COMPLIANCE AND SMOOTH-MUSCLE REACTIVITY OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) VESSELS IN-VITRO

Systemic veins have a profound influence on cardiac output in mammals. Venoregulatory mechanisms have not been adequately studied in fish an d their existence has been questioned. In the present study, two chara cteristics of vascular mechanics, compliance and agonist-induced tensi on development, were investigated in rainbow trout vessels in vitro. R apid compliance in the anterior cardinal vein and efferent branchial a rtery was calculated from step-wise changes in the volume-pressure cur ve of isolated vessel segments. Agonist-induced tension development wa s examined in four veins; anterior and posterior cardinals, intestinal and duct of Cuvier. Venous compliance was not altered in response to epinephrine, norepinephrine or angiotensin II, while efferent branchia l artery compliance was decreased by 10(-6) mol.l-1 epinephrine and no repinephrine but not angiotensin II. The ratios of venous to arterial compliance in vessels from two rainbow trout strains were similar (21: 1 and 32:1) and consistent with the ratio reported for mammalian veins (24:1). Trout veins contracted in response to agonists in both an ago nist- and vessel-specific manner. The greatest tension per vessel wet weight was produced in anterior cardinal vein. The response pattern of anterior cardinal vein and duct of Cuvier were similar; acetylcholine , arginine vasotocin, epinephrine and norepinephrine, and the thrombox ane A2 agonist, U-44,069, produced approximately identical contraction s, whereas angiotensin II was virtually ineffective. Conversely, angio tensin II was more potent than epinephrine in posterior cardinal vein. In cumulative dose-response experiments, epinephrine was equipotent i n anterior cardinal vein and duct of Cuvier, whereas the latter was le ss sensitive to acetylcholine. Both atrial natriuretic peptide and sod ium nitroprusside relaxed precontracted veins. This is the first study to determine compliance in fish vessels and the contractile nature of different rainbow trout veins. These findings suggest that venous ton e and therefore cardiac output in fish may be regulated by neural or h umoral mechanisms.