muscle
contractile
elements:
cellular
regulation.
Hypertension
17: 723-732,
1991.
Suematsu,
E., M. Resnick,
and K. G. Morgan Change of Ca2+ requirement
for myosin phosphorylation
by prostaglandin
Fzn. Am. J. Physiol.
261 (Cell Physiol.
30):
c253-c258,1991.
Winder,
S. J., and M. P. Walsh.
Smooth
muscle calponin.
Inhibition
of actomyosin
MgATPase
and regulation
by phosphorylation. J. Biol. Chem.
265: 10148-10155,
1990.
A Putative Role
for Natriuretic Peptides
in Fish Osmoregulation
The authors
acknowledge
the secretarial
assistance
of Jason Kravitz
and the artistic
and
photographic
assistance
of Al Brass and Rob
Littlefield.
The experimental
studies
were supported
by National
Heart, Lung, and Blood Institute
Grants
HL-31704
and HL-42293.
R. A. Khalil
is a fellow
of the Massachusetts
Affiliate
of
the American
Heart Association.
0886-1714/92
Morgan.
Regulation
of force in skinned,
single cells of ferret aortic smooth muscle.
Pflugers Arch. 416: 742-749,
1990.
Griendling,
K. K., S. E. Rittenhouse,
T. A.
Brock, L. S. Ekstein,
M. A. Gimbrone,
Jr.,
and R. W. Alexander.
Sustained
diacylglycerol
formation
from inositol
phospholipids in angiotensin
II-stimulated
vascular smooth
muscle
cells. J. Biol. Chem.
261:5901-5906,1986.
Hai, C. M., and R. A. Murphy.
Ca2+, crossbridge phosphorylation,
and contraction.
Annu. Rev. Physiol.
51: 285-298,
1989.
Hidaka,
H., and M. Hagiwara.
Pharmacology of the isoquinoline
sulfonamide
protein kinase C inhibitors.
Trends Pharmacok Sci. 8: 162-164,
1987.
House, C., and B. E. Kemp. Protein
kinase
C contains
a pseudosubstrate
prototope
in
its regulatory
domain.
Science Wash. DC
238:1726-1728,1987.
Jaken, S. Protein
kinase C and tumor
promoters.
Curr. Opin. Cell Biol. 2: 192-197,
1990.
Khalil,
R. A., and K. G. Morgan.
Imaging
of protein
kinase C distribution
and translocation
in living vascular
smooth muscle
cells. Circ. Res. 69: 1626-1631,1991.
Kobayashi,
E., H. Nakano,
M. Morimoto,
Sot.
peptide
hormones.
These contain
atria1 natriuretic
peptide
(ANP),
brain natriuretic
peptide (BNP) (g),
C-type natriuretic
peptide (CNP) (11,
12), and the recently described ventricular
natriuretic
peptide (14; Fig.
l), named, with the exception
of
CNP, after the respective
sites of
synthesis:
atrium, brain, and ventricle (6). Interestingly,
it is now clear
that the atrium is the major site of
BNP synthesis,
and CNP appears to
be the major peptide in the brain,
although a substantial
amount is apparently present in the dogfish shark
Volume
7/February
1992
NIPS
15
heart, suggesting that CNP may be a
circulating
hormone in fish (12).
Moreover, ANP-like
immunoreactivity has been localized in a variety
of other tissues including
the lung,
adrenal glands, gonads, gastrointestinal tract, thymus, spleen, pancreas,
eye, and salivary gland (16). Members of this hormone family produce
relaxation
of vascular smooth muscle and natriuresis
by direct glomerular and tubular effects on the kidney and indirectly
by inhibition
of
renin,
aldosterone,
and arginine
vasopressin
secretion by the kidney,
adrenals, and neurohypophysis,
respectively
(e.g., Refs, 4, 6, 9).
For obvious
biomedical
reasons,
the vast majority
of the substantial
literature
on these potentially
natural antihypertensive
hormones deals
with mammals. However,
in the past
4 years, a data base has emerged
suggesting strongly that ANP, as well
as other members of this family, is
present in fish and may play a significant role in their osmoregulation
(6) .
