Potassium-induced relaxation in vascular smooth muscle of ground squirrels and rats

This study was designed to assess differences in potassium-induced relaxation in two rodents, the Sprague-Dawley albino rat and the 13-lined ground squirrel, Citellus tridecimlineatus. Femoral arteries from both species were cut into helical strips for isometric force recording. After norepinephrine-induced contraction in potassium-free solution, the arterial strips relaxed in response to the introduction of potassium (0.25-20 mM) into the bath. Potassium-induced relaxation was greater in rat than ground squirrel arteries. The concentrations required to induce half-maximal relaxation were ~2.5 mM for both species. Potassium-induced relaxation varied with the duration of incubation in potassium-free solution, and with the contractile magnitude induced by varying norepinephrine concentrations. Ouabain inhibited potassium-induced relaxation, with the ground squirrel showing greater sensitivity to the cardiac glycoside than did the rat. Acute cooling (from 37 to 17°C) caused a reduction of the contractile response to norepinephrine in rat arteries, whereas those taken from ground squirrels maintained contractions at, or above, those attained at 37°C. In addition, potassium-induced relaxation in ground squirrel vessels was more sensitive to inhibition by cold than it was in those from the rat. The results show that the characteristics of potassium-induced relaxation (ouabain and temperature sensitivity, magnitude of response, etc.) are species related (ground squirrel vs. rat).