Calmodulin mediates the stimulatory effect of 3,5,3'-triiodothyronine on adenylate cyclase activity in rat thymocyte plasma membranes.

We have previously demonstrated that T3 increases adenylate cyclase activity in rat thymocyte plasma membranes by a mechanism that is calcium dependent. In the present studies we have examined whether calmodulin participates in this response to T3. Initial experiments provided evidence that calmodulin is involved in regulating the activity of the guanyl nucleotide-dependent adenylate cyclase in this tissue. Thus, compared with findings in freshly prepared membranes, dialysis of thymocyte plasma membrane preparations for 20 h at 4 C decreased their calmodulin concentrations from an initial value of approximately 600 ng/100 micrograms protein by about 50% and decreased adenylate cyclase activity by approximately 80%. Although storage of the same preparations under comparable conditions resulted in no change in calmodulin content, it caused an approximate 30% decrease in adenylate cyclase activity. Addition of calmodulin had no effect on adenylate cyclase activity in fresh or stored membrane preparations, but produced a dose-dependent increase in enzyme activity in preparations that had been dialyzed. Further, when added to freshly isolated membranes, three calmodulin antagonists, trifluoperazine, calmidazolium, and calmodulin antibodies, all produced a concentration-dependent inhibition of adenylate cyclase activity, and this was completely reversed in all cases by the addition of high concentrations of exogenous calmodulin. The stimulation of guanyl nucleotide-dependent adenylate cyclase activity that T3 induced in fresh membrane preparations was present in membranes that had been stored, but was absent in those that had been dialyzed. In addition, the response to T3 in fresh membranes was inhibited or abolished by all three calmodulin antagonists. Both in dialyzed membranes and in the presence of antagonists, the response to T3 was restored by the addition of exogenous calmodulin. We conclude that calmodulin influences the activity of the guanyl nucleotide-dependent adenylate cyclase in rat thymocytes and ultimately mediates the stimulation of enzyme activity that T3 produces. Such mediation very likely explains the calcium-dependent nature of the stimulatory effect of T3 on thymocyte adenylate cyclase activity.

Release of luteinizing hormone in prepubertal and middle-aged ovariectomized rats.

Concentrations of circulating LH were determined in conscious, free-moving ovariectomized rats. All of the animals had been ovariectomized at 24 days of age. Between 30 and 90 days there was an increase in mean blood LH concentrations; a more vigorous pulsatile release of LH characterized by an increase in amplitude and frequency of LH release; and an elevated responsiveness to LHRH administration. Rats which had been ovariectomized for 1 year still had elevated blood LH levels but had episodic pulses of reduced amplitude and a decrease in responsiveness to LHRH. These data suggest that important alterations occur with age in the neuroendocrine mechanisms responsible for the release of LH.