Mechanisms
of fish
osmoregulation
Extant fish can be divided into
three major systematic
groups: the
agnatha
(hagfish
and lampreys),
chondrichthyes
(sharks, skates, and
rays), and osteichthyes
(bony fish,
mostly teleosts). These are the only
primarily
aquatic
vertebrates
and
are modern representatives
of the
earliest vertebrates,
With the apparent exception of the
marine hagfish (which have plasma
NaCl concentrations
almost identical to seawater),
both marine and
freshwater
fish have all evolved
from
freshwater
ancestors
and
therefore have a plasma NaCl concentration
some one-third
that of
seawater but some 300 times that of
freshwater.
Chondrichthyes
have
similar plasma NaCl concentrations
but maintain
substantial
urea and
trimethylamine
oxide levels in their
body fluids,
and therefore
their
plasma is slightly hypertonic
to seawater.
Thus hagfish face no substantial
osmoregulatory
problems in seawater, marine chondrichthyes
face a
potential hypervolemia
and hypernatremia,
marine teleosts face hypovolemia
and hypernatremia,
and
freshwater
teleosts (and the occa16
NIPS
Volume 7/February
1992
sional shark or ray that enters freshwater) face hypervolemia
and hyponatremia (e.g., Ref. 5). Thus a variety
of osmoregulatory
problems are presented to specific fish groups, especially those that are euryhaline
and
can tolerate a range of salinities.
Contrary
to terrestrial
mammals,
where renal function dominates
osmoregulation,
fish utilize an array of
tissues to maintain
plasma tonicity
in the face of net fluxes of water and
salts (5). Briefly, chondrichthyes
balance the osmotic
influx
of water
with relatively
high glomerular
filtration rates (GFR) and urine flows.
The urine is approximately
isotonic,
however,
because the loop of Henle
is not present, so diffusional
gain of
salt is balanced by secretion of NaCl
by the unique rectal gland. Because
sharks in which the rectal gland has
been removed
still survive
in seawater, one must postulate that other
extrarenal
(probably gill) salt extrusion mechanisms
are also present.
Marine teleosts drink seawater to
counter
the osmotic loss of water,
transport
the salt (with water following osmotically)
across the intestine,
and excrete excess salt (= diffusional
+ intestinal
gain) via the gills. Teleosts also lack a loop of Henle and
therefore do not produce a urine that
is hypertonic
to the plasma, despite
the fact that, like in the shark kidney, secretion
of NaCl apparently
takes place in the proximal
tubule
(3) .
HUMAN
It is of interest to note that NaCl
transport
across the shark
rectal
gland, teleost intestine,
shark and
teleost proximal
tubule, and teleost
gill are all via the Na-K-Z1
cotransport system, which
is sensitive
to
loop diuretics
(e.g., furosemide)
and
also is present in the thick ascending
limb of the loop of Henle, as well as
a variety of other cells and epithelia
(10). The only variation
is that the
cotransporter
is apical in the intestine but basolateral
(along with NaK-activated
ATPase)
in the shark
rectal gland, shark and teleost proximal tubule, and teleost gill, all secretory
rather than absorptive
epithelia. Freshwater
teleosts excrete
large volumes
of dilute urine to
counter the osmotic influx of water
and extract needed Na and Cl from
the medium via Na-H and Cl-HCO,
exchange in the gill epithelium,
although there is some evidence that
Na and H movements
may be linked
electrically
rather
than biochemically. Thus fish utilize renal, rectal,
intestinal, and branchial epithelia in
osmoregulation
(5).
ANP effects on fish
osmoregulation
The sequences of teleost (eel and
killifish)
and shark
(dogfish)
hormones in the ANP family (Fig. 1)
have only been known for 18 mo, so
the majority
of studies
examining
a putative
role for these hormones
in fish osmoregulation
has utilized
N-S-F-R---Y-COOH
ANP
N-S-F-R---Y-COOH
--G---R-R-F-COOH
N-S---R-K-COOH
N-V-L-R-R-Y-COOH
K-V-L-R-R-H-COOH
PIG
CNP
CHICKEN
KILLIFISH
EEL
CNP
CNP
DOGFISH
EEL
G-L-S-K
VNP
G-C-F-G-L-K-L-D-R-I-G-S-M-S-G-L-G-C
CNP
COOH
G-w-N-R-G-C-F-G-L-K-L-D-R-I-G-S-M-S-G-L-G-C
IG-W-N-R-G-C-F-G-L-K-L-D-R-I-G-SI;(S-G-L-G-C
CNP
COOH
COOH
COOH
COOH
N-S-L--K-N-G-T-K-K-K-I-F-G-N-COG
FIGURE
1. Amino
acid sequences
of selected
members
of atria1
family.
