Inhibition ofin VivoSecretion of Calcitonin in the Pig by Somatostatin*

Abstract

The ability of somatostatin (somatotropin release-inhibiting factor or SRIF) to inhibit or reverse the increased secretion of calcitonin (CT) induced by Ca or pentagastrin (Pg) was examined in eight pigs. Except for Ca, test agents were administered directly into the thyroid circulation (0.1 ml/ min for 10–20 min) via a catheter in the thyroid artery. Thyroid venous effluent blood was continuously and completely collected from the surgically isolated in situ thyroid gland and CT was measured by RIA. In four of five pigs, infusion of a high dose of SRIF alone (10 μg/min) produced a small but significant fall in basal thyroid venous CT (mean decrease, 43% P < 0.005–0.001). In all five pigs, there was a significant return of CT to basal levels after the infusion of SRIF was discontinued (mean increase, 160% P < 0.05–0.001). Infusion of the high dose of SRIF (10 μg/min) together with Pg (0.25 μg/min) significantly inhibited the rise in CT observed with Pg alone in three of four pigs (mean decrease,68% P < 0.01–0.001). Infusion of a much lower dose of SRIF (0.5 μg/min) failed to inhibit the Pg-mediated rise in CT in the twopigs studied. In four studies, infusion of SRIF (10μg/min) during stimulation of CT secretion by systemic iv infusion of 0.1 M CaCl₂ produced a rapid pronounced decrease in thyroid venous CT (mean decrease, 76% P < 0.03–0.006). These results show that, in the pig, SRIF is capable of inhibiting basal and Pgmediated CT release and can largely reverse a profound increase in CT release induced by hypercalcemia.