Temporal Aspects of Ca ²⁺ and Myosin Phosphorylation during Myogenic and Norepinephrine-Induced Arteriolar Constriction

Abstract

Previous studies demonstrated that maintenance of steady-state myogenic tone requires Ca²⁺-dependent myosin phosphorylation. The present studies furthered these observations by examining temporal relationships among Ca²⁺, myosin phosphorylation and vessel diameter during acute increases in intraluminal pressure and norepinephrine stimulation. Rat cremaster muscle arterioles were cannulated and loaded with the Ca²⁺-sensitive indicator fura-2. The extent of myosin phosphorylation was measured using two-dimensional gel electrophoresis. Acute increases in intraluminal pressure caused a biphasic increase in intracellular Ca²⁺ ([Ca²⁺]_i), characterized by a transient peak followed by a decline to a steady-state level which remained significantly higher than control values. Peak [Ca²⁺]_i was significantly related to vessel distension and increased with the change in wall tension. Increased intraluminal pressure resulted in a monophasic increase in myosin phosphorylation that was significantly correlated with instantaneous wall tension. In general, norepinephrine induced larger [Ca²⁺]_i transients and a biphasic myosin phosphorylation pattern. The results demonstrate: (a) major roles for Ca²⁺ and myosin phosphorylation in arteriolar myogenic and norepinephrine-induced responses; (b) that changes in Ca²⁺ and phosphorylation during a myogenic response are related to changes in wall tension, and (c) differences in Ca²⁺ and phosphorylation patterns between the two modes of contraction reflect possible differences in underlying signaling mechanisms. The data further emphasize that spontaneous arteriolar tone represents a state of maintained smooth muscle activation that requires increases in [Ca²⁺]_i and myosin light-chain phosphorylation.