Ca2+, myosin phosphorylation, and relaxation of arterial smooth muscle.

Relaxation of tissues prepared from the swine carotid media following agonist (110 mM K+) washout was analyzed as a dual-exponential decay. The time course of the initial rapid phase (about 2 min) corresponded to myosin dephosphorylation and to the decay of the capacity to shorten isotonically. Because myosin was dephosphorylated to basal levels within 2 min, we hypothesize that the later, slow phase of relaxation (lasting up to 45 min) was due to a slow inactivation of nonphosphorylated cross bridges. Removing extracellular Ca2+ (0 mM CaCl2, 0.1 mM ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid) greatly enhanced the rate of the slow phase of relaxation, and raising extracellular CaCl2 to 5 mM slowed relaxation significantly. A slow rate of Ca2+ removal to a final concentration that maintains resting tone appears to produce the slow phase of relaxation. These results support hypotheses based on other studies of contracting muscles. There appear to be two populations of cross bridges interacting with the thin filament: 1) phosphorylated and capable of rapid cycling, and 2) dephosphorylated cross bridges that can maintain stress. The latter reflect an unidentified regulatory mechanism, which appears to have a high sensitivity for Ca2+.