MODIFICATION OF FREQUENCY AUGMENTATION-POTENTIATION BY GTP-GAMMA-S INTHE FROG NEUROMUSCULAR-JUNCTION

The effect of modifiers of guanine nucleotide-binding proteins (G prot eins) on the frequency augmentation-potentiation of transmitter releas e were studied in the frog neuromuscular junction. Using Genetransfer( R) as a carrier the mean quantal content of the endplate potential inc reased by penetration of GTP gamma S into the presynaptic nerve termin al. Neither GTP gamma S alone nor carrier alone had any effect. The re lationship of log (mean quantal content) versus stimulation frequency changed from a single linear to a dual linear function, suggesting tha t the immediately releasable pool was modified. GDP beta S + carrier a lso had similar effects, but was less potent. Aluminium fluoride was w ithout effect. Extracellularly recorded presynaptic nerve action poten tials remained unchanged with GTP gamma S + carrier. Also, GTP gamma S + carrier did not affect the action potential nor the cytosolic Ca2concentration in differentiated NG108-15 hybrid cells. It is suggested that some smg-type G protein-dependent processes are involved in dete rmining frequency augmentation-potentiation.