Abstract
The neurotransmitter in renal sympathetic nerves, norepinephrine (NE), regulates the activity of proximal tubule (PT) Na+,K+-ATPase in a bidirectional manner via stimulation of alpha- and beta-adrenoceptors. The stimulatory alpha-adrenergic pathway is mediated by calcineurin, the target molecule for FK 506 and related compounds. We examined whether the FK 506 analogue FK 520, by interrupting the calcineurin-mediated alpha-adrenergic signaling pathway, enhance the inhibitory beta-adrenergic effect of NE on PT Na+,K+-ATPase activity. The effects of three days of treatment with FK 520 were examined on rat renal PT Na+,K+-ATPase activity, measured as ouabain-sensitive ATP hydrolysis in single, microdissected PT segments. Renal function studies, including glomerular filtration rate (GFR) and urinary excretion of N-acetyl-3-D-glucoseaminidase (NAG), were examined using conventional clearance techniques after three days of treatment with FK 506. FK 520 treatment induced a pronounced and dose-dependent decrease in PT Na+,K+-ATPase activity. This effect was completely reversed by the competitive FK 520 antagonist, L 685 818, indicating that the effect was dependent on inhibition of calcineurin. To test whether the FK 520-induced decrease in Na+, K+-ATPase activity was mediated by enhanced beta-adrenoceptor signaling, the FK 520 effect was examined in rats pretreated with a beta-adrenoceptor antagonist (propranolol) or rats subjected to renal denervation. Both of these procedures prevented the FK 520-induced decrease in Na+,K+-ATPase activity. Thus, during FK 520 treatment, renal sympathetic nerves mediate an inhibitory effect on PT Na+,K+-ATPase activity via beta-adrenoceptors. Propranolol pretreatment also prevented FK 506-induced decreases in GFR and urinary excretion of NAG, an index of PT dysfunction. The results support the hypothesis that the net effect of the neurotransmitter NE on Na+,K+-ATPase activity is dependent on the balance between the alpha- and beta-adrenergic signaling pathways and suggest that agents that interfere with these pathways may, by altering the activity of tubular Na+,K+-ATPase, also alter the function of the renal tubular epithelial cell.
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