GTP-gamma-S increases the duration of backward swimming behavior and the calcium action potential in marine Paramecium.

Behavioral and electrophysiological experiments were made to examine the hypothesis that G-proteins modulate the voltage-dependent calcium channel in the marine ciliate Paramecium calkinsi. It was found that guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S), an analogue of GTP that binds to and activates G-proteins, increased the duration of backward swimming behavior in reversibly permeabilized Paramecium in an irreversible and concentration-dependent manner. At 1 mumol l-1 GTP-gamma-S, the duration of backward swimming behavior was increased fivefold. Other nucleotides and related compounds did not have a significant effect on the backward swimming behavior. To evaluate whether the behavioral effects were due to ion channel modulation, the calcium action potential in intact Paramecium was monitored before and after guanine nucleotide injection. Within 5 min after the injection of GTP-gamma-S or GTP into the cell, the duration of the calcium action potential was prolonged at least threefold. Like the behavioral response, the GTP-gamma-S effect on the calcium action potential duration was irreversible, whereas the effect of GTP began to decay after 6 min. GDP-beta-S, which binds to and inactivates G-proteins, markedly reduced the calcium action potential within 5 min after injection. These results support the hypothesis that the voltage-dependent calcium channels present in Paramecium are modulated by GTP-binding proteins.