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PubMed2006

Lipid content and fatty acid distribution in tissues from Portuguese dogfish, leafscale gulper shark and black dogfish.

The lipid characterization in tissues from the three deep-sea sharks leafscale gulper shark (Centrophorus squamosus), Portuguese dogfish (Centroscymnus coelolepis) and black dogfish (Centrocyllium fabricii) captured at Hatton Bank in the North Atlantic were examined. The objective was to determine the lipid content and the fatty acid composition in different tissues. In addition, the fatty acid composition in tissues and species was compared. The tissues examined were pancreas, heart, kidney, stomach, spleen and liver. The lipid content was high in liver (40-50%) and ranged from 1% to 5% in the other tissues. The dominant fatty acids were C16:0, C18:1 (n-9), C18:1 (n-7) and C22:6 (n-3) in all tissues. All tissues had a high content of unsaturated fatty acids.

Animals↗
PubMed1989

Mechanically stripped pigmented and non-pigmented epithelium of the shark ciliary body: morphology and transepithelial electrical properties.

Sections of intact ciliary epithelium and mechanically stripped non-pigmented (NPE) and pigmented (PE) cell layers of adult sharks (Squalus acanthias) were mounted in Ussing-type chambers (area 0.1 cm2). Addition of 10(-5) M forskolin to the aqueous side of intact epithelium significantly increased short-circuit current (Isc) within 15 min and a maximum of approx. 30 microA cm-2 was reached after 45-60 min. Transepithelial potential difference (V) increased from -0.8 mV (aqueous side negative as compared with blood/stromal side) to -1.5 mV, whereas resistance (R) was unchanged (50 omega cm2). Forskolin was without effect when applied to the blood side. In stripped PE preparations (R 15 omega cm2), 10(-5) M forskolin applied to the apical side induced a qualitatively similar change of Isc and V compared with the intact tissue. The forskolin-induced effects were fully reversed by 10(-4) M bumetanide and were not dependent on pretreatment of the tissue with 10(-3) M BaCl2. In stripped NPE preparations resistance was usually less than 10 omega cm2 and was not stable. This is consistent with the morphologic observation that although tight junctions were still demonstrable in stripped NPE cells, the apical membranes were damaged. In preparations taken for light and electron microscopy the stripped PE layer revealed intact epithelial cells. In particular, the basal thirds of the stripped PE cells were in very close contact with each other. These attachment zones may have the appearance of tight junctions. Thus the PE cells of the shark ciliary epithelium can be successfully isolated for transepithelial transport studies. The adenylate cyclase system is present in PE cells, and transepithelial transport of chloride may be regulated by intracellular cAMP.

Animals↗
PubMed1991

Coronary myointimal lesions in the dogfish shark Scyliorhinus canicula.

Proliferative lesions have been found in the coronary arteries of five specimens of dogfish shark (Scyliorhinus canicula). Three specimens lacked the internal elastic membrane and had a diffuse thickening of the intima obliterating most of the vascular lumen in several intramyocardial branches. Two specimens had intimal nodules in one of the main coronary conal trunks, where breaking and splitting of the internal elastic membrane and fibrosis of the tunica media were observed. These lesions appear similar to those described in salmonid fishes, but are less frequent. Although high concentrations of blood cholesterol and ApoB-containing lipoproteins have been related to the development of coronary lesions in these bony fishes, the dogfish sharks described in this paper showed low serum concentrations of both total cholesterol (0.33 to 1.64 mg per ml) and triglycerides (0.61 to 0.17 mg per ml).

Animals↗
PubMed1997

Electrophysiological properties of different cell types in the shark rectal gland.

