Dependence of stress on length, Ca2+, and myosin phosphorylation in skinned smooth muscle

The purpose of this study was to examine the influence of developed stress and tissue length (and presumably filament overlap) on stress hysteresis in skinned vascular smooth muscle. Stress hysteresis is defined as the level of "extra" stress maintained with reduced myosin light chain (MLC) phosphorylation after decreasing the [Ca2+] from a higher to a lower concentration and was postulated to reflect the formation of dephosphorylated, attached, slowly cycling latch bridges in smooth muscle. Detergent skinned tissues of swine carotid media were used for measurement of isometric force and MLC phosphorylation levels. The tissues were either contracted with various [Ca2+] or were first contracted with 7-10 microM Ca2+ and then exposed to a lower [Ca2+]. These two protocols were used at three tissue lengths; 0.7, 1.0, and 1.4 times the optimum length for force development (Lo). The following results were obtained: 1) the order of developed stress was 1.0 greater than 1.4 greater than 0.7 Lo; 2) the Ca2+ sensitivity of developed stress was similar at 0.7 and 1.0 Lo and decreased at 1.4 Lo; 3) the Ca2+ sensitivity and relative magnitude of stress hysteresis were similar at 0.7 and 1.0 Lo with no evidence of stress hysteresis at 1.4 Lo; and 4) the relationship between stress and MLC phosphorylation was affected by tissue length, but the Ca2+ sensitivity of MLC phosphorylation was not. These results suggest that stress maintenance by dephosphorylated crossbridges may be abolished at 1.4 Lo and that the active stress-MLC phosphorylation relationship is altered at long tissue lengths.