BNP, brain natriuretic
peptide;
Hum, human;
CNP, C-type
ventricular
natriuretic
peptide.
natriuretic
natriuretic
peptide
peptide;
(ANP)
VNP,
either heterologous
peptides or antibodies raised against mammalian
ANP or BNP. Nevertheless,
some interesting
and unexpected
patterns
have emerged (6). Contrary
to what
might be expected from the natriuretic and diuretic
action of ANP
(hereafter
used to designate the entire family) in mammals, the majority of the extant data supports
the
conclusion
that ANP functions
in
seawater rather than freshwater
osmoregulation
in fishes but may produce branchial
hemodynamic
effects that would
exacerbate
osmoregulation
in either medium. What
is the basis for these conclusions?
Initial studies (see citations 25 and
72 in Ref. 6) demonstrated
that injection
of mammalian
ANP produced natriuresis
in both the freshwater trout and the marine toadfish,
although
in both cases quite high
concentrations
(-0.1 PM) were necessary. Importantly,
the toadfish is
aglomerular,
directly demonstrating
for the first time that ANP-induced
natriuresis
could be produced without changes in GFR. At least in the
freshwater
trout,
the ANP-stimulated natriuresis
was significantly
larger than the diuresis,
somewhat
surprising
in a fish facing hyponatremia, and suggesting that salt extrusion was more sensitive to ANP than
water extrusion.
Natriuresis
in both of these species
may have been secondary
to ANP
stimulation
of proximal NaCl secretion (see above) in a manner similar
to that demonstrated
for the shark
rectal gland and teleost gill (see below), but this proposition
remains
unstudied.
More recently,
it has
been shown that physiologically
relevant concentrations
of ANP (-130
pg/ml) actual1 y produce a fall in the
GFR in the shark SquaJus acanthias,
although volume loading, caused by
placing this species in 90% seawater,
resulted in glomerular
diuresis subsequent to injection of the same dose
of mammalian
ANP (2). This suggests the interesting
possibility
that
the renal response may be keyed to
the salinity, although the shark also
faces a volume load in seawater (see
above). Nevertheless,
it is clear that
the expected
correlation
between
volume
load and diuresis
is not
supported by the extant data in fish,
and additional
studies
are warranted.
The data on other osmoregulatory
organs in fish are somewhat
clearer,
albeit limited. NaCl uptake, subsequent to ingestion of seawater, in the
intestine of the marine flounder
is
inhibited
by mammalian
ANP, but
salt extrusion
by the marine killifish
opercular epithelium
(which models
the gill epithelium)
is stimulated
by
ANP (citation
114 in Ref. 6). Salt
secretion
by the shark rectal gland
is also stimulated by ANP apparently
directly (8) as well as indirectly
via
the release of glandular
vasoactive
intestinal
polypeptide
(citation
118
in Ref. 6).
As indicated above, each of these
tissues transports
salt via the NaK-Xl
cotransporter,
although
the
transport
geometry
(basolateral
vs.
apical placement
of transporters)
of
the cells varies
somewhat
(see
above), possibly accounting
for the
divergent
effects. However,
stimulation of gill or rectal gland salt secretion is of obvious osmoregulatory
utility only in seawater,
ANP inhibition of salt uptake in the seawater
teleost intestine
is somewhat
more
difficult to rationalize.
It would certainly decrease the salt loading of
these hypotonic
fish, but it is the
only way that ingested water (critical for water balance) can be moved
from the lumen across the intestinal
epithelium.
Such an effect therefore
makes ionoregulatory,
but not osmoregulatory,
sense.
The proposition
that ANP is important in seawater
osmoregulation
in teleosts is supported
by our demonstration
that plasma levels [measured via radioimmunoassay
(RIA)
using
antibodies
against
human
ANP] decreased
in two species of
euryhaline
marine
teleosts
when
they were adapted to 20% seawater
(citation 34 in Ref. 6). Moreover,
ac-
climation
of a freshwater
fish to
higher salinity
(-35%
salt water)
also resulted in a significant
increase
in plasma immunoreactive
ANP (citation 126 in Ref. 6; Table 1). Eels
may be an exception to this pattern;
when acclimated
to seawater,
their
plasma levels of ANP (measured
by
an eel-specific RIA) fell substantially
(Takei, unpublished
observations).
