Circadian rhythm of the in vitro stimulation of adenylate cyclase in rat heart tissue.

Adenylate cyclase activity in rat heart ventricles displayed a circadian rhythm with a maximum around 8:00 h and a minimum around 20:00 h. In vitro stimulation with 0.1 mM of isoprenaline, Gpp(NH)p or forskolin increased the 24-h-mean basal adenylate cyclase activity by 1.7-, 2.7- and 10.7-fold, respectively. The magnitude of the drug-induced stimulation varied significantly with the time of day. However, the drugs did not affect the timing of the circadian maximum or the amplitude (as % of 24-h-mean) of the basal rhythm. The data are the first to show that the response to the in vitro stimulation of the cardiac adenylate cyclase also displays a pronounced circadian rhythm.