The regulation of tension in a chemically skinned molluscan smooth muscle: Effect of Mg2+ on the Ca2+ -activated tension generation

Abstract

Chemically skinned anterior byssus retractor muscle (ABRM) preparations were prepared by treatment with the nonionic detergents saponin and Triton X-100. Both maximum peak tension and rate of contraction were found to be greater in saponin-treated ABRM than in ABRM treated with Triton X-100. Active tension was initiated at a concentration of free Ca 2+ above 0.1 µM, and maximum tension development was found at a [Ca2+] ~32 µM. During exposure of the muscle preparation to optimal Ca2+ concentration, a high and almost constant tension level was sustained. The force recovery was high after a quick release during this period indicating the presence of an "active" state rather than a "catch" state. Actually, a state equivalent to the catch state in the living ABRM could not be induced, if the Ca2+ concentration was above 0.1µM. Variations in the ionic strength in the range of 0.07-0.28 M had no influence on active state and only slightly affected the maximum tension developed. The influence of Mg2+ on the Ca2+ -activated tension was examined by studying the tension-pCa relation at two concentrations of free Mg2+ (0.43 and 4.0 mM). The tension-pCa relation was found to be S-shaped with tension increasing steeply over ~1 pCa unit, indicating the existence of cooperativity between Ca2+ sites. Increasing the free concentration of Mg2+ shifted the tensionpCa relation to lower pCa as in striated muscles, demonstrating a decreasing Ca2+ sensitivity with increasing Mg2+. At [Mg2+] = 4.0 mM the half-maximum tension was found at [Ca2+] = 0.43 µM, decreasing to 0.20 µM at [Mg2+] = 0.43 raM. At both Mg2+ concentrations studied, plots of log Prel/(1 - Prel) vs. log [Ca2+] were nonlinear with a shape indicating a rather complicated model for cooperativity, probably involving four sites for Ca2+. These Ca2+ - Mg2+ interactions are most probably taking place at the myosin head itself because troponin is absent in this myosin-regulated muscle. © 1980, Rockefeller University Press., All rights reserved.