Published
and theoretical
considerations also indicate that ANP may
function
in fish osmoregulation
indirectly via interactions
with other
hormones
known
to be involved in
salt and water balance. For instance,
a very recent study (1) has found
that, both in vivo and in vitro, mammalian ANP stimulated
cortisol secretion by the trout interrenal
gland
(homologue
of the mammalian
adrenal) but only when the fish were
acclimated to seawater. Because cortisol is known
to be a major osmoregulatory hormone in seawater fish,
involved
in the stimulation
of salt
extrusion
(citation
30 in Ref. 6),
these data suggest that ANP also may
have indirect
effects
on the gill
transport
cells.
Importantly,
ANP apparently
inhibits cortisol secretion in mammals
(citation 47 in Ref. 6). In mammals,
ANP is known also to inhibit prolactin secretion by the pituitary (4), and
prolactin
has been long accepted as
a major effector in the osmoregulation of freshwater
teleosts (citation
30 in Ref. 6). However,
no data have
been published
relating ANP to prolactin secretion in fish.
Finally, it is clear that, in mammals, ANP inhibits the production
of
angiotensin
via direct or indirect
effects on kidney
renin secretion
(4). Because angiotensin
stimulates
drinking
in fish (citation 30 in Ref.
6), potential
inhibition
by ANP is
TABLE 1. Effect of salinity on immunoreactive
Species
Sculpin
Flounder
Chub
Eel
High
Salinity
102
32
347
343
t 8.0
Ifi: 4.9
k41
t59
ANP in plasma of euryhaline
Low
Salinity
fishes
Reference
9.6 k 2.1
2.5 t 0.4
146k27
689 t 184
Citation
34, Ref. 6
Citation
34, Ref. 6
Citation
126, Ref. 6
observations
Takei, unpublished
All concentrations
are means t SE in pg/ml
plasma. Chub (Gila atraria)
is a freshwater
teleost, sculpin
(Myoxocephalus
octadecimpinosus)
and flounder
(Pseudopleuronectes
americanus) are euryhaline
marine teleosts, and the eel (Anguilla
japonica)
is a euryhaline
cultured
freshwater
teleost.
Volume 7/February 1992
NIPS
17
consistent
with ANP’s inhibition
of
salt uptake across the intestine (see
above),
despite
the uncertainty
about the osmoregulatory
utility of
this effect in marine
teleosts (see
above). Studies investigating
this potential inhibitory
activity of ANP on
the renin-angiotensin
system in fish
need to be undertaken.
ANP effects
NIPS
x
+
110
[fAGFISH
TOADFISH:EEL
and function of the ANP family
peptide hormones.
WATER
SALINITY
SHARK
ANP
OR
KILLIFISH
1
A
B
x
8
n
+
A
x
?!I
+
0
0
Volume
7/February
1992
ANP
CONC.,
of
CNP
0
on hemodynamics
ANP has also been shown to be
vasoactive in fish. Infusion of mammalian ANP produced a fall in pressure in the dorsal aorta of both the
shark (2) and the eel (13) but increased the dorsal aortic pressure in
the trout, apparently
via release of
other hormones
(citation 96 in Ref.
6). Infusion of the newly described
eel ANP (13) or eel CNP (15) also
produced systemic hypotension in
the eel, with a greater efficacy than
human ANP, suggesting increased
sensitivity to homologous peptides.
Correlation between systemic blood
pressure and salinity in fish is unclear, but, intuitively, one might suspect that ANP-induced hypotension
would be physiologically relevant in
freshwater, rather than seawater,
fish.
Using vascular rings from the ventral aorta of a teleost, shark, and
hagfish, we demonstrated that rat
ANP produces relaxation
with a
half-maximal
effective concentration in the nanomolar range, a sensitivity
similar to that described
in isolated mammalian vascular
smooth muscle rings (Fig. 2). Interestingly, eel ANP and killifish CNP
do not show any greater efficacy in
relaxing ventral aortic rings from a
teleost, suggesting that there may be
as much variability
of ANP sequences within the piscine vertebrates as within the vertebrates generally. There are not enough fish
sequences published yet to determine whether this is true, but at
least eel and killifish CNP differ by
only one amino acid residue and
shark CNP by four or five from the
other two (Fig. 1). Finally, the sensitivity of the toadfish (teleost) aortic
rings to rat ANP increased lo-fold
when that species was adapted to 5%
artificial seawater (citation 34 in Ref.