Electrophysiological properties of different cell types were studied in single rectal gland cells of Squalus acanthias by the whole-cell voltage clamp technique. Based on electrophysiological characteristics and primary morphological observations (light microscope, X400), three cell types (named as I, II, and III) were found in isolated fresh cells and two cell types (I and II) in primary cultured cells of the shark rectal gland (SRG). Type I cells had both Cl- (I(Cl)) and the inwardly rectifying K+ channel (I(K1)). Type II and III cells only had I(Cl) Under X400 light microscope granular materials in the cytoplasm were found in Type I and II cells, but not in Type III cells. The data from this study show that 65 % of isolated fresh SRG cells strongly expressed the K+ channel with much less amount of the Cl- channel and 35% had only I(Cl). In sharp contrast, 11% had I(K1) and I(Cl), and 89% had only I(Cl) in cultured SRG cells. Extracellular application of 10 microM forskolin significantly enhanced I(Cl) in primary cultured SRG cells. This enhancement was influenced by intracellular Ca2+ and blocked by 50 microM Ni2+. Other compounds, such as vasoactive intestinal peptide (VIP) and 8-(4-chlorophenylthio)-adenosine3':5'-cyclic monophosphate (cpt-cAMP) also enhanced I(Cl). Interestingly, cAMP and forskolin significantly inhibited I(K1) in cultured and fresh SRG cells. I(K1) was blocked by micromolar concentrations of Ba2+ and significantly altered by extracellular K+ concentrations. The present data suggest that 1) the shark rectal gland contains different cell types which may play various roles in the process of salt secretion; 2) I(Cl) and I(K1) in SRG cells are strongly modulated by cAMP, forskolin, and VIP, as well as Ca2+, K+, and Na+ ions.

Animals↗
PubMed1994

Structural integrity of the membrane domains in extensively trypsinized Na,K-ATPase from shark rectal glands.

Removal of extramembranous portions of the integral membrane protein Na,K-ATPase from shark salt glands by trypsin in the presence of Rb+ (a K+ congener) preserves the intramembranous association of the remaining membrane-spanning tryptic peptides. This is evidenced from comparison of the rotational mobility of native and trypsinized Na,K-ATPase using saturation transfer electron spin resonance spectroscopy (ESR) and from study of the lipid-protein interactions using conventional ESR spectroscopy. The interface between the lipids and the intramembranous domains is conserved on removal of the extramembranous parts of the protein, since the population of motionally restricted boundary lipids remains essentially the same in the native and trypsinized preparations. The ability to occlude Rb+ is also retained by the trypsinized membranes, as previously observed with pig kidney Na,K-ATPase. A 19-kDa fragment remaining when Na,K-ATPase is trypsinized in the presence of Rb+ is degraded further when the trypsinization is carried out in the presence of Na+ instead of Rb+. The rotational mobility of the tryptic fragments in the Na(+)-trypsinized membranes is lower than for the Rb(+)-trypsinized membranes, indicating rearrangement of the peptides. In addition, occlusion capacity is lost when trypsinization is carried out in Na+, suggesting a correlation between structure and function in the trypsinized membranes. The sequences of four membrane-spanning tryptic fragments of shark Na,K-ATPase are found to be almost identical to corresponding sequences in pig kidney Na,K-ATPase.

Amino Acid Sequence↗
PubMed1999

Novel cardiac protective effects of urea: from shark to rat.

1. This study was carried out to investigate novel cardioprotective effects of urea and the underlying mechanisms. The cardiac functions under oxidative stress were evaluated using Langendorff perfused isolated heart. 2. Isolated dogfish shark hearts tolerated the oxidative stress generated by electrolysis (10 mA, 1 min) of the perfusion solution (n=4), and also showed normal cardiac functions during post-ischaemia reperfusion (n=4). The high concentration of urea (350 mM) in the heart perfusate was indispensable for maintaining the normal cardiac functions of the shark heart. 3. Urea at 3 - 300 mM (n=4 for each group) protected the isolated rat heart against both electrolysis-induced heart damage and post-ischaemia reperfusion-induced cardiac injury. 4. A concentration-dependent scavenging effect of urea (3 - 300 mM, n=4 for each group) against electrolysis-induced reactive oxygen species was also demonstrated in vitro. 5. Urea derivatives as hydroxyurea, dimethylurea, and thiourea had antioxidant cardioprotective effect against the electrolysis-induced cardiac dysfunction of rat heart, but were not as effective as urea in suppressing the post-ischaemia reperfusion injury. 6. Our results suggest that urea and its derivatives are potential antioxidant cardioprotective agents against oxidative stress-induced myocardium damage including the post-ischaemia reperfusion-induced injury.

Animals↗
PubMed1993

Squalamine: an aminosterol antibiotic from the shark.

