The major goal of this research program is to better understand calcium metabolism at the systemic and cellular level as regulated by different hormones and ions. The specific aims of this proposal are to study 1) the phosphaturic effect of parathyroid hormone in isolated kidney cell and its relation to cyclic AMP, intracellular calcium and H plus ion secretion; 2) the regulation in isolated kidney cells of the enzyme 25-hydroxycholecalciferol-1-hydroxylase which controls the synthesis of the hormone 1,25 dihyrdroxycholecalciferol by parathyroid hormone, calcium, phosphate and cyclic AMP; 3) the effects of the hormone 1,25-dihydroxycholecalciferol in isolated intestinal cells on RNA and protein synthesis and on transcellular calcium transport.