6), suggesting upregulation of receptor numbers correlated with a fall in
plasma ANP in lower salinities.
18
•I TOADFISH:SEA
A TOADFISH:LOW
0 DOG FISH
MOLAR
FIGURE
2.
Effect of mammalian
and fish
atria1 natriuretic
peptide
(ANP) and C-type
natriuretic
peptide
(CNP) on aortic vascular
smooth
muscle
rings
from
spiny
dogfish
(Squalus
acanthias),
hagfish
(Myxine
glutinosa), and toadfish
(Opsanus
beta). Data are
redrawn
from Refs. 7 and 11, Data for effect
of eel ANP and killifish
CNP are overlapped
because
they are nearly
identical.
It was also shown, using isolated
perfused toadfish (teleost) heads,
that rat ANP produced a net fall in
the vascular resistance of this complex vasculature,
suggesting that
ANP vasodilates the branchial vasculature, which presumably predominates in the perfused head, as
well as the ventral aorta (citation 34
in Ref. 6). Because increased perfusion of the branchial vasculature
would probably also be associated
with an increase in the surface area
for net osmotic and ionic losses, it is
difficult to see the adaptive value of
this response, at least with regard to
osmoregulation.
Could it be that the ANP family of
peptide hormones had some function in the original vertebrates, not
associated with defense against osmotic and ionic problems in nonisotonic solutions such as seawater and
freshwater? The fact that rat ANP
vasodilates in the ventral aorta of the
hagfish (7), which does not have any
substantial osmotic or ionic problems in seawater, suggests that this
might be the case.
Clearly, further studies on putative roles for ANP in fish physiology
are warranted. However, it is clear
that these “lower” vertebrates give
us an important opportunity to study
the evolution of both the structure
The restriction
of editorial
guidelines
has
limited
references
cited.
A more complete
listing of contributors
to the knowledge
about
this topic may be found in Ref. 6.
The writing
of this review,
as well as our
recent research,
has been supported
by Grant
DCB 8916413 from the National
Science Foundation
(to D. H. Evans)
and Grant
02640584
from the Ministry
of Education,
Science,
and
Culture
of Japan (to Y. Takei).
References
1. Arnold-Reed,
D. E., and R. J. Balment.
Atria1
natriuretic
factor
stimulates
invivo and in-vitro
secretion
of cortisol
in
teleosts. J. Endocrinol.
12: R17-R20,
1991.
2, Benyajati,
S., and S. D. Yokota.
Renal effects of atria1 natriuretic
peptide
in a marine teleost.
Am. 1. Physiol.
258 (Regulatory
Integrative
Camp.
Physiol.
27):
R1201-R1206,1990.
3. Beyenbach,
K. W., and M. D. Baustian.
Comparative
physiology
of the proximal
tubule.
In: Structure
and Function
of the
Kidney,
edited by R. Kinne. Basal: Karger,
1989, vol. 1, p. 103-142.
4. Brenner,
B. M., B. J. Ballermann,
M. E.
Gunning,
and M. L. Zeidel.
Diverse
biological
actions
of atria1 natriuretic
peptide. Physiol.
Rev. 70: 665-699,
1990.
5. Evans, D. H. Fish. In: Comparative
Physiology of Osmoregulation
in Animals,
edited by G. M. 0. Maloiy.
Orlando,
FL:
Academic,
1979, vol. 1, p. 305-390.
6. Evans, D. H. An emerging
role for a cardiac peptide
hormone
in fish osmoregulation.
Annu.
Rev. Physiol.
52: 43-60,
1990.
7, Evans, D. H. Rat atriopeptin
dilates
vascular smooth
muscle of the ventral
aorta
from the shark
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David F. Moffett and Alan Koch
The midgut of some insects actively transports K+ from blood to lumen.
The transporting
cells extrude K+ into an apical goblet cavity, from
which it diffuses into the gut lumen via a small valve. The reasons
why such a complicated
cytoarchitecture
envelops an ion
transport process are explored.
