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

We compared the responses to noradrenaline, serotonin, and potassium of isolated intramyocardial (flow-regulating) coronary and mesenteric resistance vessels of similar internal diameter (approximately 200 microns) from Wistar rats. The coronary but not the mesenteric resistance vessels had spontaneous basal tone, which was reduced by elimination of extracellular calcium, by increasing the extracellular potassium 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 X 10(-6) M). The noradrenaline and potassium concentration-response characteristics of the mesenteric resistance vessels were modulated by perivascular adrenergic nerve terminals, whereas no influence of nerve terminals could be shown in the coronary resistance vessels. The sensitivity of mesenteric vascular smooth muscle to noradrenaline [concentration required to give half maximal response (EC50) approximately 0.4 X 10(-6) M] and serotonin (EC50 approximately 0.3 X 10(-6) M) was higher (p less than 0.001) than the sensitivity of coronary smooth muscle (EC50 values 1.6 X 10(-6) and 1.3 X 10(-6) M, respectively). The potassium sensitivity of the coronary smooth muscle (EC50 approximately 35 mM) was higher than that of the mesenteric smooth muscle (EC50 approximately 41 mM; p less than 0.01). The respective maximal noradrenaline and serotonin responses relative to the maximal potassium response were 35 and 55% in the coronary and 127 and 120% in the mesenteric resistance vessels (p less than 0.001).