In recent years, a variety of low molecular weight antibiotics have been isolated from diverse animal species. These agents, which include peptides, lipids, and alkaloids, exhibit antibiotic activity against environmental microbes and are thought to play a role in innate immunity. We report here the discovery of a broad-spectrum steroidal antibiotic isolated from tissues of the dogfish shark Squalus acanthias. This water-soluble antibiotic, which we have named squalamine, exhibits potent bactericidal activity against both Gram-negative and Gram-positive bacteria. In addition, squalamine is fungicidal and induces osmotic lysis of protozoa. The chemical structure of the antibiotic 3 beta-N-1-(N-[3-(4-aminobutyl)]- 1,3-diaminopropane)-7 alpha,24 zeta-dihydroxy-5 alpha-cholestane 24-sulfate has been determined by fast atom bombardment mass spectroscopy and NMR. Squalamine is a cationic steroid characterized by a condensation of an anionic bile salt intermediate with spermidine. The discovery of squalamine in the shark implicates a steroid as a potential host-defense agent in vertebrates and provides insights into the chemical design of a family of broad-spectrum antibiotics.

Amino Acid Sequence↗
PubMedDate not supplied

Vasoactive receptors in abdominal blood vessels of the dogfish shark, Squalus acanthias.

Previous studies have demonstrated that the ventral aorta of the dogfish shark, Squalus acanthias, responds to a variety of cell-signaling agents. To investigate the generality of vasoactive receptors in the shark vasculature, in particular a conductance artery (anterior mesenteric) and vein (posterior intestinal), I measured the effect of acetylcholine, endothelin, nitric oxide, natriuretic peptides, and prostaglandins on tension in isolated rings from these vessels. Both vessels responded to these agents, and responses to receptor-specific ligands for endothelin and natriuretic peptide receptors suggest that B-type endothelin receptors are expressed in both vessels and that the artery expresses both A- and B-type natriuretic peptide receptors; however, the vein (like the ventral aorta) expresses only the B-type natriuretic peptide receptor. My data suggest that a suite of signaling systems is ubiquitous in both arteries and veins in at least this elasmobranch species. Their role in hemodynamics and osmoregulation (perfusion of gill and rectal gland) remains to be determined.

Animals↗
PubMed1999

Mutational pattern of the nurse shark antigen receptor gene (NAR) is similar to that of mammalian Ig genes and to spontaneous mutations in evolution: the translesion synthesis model of somatic hypermutation.

The pattern of somatic mutations of shark and frog Ig is distinct from somatic hypermutation of Ig in mammals in that there is a bias to mutate GC base pairs and a low frequency of mutations. Previous analysis of the new antigen receptor gene in nurse sharks (NAR), however, revealed no bias to mutate GC base pairs and the frequency of mutation was comparable to that of mammalian IgG. Here, we analyzed 1023 mutations in NAR and found no targeting of the mechanism to any particular nucleotide but did obtain strong evidence for a transition bias and for strand polarity. As seen for all species studied to date, the serine codon AGC/T in NAR was a mutational hotspot. The NAR mutational pattern is most similar to that of mammalian IgG and furthermore both are strikingly akin to mutations acquired during the neutral evolution of nuclear pseudogenes, suggesting that a similar mechanism is at work for both processes. In yeast, most spontaneous mutations are introduced by the translesion synthesis DNA polymerase zeta (REV3) and in various DNA repair-deficient backgrounds transitions were more often REV3-dependent than were transversions. Therefore, we propose a model of somatic hypermutation where DNA polymerase zeta is recruited to the Ig locus. An excess of DNA glycosylases in germinal center reactions may further enhance the mutation frequency by a REV3-dependent mutagenic process known as imbalanced base excision repair.

Amino Acid Sequence↗
PubMed2003

Shark skin: a function in feeding.

Dermal denticles are unique tooth-like structures embedded in the skin of sharks and rays that protect them from predators and ectoparasites, reduce mechanical abrasion and possibly minimize swimming-induced drag. Here, we show that juvenile lesser spotted dogfish (Scyliorhinus canicula) also use this body armour to anchor food items near their tail so that bite-sized pieces can be torn away by rapid jaw and head movements. This scale-rasp behaviour is novel among fishes and suggests a new role for skin in the feeding ecology of sharks. Scale rasping may be important ecologically because it could function to increase the dietary breadth and growth potential of juveniles.

Animals↗
PubMed1992

Na-K-Cl cotransport in the shark rectal gland. I. Regulation in the intact perfused gland.

