Inhibition by hypertonic solutions of Ca-dependent electrogenesis in single crab muscle fibers.

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Abstract
The effect of hypertonic solutions on isolated muscle fibers of Callinectes danae was studied. Solutions of twice normal tonicity (2.0 T) inhibit the normal graded membrane responses and the spikes induced by procaine, tetraethylammonium or Ba. The inhibition is maintained throughout exposure to hypertonic solutions prepared by addition of impermeant solutes such as NaCl, sucrose or Tris-propionate, but is reversible on their withdrawal. In the presence of permeant solutes such as glycerol or acetamide, the inhibition is transient. In both cases the onset of inhibition of the depolarizing Ca electrogenesis is correlated with fiber shrinkage. In the case of permeant solutes, the time course of recovery of the graded responses or the spikes follows recovery of the fiber volume. Changes in the passive electrical characteristics of the fibers due to hypertonic solutions were unrelated to the blockade of membrane Ca activation. The current-voltage relationship in hypertonic solution revealed no increase in depolarizing K activation. Inhibition of the graded membrane responses and spikes appears to be associated with depression of Ca conductance. Hypertonic solutions might affect the activation of Ca conductance through reduction of the electric field generated by fixed negative surface changes and/or morphological changes in the T tubules. Membrane depolarization elicited little or no tension in 2.0 T solutions while caffeine contractures (10 mM) with an amplitude of 76% of the maximal contractile ability could still be elicited. This indicates that direct effects of hypertonic solutions on the contractile apparatus were not responsible for loss of tension. The latter is attributed to the inhibition of the transmembrane Ca currents.