Effects of calcitonin, katacalcin, and calcitonin gene‐related peptide on basal and TSH‐stimulated thyroid hormone secretion in the mouse

It is known that transcripts of the calcitonin gene in the thyroid C cells are processed to mRNAs generating precursors for the three peptides, calcitonin, katacalcin, and calcitonin gene-related peptide (CGRP), and that these peptides are produced and secreted from the C cells. The present study investigated the effects of these three peptides on basal and TSH-stimulated thyroid hormone secretion in the mouse, according to the McKenzie technique. Mice were pre-treated with 125I and thyroxine. Calcitonin (3 nmol animal-1) reduced the basal blood radioiodine levels to 72 +/- 5% compared to 95 +/- 6% in controls (P less than 0.01). This effect is likely to be renal, since calcitonin significantly increased the renal excretion of a concomitantly injected 125I load. In contrast, katacalcin and CGRP were without effect on basal blood radioiodine levels. All three peptides (1 or 3 nmol animal-1) were without effect on TSH (70 microU animal-1)-induced increase in blood radiodine levels. However, when the peptides were given together (either two and two concomitantly, or all three together), the TSH-induced increase in blood radioiodine levels was inhibited. This inhibition of TSH-induced thyroid hormone secretion might represent an intrathyroidal regulatory mechanism of the follicular cell activity.