Calcitonin, Serotonin, and Parafollicular Cell Granules during the Hibernation Activity Cycle in the Bat

Abstract

A seasonal covariation in calcitonin and serotonin content of bat [Myotis lucifugus] thyroid was found. Whereas the mean level of thyroidal calcitonin in active bats ranged between 30-48 MRC mU[mouse radius calcifying milliunits]/mg of protein, a 2 to 3-fold increase in this level was observed prior to and during early hibernation. Associated with this increase, a relative hypocalcemia was observed. In late hibernation, thyroidal calcitonin level returned to the range observed in active bats. A parallel seasonal profile was found for bat thyroid serotonin content, which varied from a mean of 4.1 .mu.g/g during the active phase of the annual life cycle to a peak close to 13 .mu.g/g observed in early hibernation. The seasonal covariation of calcitonin and serotonin reflects ultrastructural changes observed in the secretory granules of thyroid parafollicular cells during the annual life cycle of the bat.