Modulation of renal basolateral Na-phosphate cotransporter by protein kinase A and Ca-dependent protein kinases

Renal basolateral membranes contain protein kinase A (PKA) and Ca-dependent protein kinases. We studied the effect of cyclic adenosine monophosphate (cAMP), the active phorbol ester phorbol 12-myristate 13-acetate (PMA) and calmodulin on Na-phosphate cotransporter. Rabbit renal basolateral membranes, enriched 15-fold in Na-K-ATPase activity, were phosphorylated with 50 microM ATP, and 32P uptake was measured in the presence of Na or K. 32P uptake was greater in the presence of Na than in the presence of K, indicating the existence of Na-dependent phosphate uptake, i.e., Na-phosphate cotransporter. cAMP, 1 microM, and the catalytic subunit of cAMP-dependent protein kinase (PKA-CSU, 15 mU/ml) inhibited Na-phosphate cotransporter activity by 30-50%, respectively. The effect of CSU was prevented by the PKA inhibitor (1 microgram/ml). Calmodulin, 1 microM, also inhibited Na-phosphate cotransporter by 48% (p < 0.05), and this effect of calmodulin was prevented by the inhibitor naphthalene sulfonamide W-13 (100 microM). In contrast, the active phorbol ester PMA, 1 microM, increased the Na-phosphate cotransporter by 62%, while the inactive analog 4-alpha phorbol failed to elicit such a stimulation. The results demonstrate the presence of Na-dependent phosphate transport in rabbit renal basolateral membranes which is modulated by PKA and by Ca-dependent protein kinases.