Evidence for Ca-2+ control of the transducer mechanism in crayfish stretch receptor.

Recording from the dendrite membrane indicated a resting potential of minus-51.6 mV, which was reduced by inhibition of the Na+/K+ pump. Voltage clamp at rest revealed a small inward current between minus-50 and minus-80 mV and a larger outward current at clamp potentials of minus-40 to +30 mV. Using ramp-changes of muscle tension as stimuli a time-variant tension-induced inward current (TIC) became apparent, the amplitude of which decreased towards larger depolarizing voltages until at +18 mV the current reversed the direction. The time course of the conductance changes corresponds to similar phases in the generator potential. The outward current only responded to fast reductions in tension, decreasing transiently. A contribution of the active Na+/K+ pump to the hyperpolarizing potential response is suggested by the effects of K-removal or Na-substitution by Li+. In Na-free choline chloride media the generator potential and the TIC was depressed by 70-85%. Additional removal of Ca-2+ abolished the TIC. In contrast, lowering the Ca-2+ level in presence of Na+ decreased the membrane resistance and markedly enhanced the TIC (maximally eightfold at 10-minus 5M Ca-2+) while 75-150 mM Ca-2+ or intracellular application of a Ca-ionophore had the reverse effect.