Regulation of Calcium Transport in Bone by Parathyroid Hormone

Publisher Summary This chapter discusses regulation of calcium transport in bone by parathyroid hormone. It attempts to review current concepts concerning the mode of action of parathyroid hormone, and points out apparent similarities between in vivo effects of this hormone on a variety of metabolic functions and those produced by elevated circulating calcium concentrations. It also presents a model to account for parathyroid control of calcium fluxes in bone. In this chapter, it is proposed that by enhancing entry of calcium into the interior of the cell, parathyroid hormone produces two divergent effects on the overall metabolism and physiology of bone and other target tissues. The first effect is that by allowing increased intracellular ionic calcium levels, the hormone increases the amount of calcium transported across specific layers of target cells. This net movement of calcium is unidirectional or “polarized” and occurs against a concentration gradient via a “pump” mechanism. The second is that if the intracellular concentration of ionic calcium is appreciably increased, many enzymatically controlled processes within the cell may, in turn, be affected. This secondary function of parathyroid hormone can be termed “stimulation of calcium-influenced metabolic activities” and may occur in any cell that responds to the hormone by allowing increased entry of calcium.