Thyroid venous plasma samples from pigs given 1) CaCl2, iv or into the stomach or jejunum, 2) acetylcholine intragastrically, and 3) porcine gastrin iv were analyzed for both gastrin and thyrocalcitonin (TCT) using specific radioimmunoassays. Changes in TCT secretion were found to correlate with alterations in the level of gastrin in blood traversing the thyroid gland. In 10 of the 11 pigs correlation coefficients (r) were highly significant (p < 0.05 to p < 0.01), ranging from 0.61 to 0.95. In individual experiments, 2 to 3-fold increases in TCT usually were associated with small (2 to 3-fold) increases in circulating gastrin. Larger increases in TCT (> 10-fold) occurred after iv doses of gastrin sufficient to raise the concentration of gastrin in thyroid venous blood to levels ≤ 1000 pg/ml. The findings extend previous studies showing that the native gastrointestinal hormones, gastrin and cholecystokinin-pancreozymin, and their active synthetic analogs are potent stimuli for secretion ofTCT in several species and support our hypothesis that gastrin, acting as a stimulus for TCT secretion, may participate in restricting hypercalcemia after feeding and during intestinal absorption of calcium.
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