To investigate regulation of the Na-K-Cl cotransport system in the rectal gland of the dogfish shark Squalus acanthias, we examined binding of the loop diuretic [3H]benzmetanide to the intact gland. Glands were perfused with a shark Ringer solution, either in a basal state or stimulated with vasoactive intestinal peptide (VIP). [3H]benzmetanide was added to the perfusion solution for the last 25 min of perfusion, after which the gland was homogenized and the amount of bound [3H]benzmetanide was determined in the membrane fraction. Most of the membrane-associated [3H]-benzmetanide appeared to be associated with the Na-K-Cl cotransporter as judged by the dissociation rates at 0 degree C and 20 degrees C, by labeling with a photosensitive analogue, and by continued association of [3H]benzmetanide with membrane protein on solubilization. With the use of [3H]4-benzoyl-5-sulfamoyl-3-(3- thenyloxy)benzoic acid, a photosensitive analogue of benzmetanide, a 200-kDa protein was selectively labeled on exposure to ultraviolet light. It was also possible to detect [3H]-benzmetanide binding during the perfusion period as an arterial-venous difference, thereby providing a time course of the binding process. In comparing two groups of five glands each, VIP stimulated NaCl secretion 20-fold and [3H]benzmetanide binding 16-fold, providing strong evidence that the Na-K-Cl cotransport system is activated as part of the process of stimulation of secretion. The VIP-stimulated increase in [3H]benzmetanide binding was completely inhibited when Ba was added to the perfusate to block K channel-mediated K exit across the basolateral membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1998

Excretory transport of xenobiotics by dogfish shark rectal gland tubules.

Marine elasmobranch rectal gland is a specialized, osmoregulatory organ composed of numerous blind-ended, branched tubules emptying into a central duct. To date, NaCl excretion has been its only described function. Here we use isolated rectal gland tubule fragments from dogfish shark (Squalus acanthias), fluorescent xenobiotics, and confocal microscopy to describe a second function, xenobiotic excretion. Isolated rectal gland tubules rapidly transported the fluorescent organic anion sulforhodamine 101 from bath to lumen. Luminal accumulation was concentrative, saturable, and inhibited by cyclosporin A (CSA), chlorodinitrobenzene, leukotriene C4, and KCN. Inhibitors of renal organic anion transport (probenecid, p-aminohippurate), organic cation transport (tetraethylammonium and verapamil), and P-glycoprotein (verapamil) were without effect. Cellular accumulation of sulforhodamine 101 was not concentrative, saturable, or inhibitable. Rectal gland tubules did not secrete fluorescein, daunomycin, or a fluorescent CSA derivative. Finally, frozen rectal gland sections stained with an antibody to a hepatic canalicular multispecific organic anion transporter (cMOAT or MRP2) showed heavy and specific staining on the luminal membrane of the epithelial cells. We conclude that rectal gland is capable of active and specific excretion of xenobiotics and that such transport is mediated by a shark analog of MRP2, an ATP-driven xenobiotic transporter, but not by P-glycoprotein.

Animals↗
PubMed1999

Mode of activation of salt secretion by C-type natriuretic peptide in the shark rectal gland.

We studied the modes of activation of the salt-secreting rectal gland of the spiny dogfish, Squalus acanthias, by the native cardiac peptide CNP. The stimulatory action of CNP in isolated perfused glands is inhibited by 10 mM procaine, presumably by blocking release of vasoactive intestinal peptide (VIP) from nerves. Procaine reduces the slope of the dose-response curve of human CNP and that of shark CNP (each P < 0.0001). CNP increases short-circuit current in cultured rectal gland cells from 4.8 +/- 1.6 to 27.0 +/- 7.8 microA/cm2. It also stimulates the secretion of chloride in isolated perfused glands in the presence of 10 mM procaine from 72 +/- 31 to 652 +/- 173 microeq. h(-1). g(-1). These results suggest that CNP has a direct cellular action not mediated by the neural release of VIP. The residual stimulation of perfused glands in the presence of procaine was almost completely inhibited by staurosporine [10 nM; an inhibitor of protein kinase C (PKC)] from 652 +/- 173 to 237 +/- 61 microeq. h(-1). g(-1). Although CNP stimulates guanylyl cyclase in shark rectal gland, chloride secretion of perfused glands was not elicited by 8-bromoadenosine-cGMP (8-BrcGMP) alone nor by the activator of PKC phorbol ester. The combination of PKC activation and 8-BrcGMP infusion, however, stimulated chloride secretion in perfused glands from 94 +/- 30 to 506 +/- 61 microeq. h(-1). g(-1), a level comparable to that observed in glands blocked with procaine. Several parallel pathways appear to be synergistic in activating chloride secretion stimulated by CNP in the rectal gland.

Animals↗
PubMed2005

Selective permeability barrier to urea in shark rectal gland.

