Comparative effects of eel calcitonin, salmon calcitonin and [asu^1,7]eel calcitonin on hypophyseal and osteoblastic function

Three different calcitonins: salmon calcitonin, eel calcitonin and the semi-synthetic analog [Asu ^{1, 7}]eel calcitonin have been evaluated for their ability to affect phosphoinositide hydrolysis in primary cultures of anterior pituitary cells and in the osteoblast-like UMR-106 cells. In both cellular systems a repeated treatment with any form of calcitonin induced an inhibition of inositol phospholipid turnover. Eel calcitonin and its analog were always more potent than salmon calcitonin, but the efficacy of the three polypeptides was comparable. In cultured anterior pituitary cells, the inhibitory effect on phosphoinositide hydrolysis observed after chronic treatment with calcitonin was accompanied by a reduction of prolactin release. In contrast, a single treatment of cultured anterior pituitary cells with eel calcitonin or its analog [Asu^{1, 7}]eel calcitonin induced an increase of inositol phosphate accumulation, while salmon calcitonin was inactive. Accordingly, eel and [Asu^{1, 7}]eel calcitonin, but not salmon calcitonin, induced a slight but significant stimulation of prolactin secretion. In UMR-106 cells, the three calcitonins exhibited similar potency and efficacy in reducing parathyroid hormone-stimulated 4ß[³H]-phorbol- 12,13-dibutyrate ([³H]PdBu) binding, an indirect index of protein kinase C activation. Taken together, these results suggest that, either at the pituitary or in osteoblastlike cells, some of the effects exerted by calcitonin may be ascribed to an interference with the intracellular events initiated by modulation of phosphoinositide turnover.