Myoplasmic Ca2+ determines myosin phosphorylation in agonist-stimulated swine arterial smooth muscle.

Our objective was to test the hypotheses that 1) myoplasmic Ca2+ is the primary determinant of crossbridge phosphorylation and that 2) phosphorylation is the primary determinant of crossbridge interactions with the thin filament in swine carotid arterial smooth muscle. We tested these hypotheses by evaluating the relation between aequorin-estimated myoplasmic Ca2+, myosin light chain phosphorylation, shortening velocity at zero load (V0), and stress at various times after stimulation with histamine, phenylephrine, and depolarization with KCl. Agonist-induced changes in myoplasmic Ca2+ were associated with predictable changes in myosin phosphorylation. Depolarization required proportionally higher changes in myoplasmic Ca2+ for a given change in myosin phosphorylation. The relation between phosphorylation and V0 or steady-state stress was invariant with all tested stimuli. This suggests that Ca2+-dependent crossbridge phosphorylation is the primary determinant of the mechanical response.