Regulation of the renal Na-HCO(3) cotransporter X. Role of nitric oxide and intracellular calcium.

Cholinergic agents increase the activity of the renal Na-HCO(3) cotransporter and have been shown to stimulate the production of nitric oxide (NO) in other cells. To study the role of NO in mediating the effect of carbachol on Na-HCO(3) cotransporter, we measured the activity of the cotransporter in rabbit proximal tubule cells treated with carbachol (10(-4 )M) or the NO inhibitor, L-NAME (10(-3) M), or carbachol+L-NAME. The activity of NaHCO(3) cotransporter was measured by recovery of intracellular pH (pH(i)) in cells loaded with pH-sensitive dye, BCECF. In control cells, carbachol significantly increased Na-HCO(3) cotransporter activity while L-NAME did not affect the activity of the cotransporter but completely blocked the enhancement induced by carbachol. Carbachol increased NO production by proximal tubule cells. We also studied the effect of the NO donor, SNAP (10(-3) M), on the cotransporter incubated for 1 h in cultured proximal tubule cells. SNAP caused a similar enhancement in the activity of the cotransporter suggesting that a different NO donor is capable of enhancing the activity of the cotransporter to the same extent as that observed with carbachol. Because the effect of NO is thought to involve cGMP, we examined the effect of 8-Br-cGMP (10(-3 )M) on the cotransporter. 8-Br-cGMP caused stimulation of the Na-HCO(3) cotransporter activity although to a lesser degree than carbachol. We have previously shown that carbachol increases cytosolic calcium but the role of intracellular calcium (Ca(i)) per se on the cotransporter has not been studied. We therefore studied the role of Ca(i) on the activity of Na-HCO(3) cotransporter in rabbit proximal tubule cells by utilizing the calcium ionophore, ionomycin, the microsomal Ca-ATPase inhibitor, thapsigargin, and the calcium chelator, BAPTA. Ionomycin, 5 microM, caused a significant stimulation of Na-HCO(3) cotransporter which was prevented by BAPTA. The microsomal Ca-ATPase inhibitor, thapsigargin, also increased the cotransporter activity. As expected both ionomycin and thapsigargin caused a significant increase in Ca(i). Calyculin A, an inhibitor of protein phosphatase 2A prevented the stimulation of the cotransporter by calcium (in pH units/min: control 1.8+/-0.13; Ca 2.22+/-0.07; p<0.05; Ca+calyculin A 1.9+/-0.09, p<0.025) suggesting that calcium acting through kinases/phosphatases, plays a role in the phosphorylation of the cotransporter. These results demonstrate that NO and Ca(i) modulate the activity of the cotransporter.