VOLTAGE CONTROLLED CONTRACTIONS AND CURRENT VOLTAGE RELATIONS OF CRAYFISH MUSCLE FIBERS IN CHLORIDE-FREE SOLUTIONS

In single crayfish muscle fibers isometric force development was measured as function of the clamped membrane potential while extracellular chloride was substituted by various anions. In propionate and isethionate saline relaxation was remarkably slowed and its onset delayed while the rise of contraction was not much affected. At prolonged exposure to these solutions amplitude of contraction decreased and sometimes the mechanical threshold shifted to a less negative potential. In contrast, methylsulphate and nitrate saline had little effect on contraction, rise and fall of contraction being only somewhat accelerated in methylsulphate. The change in membrane conductance due to chloride replacement was studied by measuring current voltage relations. GABA was applied to determine the Cl- equilibrium potential under the various conditions. The findings suggest that in propionate saline the fibers lose chloride while propionate does not permeate the membrane. Also in methylsulphate intracellular chloride is lost, but permeating CH3OSO3- can replace Cl-. So does also nitrate, the permeability of the membrane to this ion being increased by GABA as it is to chloride. It seems that relaxation is connected to the flow of chloride and it is suggested that a high chloride conductance in the transverse tubular system is necessary for its repolarisation. Methylsulphate and nitrate can replace chloride for this function. © 1969 Springer-Verlag.