Effects of synthetic salmon calcitonin on C cells of the thyroid

The effects of long-term administration of synthetic salmon calcitonin (TZ-CT) on thyroid C cells of rats were evaluated immunohistochemically and electron microscopically. Low dose (0.075, 0.75 and 7.5 IU/kg/day) administration caused no significant changes in the C cell population, whereas high doses (30 and 120 IU/kg/day) caused C cell suppression. The area ratio of C cells to total cells in the thyroid was measured on sections immunoperoxidase stained for calcitonin. The average C cell ratio was compared among the experimental groups. This morphometric analysis of the high-dose and control groups confirmed the differences in C cell populations statistically (30 IU/kg vs. control, p less than or equal to 0.05; 120 IU/kg vs. control, p less than or equal to 0.05). The suppressive effect of TZ-CT on C cells was reversible after cessation of administration for one month. Electron microscopic examination on the C cells in the high-dose groups revealed no particular ultrastructural differences. These results may suggest a negative feedback mechanism on C cells by exogenous CT.