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 and their existence has been questioned. In the present study, two characteristics of vascular mechanics, compliance and agonist-induced tension development, were investigated in rainbow trout vessels in vitro. Rapid compliance in the anterior cardinal vein and efferent branchial artery was calculated from step-wise changes in the volume-pressure curve of isolated vessel segments. Agonist-induced tension development was 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 branchial artery compliance was decreased by 10^-6 mol·l^-1 epinephrine and norepinephrine 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 viens (24:1). Trout veins contracted in response to agonists in both an, agonist- and vesselspecific 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 thromboxane A₂ agonist, U-44,069, produced approximately identical contractions, whereas angiotensin II was virtually ineffective. Conversely, angiotensin II was more potent than epinephrine in posterior cardinal vein. In cumulative dose-response experiments, epinephrine was equipotent in anterior cardinal vein and duct of Cuvier, whereas the latter was less sensitive to acetylcholine. Both atrial natriuretic peptide and sodium 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 tone and therefore cardiac output in fish may be regulated by neural or humoral mechanisms.Abbreviations ACH acetylcholine - ACV anterior cardinal vein - ANG II salmon asn¹-val⁵ angiotensin II - ANP rat atrial natriuretic peptide - AVT arginine vasotocin - DNR Department of Natural Resources - DOC duct of Cuvier - EBA efferent branchial artery - EC₅ threshold dose producing 5% maximal contraction - EC₅₀ dose producing 50% maximal contraction - EPI epinephrine - HI K⁺ 80 mmol·l^-1 - KCl IV, intestinal vein - NEPI norepinephrine - PBS phosphate buffered saline - PCV posterior cardinal vein - SNP sodium nitroprusside - U-44,069 thromboxane A₂ agonist