Elasmobranchs such as the dogfish shark Squalus acanthius achieve osmotic homeostasis by maintaining urea concentrations in the 300- to 400-mM range, thus offsetting to some degree ambient marine osmolalities of 900-1,000 mosmol/kgH(2)O. These creatures also maintain salt balance without losing urea by secreting a NaCl-rich (500 mM) and urea-poor (18 mM) fluid from the rectal gland that is isotonic with the plasma. The composition of the rectal gland fluid suggests that its epithelial cells are permeable to water and not to urea. Because previous work showed that lipid bilayers that permit water flux do not block flux of urea, we reasoned that the plasma membranes of rectal gland epithelial cells must either have aquaporin water channels or must have some selective barrier to urea flux. We therefore isolated apical and basolateral membranes from shark rectal glands and determined their permeabilities to water and urea. Apical membrane fractions were markedly enriched for Na-K-2Cl cotransporter, whereas basolateral membrane fractions were enriched for Na-K-ATPase. Basolateral membrane osmotic water permeability (P(f)) averaged 4.3 +/- 1.3 x 10(-3) cm/s, whereas urea permeability averaged 4.2 +/- 0.8 x 10(-7) cm/s. The activation energy for water flow averaged 16.4 kcal/mol. Apical membrane P(f) averaged 7.5 +/- 1.6 x 10(-4) cm/s, and urea permeability averaged 2.2 +/- 0.4 x 10(-7) cm/s, with an average activation energy for water flow of 18.6 kcal/mol. The relatively low water permeabilities and high activation energies argue strongly against water flux via aquaporins. Comparison of membrane water and urea permeabilities with those of artificial liposomes and other isolated biological membranes indicates that the basolateral membrane urea permeability is fivefold lower than would be anticipated for its water permeability. These results indicate that the rectal gland maintains a selective barrier to urea in its basolateral membranes.

Animals↗
PubMed1987

Atrial natriuretic peptide stimulates salt secretion by shark rectal gland by releasing VIP.

Salt secretion by the isolated perfused rectal gland of the spiny dogfish shark, Squalus acanthias, is stimulated by synthetic rat atrial natriuretic peptide (ANP II) as well as extracts of shark heart, but not by 8-bromo-cyclic guanosine 5'-monophosphate. Cardiac peptides have no effect on isolated rectal gland cells or perfused tubules, suggesting that stimulation requires an intact gland. The stimulation of secretion by ANP II is eliminated by maneuvers that block neurotransmitter release. These include: perfusion with procaine (10(-2) M), perfusion with high Mg2+ (9.5 mM) and low Ca2+ (0.5 mM) concentrations, and addition to the perfusate of the calcium channel blockers nifedipine (10(-6)M), diltiazem (5 X 10(-5)M), or verapamil (10(-4)M). Cardiac peptides stimulate the release of vasoactive intestinal peptide (VIP), known to be present in rectal gland nerves, into the venous effluent or perfused glands in parallel with their stimulation of salt secretion, but the release of VIP induced by ANP II is prevented by perfusion with procaine. Cardiac peptides thus appear to regulate rectal gland secretion by releasing VIP from neural stores within the gland. It is possible that other physiological effects of these hormones might be explained by an action to enhance local release of neurotransmitters.

Adenosine↗
PubMed1992

Hypotonicity and cell volume regulation in shark rectal gland: role of organic osmolytes and F-actin.

Hypotonic stress (reduction of external tonicity from approximately 900 mosM and 295 mM NaCl to approximately 600 mosM and 135 mM NaCl) produced a relatively slow regulatory volume decrease (RVD) in dogfish shark (Squalus acanthias) rectal gland cells. During the 5-h experiment, cell K+ content remained unchanged; cell content of Na+ and Cl- dropped in the initial swelling phase by some 50% (reflecting the corresponding reduction in medium NaCl), and then remained unchanged during volume recovery phase. Also, cellular fluxes of 86Rb+ and urea were not affected by hypotonic stress. However, hypotonicity enhanced 10- to 20-fold the efflux of organic cell osmolytes taurine, betaine, and trimethyloxamine, and this accounted for the loss of osmotically obliged water during RVD. Enhancement of osmolyte efflux by hypotonic stress was abolished by readjusting the low-Na+ saline to isotonicity (approximately 900 mosM) with innocuous cations (choline+, Li+, or N-methylglucamine+). The results suggest that reduction of medium tonicity may be the determinant for the RVD response to hypotonic stress. The above properties of the observed RVD were also displayed when studying changes on cell F-actin at the basolateral cell face; hypotonic stress (medium with 135 mM NaCl) produced a rapid disappearance of fluorescence related to this cytoskeletal component, whereas no such changes were seen in low-Na+ salines made isotonic with choline or N-methylglucamine chloride nor in a saline made hyposmolar by omitting urea. Hence, hypotonicity is required to affect F-actin organization (depolymerization?). These changes of F-actin fluorescence are transient; they were completed within 5-10 min of hypotonic stress, and afterwards a gradual reconstitution of cell F-actin organization was seen. The above observations are consistent with the assumption that, in shark rectal gland cells, transient loss of cytoskeleton (F-actin) organization at the basolateral cell face, induced by hypotonicity, brings about a selective efflux of organic osmolytes, thus producing the observed RVD.

