The Journal of General Physiology, Vol 75, 709-725, Copyright © 1980 by The Rockefeller University Press
The regulation of tension in a chemically skinned molluscan smooth muscle: effect of Mg2+ on the Ca2+-activated tension generation
F Cornelius
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 Ca2+ above 0.1
microM, and maximum tension development was found at a [Ca2+] =
approximately 32 microM. 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 0.1 microM.
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 approximately 1 pCa unit, indicating the
existence of cooperativity between Ca2+ sites. Increasing the free
concentration of Mg2+ shifted the tension-pCa 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 microM, decreasing to 0.20 microM at [Mg2+] = 0.43 mM. 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.
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