In 1961, when epithelial physiology
was a relatively young science, William Harvey, then a visiting scientist
with Karl Zerahn at the Institute of
Biological Chemistry of the University of Copenhagen, made a sac preparation of the midgut of a larva of
the silkworm moth Samia cecropia
in the manner Zerahn was using to
study sodium transport by toad urinary bladder. To their surprise, the
tissue developed a transepithelial
potential of X00
mV.
In a series of papers (reviewed in
Ref. 8), Harvey, Zerahn, and their
associates characterized
this new
transport system, showing that 1) the
midgut secretes K+ at a very high
rate (as much as 2 ~eqcm-zomin-l),
2) K+ secretion accounts for almost
all of the short-circuit
current, 3)
K+ transport does not require Na+
or involve Na+-K+-adenosinetriphosphatase (ATPase), and 4) the transport process is electrogenic. Intensive study of this transport system in
several laboratories had made it one
of the best understood of invertebrate epithelia, but recent studies
have raised some perplexing new
questions.
Cellular basis of active
K+ secretion
The midgut of lepidopteran insect
larvae (the caterpillar
larvae of
moths and butterflies) contains two
major cell types (Fig. 1). The most
numerous are columnar cells possessing a tuft of apical microvilli. In
addition, there are goblet cells, in
which the apical membrane is invaginated to form an apical cavity.
The unusual structure of goblet
cells suggested they might be responsible for active K+ secretion (4,
8). The cavity accounts for 40-70%
of the volume of the cell and contains a “matrix,” suggested by histochemistry to consist of polyanion (reviewed in Ref. 5). Goblet cells from
the middle and anterior midgut are
distinguishable from posterior midgut goblet cells in having larger cavities that extend further toward the
basal pole of the cell.
The interior of the goblet cavity is
lined with microvilli
that project
into the cavity. In the anterior and
D. F. Moffetf
and A. Koch are in the Laboratory
of A/lolecular
Physiology,
Dept. of Zoology,
Washington
State University,
Pullman,
WA
0886-1714/92
USA.
$2.00 0 1992 Int. Union
Phvsiol.
Sci./Am. Phvsiol.
1990.
16.
1991.
The Insect Goblet Cell: A
Problem in Functional
Cytoarchitecture
,992 64-4220,
15. Takei, Y., A. Takahashi,
T. X. Watanabe,
K. Nakajima,
S. Sakakibara,
T. Takao, and
Y. Shimonishi.
Amino
acid sequence
and
relative
biological
activity
of a natriuretic
peptide
isolated
from eel brain. Biochem.
Biophys.
Res. Commun.
170:
883-891,
Sot.
Vollmar,
A. M. Atria1 natriuretic
in peripheral
organs other than
Klin. Wochenschr.
68: 699-708,
peptide
the heart.
1990.
middle regions of the midgut, each
microvillus
contains a mitochondrion; this close relationship of mitochondria with the goblet cell apical membrane (GCAM) does not occur in the posterior midgut. The
most apical part of the goblet cavity
forms a narrow, tortuous passage
surrounded by interdigitated microvilli (Fig. 1). This structure has been
termed the apical valve, since in
electron
micrographs
individual
passagesmay appear open or closed
(reviewed in Ref. 5). Such valves are
not characteristic of other secretory
cells with apical crypts or cavities,
such as the vertebrate goblet cell and
gastric parietal cell or the chloride
cell of fish gill. Furthermore,
although somewhat similar cells have
been reported in the gut and integument of other orders of insects,
none of these have apical valves.
Electrophysiology
of goblet cells
Penetrations with microelectrodes
of the isolated midgut under open
circuit showed that the transbasal
potential (Vb) is of a magnitude not
unexpected for gut epithelial cells
(-20 to -40 mV) (Fig. 2). It is not
possible to distinguish two modes of
Vb, and other evidence suggests that
goblet cells are electrically coupled
to surrounding columnar cells (9).
The potential across the GCAM (Vam)
is large (70-140 mV) (Fig. a), suggesting that this is the site of the electrogenie K+ pump. Under short circuit
(Fig. 2A), the potential of the goblet
cavity relative to the luminal solution (VJ is positive by -50 mV; VP is
reduced to a few millivolts under
open circuit (Fig. 2B).
Conclusive evidence for active K+
transport across the GCAM was provided by recent studies in our laboratory (9, 10). Goblet cells and goblet
cavities in posterior midgut were
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1992
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