Actins↗
PubMedDate not supplied

Synthesis of nonmammalian angiotensins and their comparative pressor properties in dogfish shark, domestic chicken, and rat.

To understand how vertebrates utilize angiotensins during evolutionary development, we undertook studies to synthesize and/or characterize angiotensin-like peptides from nonmammalian species. The present paper describes the synthesis of [Asp1,Val5,Asn9] angiotensin I (bull frog, Rana catesbeiana) (I), [Asn1,Val5,His9] angiotensin I (Japanese goosefish, Lophius litulon) (II), [Asn1, Val5,Asn9] angiotensin I (chum salmon, Oncorhynchus keta) (III), and [Asn1, Val5,Tyr9] angiotensin I (related to native angiotensin in snake, Elaphe climocophora (IV). Pressor properties of these peptides were compared with the peptides isolated from other species and related synthetic analogs in one representative species from three distinct classes of vertebrates: 1) elasmobranchs: spiny dogfish shark; 2) birds: domestic chicken; and 3) mammals: rat. The effect of angiotensins on short circuit current (to compare sodium and water permeability) was studied by adding these on the dermal side of the isolated frog skin. In the rat pressor bioassays, the above peptides possessed, respectively, I, 87.8%; II, 51.5%; III, 65.2%; and IV, 60.3% pressor activity of [Ile5] angiotensin II, which was blocked with a converting-enzyme inhibitor, captopril. In the conscious dogfish shark, the percentage increase of blood pressure based on preinjection level (= 100) in the dorsal aortic pressure was 35% to 60% for [Asp1,Ile5,His9] angiotensin I (human) (3 micrograms/kg), [Asp1,Val5,Ser9] angiotensin I (chicken) (3 micrograms/kg), [Asp1,Ile5] angiotensin II (3.6 micrograms/kg), and [Asn1,Val5] angiotensin II (6 micrograms/kg). Likewise, a 30% to 35% increase in blood pressure was obtained with angiotensin III (3 micrograms/kg), [Ile8] angiotensin II (4.4 micrograms/kg), and [Sar1,Ile8] angiotensin II (9.1 micrograms/kg). [Sar1,Thr8] angiotensin II and [Ile8] angiotensin I did not produce a significant pressor response even at high dose-level (8 micrograms/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin I↗
PubMed1999

Characterization of an endothelin ET(B) receptor in the gill of the dogfish shark Squalus acanthias.

Endothelins (ETs) are potent vasoconstrictive peptides that are secreted by the vascular endothelium and other tissues in vertebrates. Previous studies have demonstrated that ETs are expressed in a variety of fish tissues and contract various blood vessels. In order to determine if receptors for ET are expressed in fish gill tissue, we examined the binding kinetics of (125)I-labeled, human ET-1 to membrane fragments isolated from the gill of the dogfish shark, Squalus acanthias. (125)I-ET-1 bound at a single site, with a dissociation constant (K(d)) and binding site number (B(max)) very similar to those described in a variety of mammalian blood vessels. ET-1 and ET-3 competed equally with (125)I-ET-1, suggesting that the receptor was ET(B), which has been shown in mammalian systems to bind to both ligands equally. The ET(B)-specific agonists sarafotoxin S6c, IRL-1620, and BQ-3020 also competed against (125)I-ET-1 at a single site, supporting this hypothesis. We conclude that the shark gill expresses an ET(B) receptor with substantial homology to the mammalian receptor and that ET may play an important role in modulating such vital gill functions as gas exchange, ion regulation, acid-base balance, and excretion of nitrogen.

Animals↗