Desensitization to gamma-aminobutyric acid in crustacean muscle fibres.

1. Desensitization to gamma-aminobutyric acid (GABA) was demonstrated in crab muscle fibres by measurements to conductance changes induced by the drug. 2. In standard solutions (containing 34 mM-Ca2+), complete desensitization occurred in the presence of 5 X 10(-4) M-GABA. In Ca2+-free solutions, however, only partial (70-80%) desensitization was observed after the application of the same concentration of GABA. 3. Complete recovery of the membrane sensitivity to GABA was achieved after less than 60 min of continuous washout with standard solution. Only partial (10%) recovery was observed after 1 hr of washout with Ca2+-free solution. 4. The results support the 'receptor hypothesis' of desensitization which assumes transformation of the receptor from an effective to refractory state, as opposed to the 'ionophore hypothesis' which assumes direct inactivation of the synaptic current. 5. The possible mechanism by which Ca ions effect desensitization is discussed in terms of the 'cyclic model' of Katz & Thesleff (1957).