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Temporal Aspects of Ca2+ and Myosin Phosphorylation during Myogenic and Norepinephrine-Induced Arteriolar Constriction

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.