Ca2+-dependent stress maintenance without proportional myosin light chain (MLC) phosphorylation was demonstrated in the detergent-skinned swine carotid media when [Ca2+] was reduced from high to intermediate concentrations (Chatterjee, M., and R. A. Murphy. Science Wash. DC 221: 464-466, 1983). In this study of the same preparation, we examined the influence of the initial [Ca2+] and MLC phosphorylation levels on the formation of a stress maintaining state (the "latch" state). Fibers were contracted with 0.82, 1.3, 2.1, or 7.2 microM Ca2+ and then exposed to a lower [Ca2+] to determine the magnitude and Ca2+ sensitivity of stress maintenance. MLC phosphorylation levels were measured in all strips. The magnitude of the stress that rapidly developed was dependent on the initial activating [Ca2+]. The Ca2+ sensitivity of stress maintenance appeared to be independent of the initial levels of stress and MLC phosphorylation. However, the magnitude of the maintained stress was dependent on the initial levels of Ca2+. Only two values for half-maximal responses were evinced in all Ca2+-dependent stress curves: 1.4 X 10(-6) M Ca2+ for rapid stress development and 3.1 X 10(-7) M for stress maintenance. Cumulative [Ca2+]-response curves were shown to result in a decreased maximal stress response and an increase in the apparent Km compared with curves determined by responses to single [Ca2+] exposures. This was associated with a time- and stress-related deterioration of the preparation. The latter was not prevented by exogenous calmodulin or leupeptin (an effective inhibitor of an endogenous Ca2+-dependent protease shown to disrupt the cellular cytoskeleton).(ABSTRACT TRUNCATED AT 250 WORDS)
