Different nerve responses in consecutive sections of the arterial system.

Isometric responses to nerve stimulation and to applied noradrenaline were compared in isolated ring preparations of three consecutive types of rat arteries, viz. the aorta, the superior mesenteric artery and 200-micron branches of the latter. Intramural nerves were activated by graded electrical field stimulation; obtained responses were blocked by tetrodotoxin, phentolamine or prazosin. Also direct muscle activation could be accomplished, using impulses of long duration. In the aorta responses to nerve stimulation were sluggish, could reach only 30-40% of the maximal noradrenaline response and single impulses were ineffective. In the small resistance arteries, neurogenic responses matched the maximal noradrenaline responses and distinct, rapid contractions occurred to single nerve impulses. The superior mesenteric artery was intermediate in these respects. By contrast, on direct muscle activation all vessels responded to single impulses with rapid contractions that differed little in velocity. Pharmacological inhibition of transmitter reuptake increased noradrenaline sensitivity most in the small arteries, but nerve responses were most enhanced in the superior mesenteric artery. These regional differences, probably reflecting decreasing neuromuscular distance with decreasing vessel size, imply that generalizations concerning the relation between responses to nerve stimulation and to exogenous noradrenaline cannot be made from one vascular model only.