Influence of blood calcium on calcitonin receptors in isolated chick osteoclasts.

Isolated osteoclasts obtained from young chickens fed a normal (+Ca) or deficient (-Ca) calcium and vitamin D diet for 3 weeks, were studied for their ability to bind salmon calcitonin (sCT). Osteoclasts obtained from -Ca chickens, when incubated with 0.1 mumol sCT/l, doubled cyclic (c)AMP production and retracted from a glass support, as observed by scanning electron microscopy. The presence of receptors was also demonstrated by autoradiography and competition analysis of 125I-labelled sCT binding. The number of receptors per cell was 0.9 +/- 0.1 x 10(4). In contrast, osteoclasts obtained from +Ca chickens did not increase cAMP production and did not retract in the presence of 0.1 mumol sCT/l. No specific binding of 125I-labelled sCT could be demonstrated on these osteoclasts. Plasma levels of calcium and calcitonin were measured in +Ca and -Ca chickens. The plasma concentration of calcium was markedly lower at 3 weeks in -Ca than in +Ca chickens. The plasma concentration of calcitonin was decreased in -Ca chickens compared with +Ca chickens at the first week and kept decreasing during the 3 weeks. These results strongly support the hypothesis that calcium and vitamin D intake regulate plasma calcitonin levels in chickens, and that calcitonin receptors can be detected on chicken osteoclasts only when blood calcium is decreased by a diet deficient in calcium and vitamin D.