In Vitro Studies on Responses to Noradrenaline, Serotonin, and Potassium of Intramyocardial and Mesenteric Resistance Vessels from Wistar Rats

Abstract

The responses to noradrenaline [norepinephrine], serotonin and K of isolated intramyocardial (flow-regulating) coronary and mesenteric resistance vessels of similar internal diameter (.apprx. 200 .mu.m) were compared from Wistar rats. The coronary but not the mesenteric resistance vessels had spontaneous basal tone, which was reduced by elimination of extracellular Ca, by increasing the extracellular K concentration up to 22 mM and by stimulation of coronary vessels with noradrenaline. Noradrenaline always caused contraction of mesenteric resistance vessels and in coronary vessels after incubation with propranolol (3 .times. 10-6 M). The noradrenaline and K concentration-response characteristics of the mesenteric resistance vessels were modulated by perivascular adrenergic nerve terminals, whereas no influence of nerve terminals were shown in the coronary resistance vessels. The sensitivity of mesenteric vascular smooth muscle to noradrenaline [concentration required to give half maximal response (EC50) .apprx. 0.4 .times. 10-6 M] and serotonin (EC50 .apprx. 0.3 .times. 10-6 M) was higher (P < 0.001) than the sensitivity of coronary smooth muscle (ED50 values 1.6 .times. 10-6 and 1.3 .times. 10-6 M, respectively). The K sensitivity of the coronary smooth muscle (EC50 .apprx. 35 mM) was higher than that of the mesenteric smooth muscle (EC50 .apprx. 41 mM; P < 0.01). The respective maximal noradrenaline and serotonin responses relative to the maximal K response were 35 and 55% in the coronary and 127 and 120% in the mesenteric resistance vessels (P < 0.001).