Modification of frequency augmentation-potentiation by GTP gamma S in the frog neuromuscular junction.

The effect of modifiers of guanine nucleotide-binding proteins (G proteins) on the frequency augmentation-potentiation of transmitter release were studied in the frog neuromuscular junction. Using Genetransfer as a carrier the mean quantal content of the endplate potential increased by penetration of GTP gamma S into the presynaptic nerve terminal. Neither GTP gamma S alone nor carrier alone had any effect. The relationship of log (mean quantal content) versus stimulation frequency changed from a single linear to a dual linear function, suggesting that the immediately releasable pool was modified. GDP beta S + carrier also had similar effects, but was less potent. Aluminium fluoride was without effect. Extracellularly recorded presynaptic nerve action potentials remained unchanged with GTP gamma S + carrier. Also, GTP gamma S + carrier did not affect the action potential nor the cytosolic Ca2+ concentration in differentiated NG108-15 hybrid cells. It is suggested that some smg-type G protein-dependent processes are involved in determining frequency augmentation-potentiation.