The effects of salt-induced hypertension on [[alpha].sub.1]-adrenoreceptor expression and cardiovascular physiology in the rainbow trout (Oncorhynchus mykiss)

Experiments were conducted on rainbow trout to determine the impact of dietary salt on arterial blood pressure. After 4-6 wk, fish fed a salt-enriched diet exhibited a 37% elevation of dorsal aortic pressure (from 23.8 [+ or -]1.2 to 32.6 [+ or -] 1.4 mmHg) and an 18% increase in ventral aortic pressure (from 33.0 [+ or -] 1.5 to 38.9 [+ or -] 1.3 mmHg). The hypertension presumably reflected the increase in cardiac output (from 31.0 [+ or -] 0.8 to 36.4 [+ or -] 2.2 ml.[min.sup.-1]. [kg.sup.-1]) because systemic and branchial resistances were statistically unaltered by salt feeding. The chronic hypertension was associated with a decrease in the pressor responses of the systemic vasculature to catecholamines and hypercapnia in the salt-fed fish. The reduction in [alpha]-adrenergic responsiveness of the systemic vasculature is consistent with desensitization or loss of functional [alpha]-adrenoceptors ([alpha]-ARs). In support of this idea, the salt-fed fish exhibited significantly decreased levels of [[alpha].sub.1D]-AR mRNA in the dorsal aorta and the afferent (ABA) and efferent branchial arteries (EBA). In contrast, however, the results obtained from norepinephrine doseresponse curves for EBA and ABA vascular rings in vitro did not provide evidence for loss of function of branchial artery [[alpha].sub.1]-ARs in the salt-fed fish. Indeed, the [EC.sub.50] for the EBA norepinephrine doseresponse curve was significantly reduced (from 3.75 x [10.sup.-7] to 2.12 x [10.sup.-7] M) in the salt-fed fish, indicating an increase in the binding affinity of the [[alpha].sub.1]-ARs. norepinephrine; epinephrine; blood pressure; systemic resistance; salt feeding; hypercapnia; efferent branchial artery; afferent branchial artery