Renal regulation of avian calcium and phosphorus metabolism

Avian plasma calcium (Ca) and inorganic phosphate (Pi) concentrations are coregulated by parathyroid hormone (PTH) and vitamin D acting directly or indirectly on the intestine, bone and kidneys. Within the overall scheme of chemical, hormonal and target tissue interactions, the kidneys serve as a safety valve, operating on a minute-to-minute basis to excrete Ca and Pi when these minerals are present in excess or to conserve Ca and Pi when they are needed for eggshell formation or bone mineral replenishment. This key role of the kidneys is directly mediated by PTH, acting to stimulate Ca reabsorption and increase Pi excretion at the tubular level. By dissociating Ca from Pi transport, the kidneys serve to maintain appropriate blood Ca:Pi concentration ratios during extremes of dietary challenge or bone mineral mobilization. Direct regulatory roles for vitamin D metabolites and calcitonin on avian urinary Ca and Pi excretion have yet to be firmly established.