CARDIOVASCULAR EFFECTS OF ARGININE VASOTOCIN IN THE RAINBOW-TROUT ONCORHYNCHUS-MYKISS

The physiological functions of the neurohypophyseal hormone arginine v asotocin (AVT) in teleosts are not clear, In the present studies, the sites and mechanisms of action of AVT on the rainbow trout Oncorhynchu s mykiss cardiovascular system were examined in unanesthetized instrum ented fish, perfused organs and isolated vessels, Injection of AVT (1, 10 or 100 pmol kg(-1) body mass) into trout with dorsal aortic cannul as produced a modest, but dose-dependent, increase in dorsal aortic pr essure (P-DA). Bolus injection of AVT (100 pmol kg(-1) body mass), or continuous infusion (6.7 pmol kg(-1) min(-1)), into trout instrumented with dorsal aortic, ventral aortic and central venous cannulas and a ventral aortic flow probe significantly increased P-DA as well as vent ral aortic (P-VA) and central venous (P-VEN) blood pressure, Bradycard ia accompanied the rapid rise in P-VA while gill resistance (R-G) incr eased, Maximum response to the AVT bolus was reached within 13-21 min and the response decayed slowly over the ensuing 90 min, AVT infusion (6.7 pmol kg(-1) min(-1)) significantly increased P-VEN and mean circu latory filling pressure and decreased unstressed blood volume, whereas venous compliance was unaffected, These iii vivo studies indicate tha t AVT increases venous tone, thereby mobilizing blood from the unstres sed compartment into the stressed compartment, This increases P-VEN, w hich increases venous return and helps maintain, or slightly elevate, cardiac output, This, combined with an elevated R-G and slightly eleva ted systemic resistance (R-S), increases both P-VA and P-DA; however, the rise in P-DA is mitigated by a disproportionate increase in Re rel ative to R-S, In vitro, the effects of AVT are consistent with in vivo responses, AVT increased vascular resistance in the perfused gill and perfused trunk and contracted isolated vascular rings from both rainb ow and steelhead trout, The general order of sensitivity of isolated v essels to AVT was (in decreasing order): anterior cardinal vein, celia comesenteric artery, ductus Cuvier, efferent branchial artery, ventral aorta and coronary artery, Extracellular Ca2+ accounted for over 70 % of the tension in the AVT-contracted efferent branchial artery, but o nly 57 % of the tension in the anterior cardinal vein, Vascular AVT re ceptor sensitivity (EC50) in vitro ranged from 0.3 to 6 nmol l(-1) and was similar to the estimated ED50 for the dose-dependent increase in P-DA in vivo (approximately 1 nmol l(-1)). AVT was not inotropic in pa ced ventricular rings nor did it exhibit vasorelaxant activity in perf used organs or vascular rings, These results show that AVT is a potent vasoconstrictor in trout and that its two primary cardiovascular targ ets are the systemic veins and the branchial vasculature.