Influence of the interaction of calcitonin and 1,25(OH)2-vitamin D3 on prolactin secretion.

Calcitonin inhibits and 1,25(OH)2-Vitamin D3 (1,25(OH)2D3) stimulates prolactin and thyrotropin secretion. During concurrent administration of calcitonin and 1,25(OH)2D3 the action of the two calciotropic substances on thyrotropin secretion antagonizes each other. In the submitted investigation the effect of a single dose of synthetic salmon calcitonin on the prolactin secretion at rest and TRH-induced secretion was compared in untreated healthy subjects and in subjects given for four days previously 1,25(OH)2D3 in doses which have a stimulating effect on prolactin secretion (3 micrograms/d). Calcitonin, 50 U, administered by the i.v. route by infusion inhibited the secretion of prolactin at rest (p less than 0.05) and the TRH-stimulated secretion during the 20th, 30th, 40th and 60th minute after TRH administration (p less than 0.01 at all time intervals) in untreated subjects but also the secretion at rest (p less than 0.01) and TRH - stimulated secretion at times 20, 30, 40 and 60 mins following TRH administration (p less than 0.01, p less than 0.05, p less than 0.05 and p less than 0.05, respectively) in subjects pretreated with 1,25(OH)2D3. Compared with the thyrotropic system, the inhibiting effect of calcitonin on the lactotropic system is not cancelled by the administration of 1,25(OH)2D3. It is assumed that differences in the response of the lactotropic and thyrotropic system to the concurrent administration of calcitonin and 1,25(OH)2D3 are conditioned by the different ratio of the number of receptors for calcitonin and 1,25(OH)2D3 in the lactotropic and thyrotropic cell.