Inhibitory effect of dibutyryl cyclic GMP on potassium-stimulated 45Ca uptake by synaptosomes from rat brain

The effects of dibutyrl cyclic GMP (db-cGMP) and dibutyryl cyclic AMP (db-cAMP) on potassium-stimulated 45Ca uptake by the P2 fraction of Gray and Whittaker were investigated with the following results. (1) db-cGMP inhibited the initial rate of potassium-stimulated 45Ca-uptake in a dose-dependent manner in 0.1 mM Ca0 medium, but had no effect on the uptake in low K+ medium. In 0.1 mM Ca0 medium, the concentration of db-cGMP causing 50% inhibition was about 3 mM. db-cAMP (5 mM) had no effect on the uptake. (2) db-cGMP-inhibited potassium-stimulated 45Ca uptake in 1 mM Ca0 medium, though less than in 0.1 mM Ca0 medium. (3) db-cGMP inhibited potassium-stimulated 45Ca uptake by synaptosomes (pinched-off nerve terminals) more than the uptakes by other subfractions of P2 fraction. It is suggested from the results that cGMP inhibits the Ca influx resulting from depolarization of the nerve endings in situ.