Arginine vasotocin (AVT) is present in the neurohypophysis of all nonmammalian vertebrates and it appears to be the antecedent of the neurohypophysial nonapeptide hormones. Relatively little is known about AVT receptors in lower vertebrates, especially fish, and the present study was designed to examine AVT receptor interactions in trout vascular and nonvascular smooth muscle in vitro. AVT produced dose-dependent contraction of isolated rings from celiacomesenteric, coronary, and efferent branchial arteries, ventral aorta, anterior cardinal vein, and strips of ductus Cuvier. The greatest efficacy (magnitude of contraction per unit tissue weight) and sensitivity (effective concentration for half-maximal response, EC50) to AVT was found in the efferent bronchial artery (EBA) and its receptors were characterized further. Other neurohypophysial peptides, including arginine vasopressin (AVP), lysine vasopressin (LVP), isotocin (IST), and oxytocin (OXY), contracted EBA with an efficacy order of (most to least) AVT = AVP = OXY > LVP > IST and a sensitivity order of AVT > OXY ≥ AVP IST > LVP. Neither Desmopressin, an AVP V2-receptor agonist, nor the AVP ring fragment, AVP4-9, contracted EBA nor did they inhibit AVT contraction. Pretreatment of EBA rings with the selective AVP V1-receptor antagonists (deamino-Pen1, O-Me-Tyr2, Arg8-vasopressin and deamino-Pen1, Val4, Arg8-vasopressin), the selective V2-receptor antagonist (adamantaneacetyl1, O-Et-D-Tyr0, Val4, aminobutyryl6, Arg8,9-vasopressin), or the combined V1-oxytocin receptor antagonist (d(CH2)5[Tyr(Me)2, Orn8-AVT]) competitively inhibited AVT contractions without affecting AVT efficacy. Receptor affinity constants (pA2) determined by Schild analysis were in the range of 6.8-7.3, with slightly higher constants for the AVP V1-/oxytocin receptor antagonists than for the selective V2-receptor antagonist. Endothelium removal had no effect on EBA sensitivity to AVT. EBA rings were an order of magnitude more sensitive to AVT than nonvascular gastrointestinal and urinary bladder smooth muscle rings or strips. However, AVT (10-7 M) was as efficacious as acetylcholine (10-5 M) in gastrointestinal, gallbladder, and urinary bladder smooth muscle. It is concluded that trout EBA possess an AVT smooth muscle receptor that shares a similar pharmacological profile with the mammalian vascular AVP V(1a)-receptor and the OXY-receptor, but it is distinct from the previously reported gill epithelial cell receptor.
- Urinary bladder
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