Effect of human calcitonin on the secretion of luteinizing hormone in orchiectomized rats

The effect of human calcitonin (hCT) on the secretion of luteinizing hormone (LH) was examined in castrated rats. Male rats were orchiectomized (Orch) 2 weeks before they were treated with a single bolus injection of gonadotropin-releasing hormone (GnRH, 1 microgram/kg of body weight), hCT (3.4 ng/kg), or the combination of GnRH and hCT through a right jugular vein catheter. Blood samples (0.5 ml each time) were collected immediately before (0-min) and then at 15, 30, 60, 120 and 180 min after hormone injection. Plasma LH concentration was measured with a radioimmunoassay technique. Plasma calcium concentration was determined in an automatic analyzer. hCT significantly diminished both the basal and GnRH-stimulated levels of plasma LH without any relation to the reduced plasma levels of calcium. These results clearly demonstrated a plasma calcium-unrelated inhibitory effect of hCT on LH secretion.