THE ROLE OF THE PARATHYROIDS FOR THE ADAPTATION TO A LOW CALCIUM INTAKE.

One‐year‐old selectively parathyroidectomized rats responded with a plasma calcium reduction to below 4.1 mEq./1 in 86 per cent at a normal and in 100 per cent at a low dietary calcium intake. Among the former, 17 per cent normalized their plasma calcium level within 8 weeks and another 40 per cent between the 8th and 27th week of observation. Among the latter, only 10 per cent showed a normalization and first after the long‐term period of observation. On the normal level of dietary calcium the parathyroidectomized animals with persistently reduced plasma calcium showed a significantly increased bone mass which was paradoxical in view of their inability of adaptation. On the low level of dietary calcium, a normal bone resorptive activity was maintained despite parathyroidectomy possibly through the action of increased levels of 1,25‐dihydroxycholecalciferol provoked by the profoundly reduced plasma calcium. This was, however, insufficient for adaptation and no osteoporosis developed. For the adaptation to a reduced calcium intake skeletal calcium reserves had to become mobilized through the action of the parathyroids with resulting osteoporosis. This was brought about by increased mobilization of skeletal calcium with resulting osteoporosis, a phenomenon which could be seen also among the parathyroidectomized animals on a low dietary calcium and with a normalized plasma calcium level. This osteoporosis was similar to that found in the calcium deficient intact animals. The described bone changes were progressing in character and there was no major influence by any eventual effect on bone growth. At histological and morphometric analyses of the adrenal cortex no apparent changes were found after parathyroidectomy.