Enzymes of guanine nucleotide metabolism and the diurnal cycle in cGMP content of the chick pineal gland.

Levels of cGMP phosphodiesterase, guanylate cyclase, and GTPase activities were determined in homogenates of chick pineal glands. Only small variations in vivo were observed with glands removed at different times of the day from birds under a standard cycle of illumination. Glands cultured under the cycle of illumination from late in the photoperiod showed a progressive loss of about half the phosphodiesterase activity in 24 h, and an increase of roughly 75% in GTPase activity within 12 h. No simple correlations were found between variations in levels of enzyme activity and the diurnal cycles in pineal content of cGMP and level of serotonin N-acetyltransferase (NAT) activity. However, onset of rapid increases in 3',5'-cyclic GMP (cGMP) content and NAT activity was correlated with a transient decrease of about 30% in the phosphodiesterase activity, both in vivo and in culture. Further, known inhibitors of phosphodiesterase activity previously shown to elicit increase of cGMP content and marked elevation of NAT activity in cultured glands only inhibited phosphodiesterase activity of homogenates by 25-30%. It was therefore concluded that the transient decrease in level of phosphodiesterase may facilitate onset of increase in pineal cGMP content. However, it seems improbable that changes in pineal content of enzymes of guanine nucleotide metabolism are essential to regulation of diurnal cycles in cGMP content or level of NAT activity.