Actions of parathyroid hormone on isolated renal tubules

Studies of the action of PTH on transport events in specific segments of the rabbit nephron correlate accurately with observations on the presence or absence of PTH-sensitive adenylate cyclase activity. Uncertainties remain, however, as to whether PTH via cyclic AMP affects the transport of phosphate or of calcium directly or whether these effects are secondary events. In the proximal tubule, the most striking effect of PTH is the inhibition of bicarbonate transport. The response of phosphate transport is modified by the dietary intake of phosphate. In the cortical ascending limb, PTH may stimulate calcium absorption directly or alter membrane permeability and the voltage-dependent transfer of calcium. At this point, we recognize no single transport event that underlies the common response of all sensitive segments in terms of increased cyclic AMP.