Activation of renal adenylate cyclase by forskolin: assessment of enzymatic activity in animal models of the secondary hyperparathyroid state

The effects of forskolin on kidney slice cyclic AMP content and membrane adenylate cyclase activity were studied in order to determine whether or not activation of the enzyme by forskolin was affected in experimental animal models of the secondary hyperparathyroid state. Forskolin was found to be a potent activator of renal adenylate cyclase in rats and chicks, and the diterpene produced a marked potentiation of the cyclic AMP response to parathyroid hormone (PTH). The diterpene had no effect on the binding of PTH to renal receptors. Activity of adenylate cyclase in the presence of forskolin was similar in renal membranes from either vitamin D-deficient rats or chicks compared to control. Forskolin did not restore full responsiveness to PTH in renal slices from chicks raised on diets that were deficient in either vitamin D or calcium although the diterpene was capable of potentiating the cyclic AMP response to PTH in these tissues. Forskolin also augmented the activation of membrane adenylate cyclase by PTH although this effect of the diterpene was much less prominent in membrane preparations than that observed in renal slices. This study provided additional evidence that the downregulation of renal PTH-dependent adenylate cyclase in experimental models of secondary hyperparathyroidism is due to a specific reduction in receptor-mediated regulation of cyclic AMP formation. Adenylate cyclase activity as assessed by forskolin-stimulated enzyme activity was fully maintained in kidney membranes from these animal models. Thus, forskolin appears to be a useful drug for measuring total enzymatic activity in situations where altered responsiveness of adenylate cyclase to hormones has been demonstrated to be mediated by changes in hormone receptors.