The effects of infusions of synthetic adrenocorticotrophin in the conscious calf.

1. A technique is described by which the whole of the effluent blood from the right adrenal gland can be collected as required from conscious, unrestrained calves. The technique may be used to measure adrenal blood flow gravimetrically and to compute the output of adrenal hormones under various conditions in the normal calf.2. In a group of seven calves mean cortisol output from the right adrenal gland was found to vary between 20 and 40 ng.kg(-1) min(-1) and corticosterone between 6 and 18 ng.kg(-1) min(-1) during a 2 hr period, 24 hr after surgery.3. Intravenous infusions of synthetic adrenocorticotrophin (5 ng.kg(-1) min(-1)) produced a significant increase in the output of both cortisol and corticosterone within 5 min. The output of both hormones rose to maximal values within 10-20 min and mean values of approximately 300 ng.kg(-1) min(-1) (cortisol) and 120 ng.kg(-1) min(-1) (corticosterone) were maintained thereafter for the duration of the infusion (120 min). The output of both steroids fell to values comparable with those observed initially within 45-60 min after the infusion was discontinued.4. These changes in glucocorticoid output in response to adrenocorticotrophin produced a significant rise in the concentration of both cortisol and corticosterone in peripheral plasma. It is noteworthy that the rise in the mean corticosterone concentration in the peripheral plasma was substantially less than that which might be expected from relating the rise in mean plasma cortisol concentration to cortisol output.5. The results of control experiments have eliminated the possibility that the sampling procedure might itself increase steroid output or peripheral plasma concentration. Comparison of results from calves of widely disparate ages (8-38 days) provided no evidence that either the resting output of cortisol or corticosterone or the response to adrenocorticotrophin changes with age within the range examined.6. Infusion of adrenocorticotrophin (5 ng.kg(-1) min(-1)) also stimulated an abrupt rise in adrenal blood flow; mean resting flow (210 +/- 23 mul.kg(-1)) increased by approximately 30% within 5 min and attained peak values (355-365 mul.kg(-1) min(-1)) between 10 and 30 min. Thereafter, adrenal blood flow steadily decreased and then fell rapidly to within the resting range when the infusion was terminated. No significant changes in heart rate or aortic blood pressure occurred during these infusions.7. The results are discussed in relation to those obtained in other species and under differing conditions by other workers.