The Journal of General Physiology, Vol 79, 821-834, Copyright © 1982 by The Rockefeller University Press
Tonic contraction and the control of relaxation in a chemically skinned molluscan smooth muscle
F Cornelius
The same functional states that characterize the living anterior byssus
retractor muscle (ABRM) from Mytilus edulis can be initiated in the
saponin-treated (chemically skinned) muscle preparation under controlled
biochemical conditions. A tonic contraction was induced if the
concentration of free Ca2+ was above approximately 10(7) M in the presence
of Mg2+ and ATP. Maximum tension development was achieved at a Ca2+
concentration of approximately 10(4) M. Within these Ca2+ concentrations
tension was always associated with the presence of 'active state," as
indicated by a high recovery of tension after a quick release in muscle
length. Tonic tension, and the associated active state was maintained for
hours during these conditions irrespective of variations in both ionic
strength and pH. Reduction of the Ca2+ concentration to below threshold for
tension initiation during a tonic contraction immediately switched off the
active state and relaxation of the muscle preparation resulted. However,
the rate of relaxation was extremely low, leaving a substantial fraction of
tension in the absence of active state. Both 5-hydroxytryptamine (5-HT) and
cAMP accelerated this slow relaxation in the absence of Ca2+. Thus, this
state was considered equivalent to the 'catch state" in the living ABRM. In
the presence of Ca2+ concentrations above 10(7) M, cAMP did not affect
either the maximum tension developed or the Ca2+ sensitivity of the
chemically skinned muscle preparation.
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