Relationship between cycles in level of serotonin N-acetyltransferase activity and cyclic GMP content of cultured chick pineal glands.

We examined effects of supplements on cycles in cyclic GMP content and serotonin N-acetyltransferase (NAT) activity in cultured chick pineal glands. Increases in cyclic GMP content and NAT activity were stimulated by 1-ethyl-4(isopropylidene-hydrazino)-1H-pyrazolo[3,4-b]pyridene-5-c arboxylic acid, ethyl ester, hydrochloride and isobutylmethyl xanthine under diurnal illumination and in constant darkness, but subsequent decreases were not inhibited. Hypoxanthine had little effect on NAT activity under all lighting conditions, or on the content of cyclic GMP in glands cultured in the dark. However, it markedly stimulated accumulation of cyclic GMP in illuminated cultures. EGTA or additional Ca2+ had no effect on pineal NAT activity. However, EGTA markedly stimulated accumulation of cyclic GMP both in the light and in the dark. Supplementary Ca2+ slightly retarded accumulation of cyclic GMP in the dark but stimulated slightly in the light.