Activation of Bovine Renal Cortex Membrane Adenylyl Cyclase by Low Concentrations of Parathyroid Hormone

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Abstract
Under conditions designed to reduce non-specific adsorption of parathyroid hormone (PTH), the sensitivity of the renal membrane adenylyl cyclase to PTH was significantly enhanced. Stimulation of the enzyme could be observed at hormone concentrations as low as 2 x 10-11M. In addition, kinetic analysis of hormone activation revealed that under conditions where non-specific adsorption is great, downward concavity of Eadie-Hofstee plots is observed, whereas when such adsorption is reduced these plots become upwardly concave. These results suggest that in the absence of non-specific adsorption, PTH activation of kidney membrane adenylyl cyclase can occur at hormone concentrations approaching physiological. In addition, at low hormone concentrations PTH activation is probably more complex than previously recognized.