Increased susceptibility of newborn puppy mesenteric arteries to ouabain and reduced extracellular K+ concentration.

Responses to the inhibition of Na+, K+ exchange pump activity by a reduction in extracellular K+ concentration and by an addition of ouabain in isolated mesenteric arteries were compared between the newborn puppy and adult dog. In the arteries treated with phentolamine (10(-6)M) or reserpine (0.5 mg/kg), K+-free solution elicited a gradually developed contraction. The tension started to increase much sooner after the substitution with K+-free solution in the newborn than in the older groups; the time to reach 1/2 of the peak contraction was thus directly related to the age. In the newborn artery treated with phentolamine or reserpine, ouabain at a low concentration of 5 X 10(-8)M produced a prominent contraction, while it did not contract the adult artery. The ouabain-induced contraction in the newborn, as well as contractions induced by K+-free solution, was not inhibited by atropine, chlorpheniramine, saralasin, methysergide or aspirin. Contractile responses to norepinephrine and prostaglandin F2 alpha and relaxant responses to nitrite did not change with age in terms of the EC50 and the relative maximum response. Thus, Na+, K+ pump of mesenteric arterial smooth muscle at a resting tension seems more susceptible to the pump inhibition in the newborn than in the adult artery.