Na,K-ATPase activity and 25(OH)vitamin D3 hydroxylation in rat proximal tubules

The proximal tubule is the target site for parathyroid hormone (PTH), and conversion of 25(OH)vitamin D₃ hormones which impinge on calcium (Ca) homeostatis, as well as a major site for sodium (Na) reabsorption. The effect of changes in PTH and vitamin D status on Na,K-ATPase activity, as a measure of Na transport, were studied in the proximal tubules of adult rat kidneys where Na and Ca reabsorption rates are in parallel. Na,K-ATPase activity and 25(OH)D₃ metabolism were determined in cortical and juxtamedullary proximal tubule segments from normal, parathyroidectomized (PTX), and vitamin D-deficient (-D) rats. Na,K-ATPase activity was highest in cortical segments. PTX led to a decrease in activity in convoluted segments but increased activity in straight segments. In-D rats, Na,K-ATPase activity decreased in cortical segments but increased in juxtamedullary segments. 25(OH)D₃ was metabolized more to 24,25(OH)₂D₃ than to 1,25(OH)₂D₃ in all normal segments. Juxtamedullary segments were more sensitive to PTX and-D conditions. These findings suggest that cortical and juxtamedullary nephrons are inherently different in basal Na,K-ATPase activity, in conversion of 25(OH)D₃ to active metabolites, and in response to altered PTH and vitamin D₃ status.