Glucocorticoids and the renal Na-H antiporter: role in respiratory acidosis

Regulation of the renal Na-HCO(3) cotransporter. XI. Signal transduction underlying CO(2) stimulation

We have previously shown that CO(2) stimulation of the renal Na-HCO(3) cotransporter (NBC) activity is abrogated by general inhibitors of protein tyrosine kinases. The more selective inhibitor herbimycin also blocked this effect at concentrations known to preferentially inhibit Src family kinases (SFKs). We therefore examined a role for SFKs in CO(2)-stimulated NBC activity. To this end, we engineered OK cells to express the COOH-terminal Src kinase (Csk), a negative regulator of SFKs. CO(2) stimulated NBC activity normally in beta-galactosidase-expressing and untransfected control cells. In contrast, Csk-expressing cells had normal baseline NBC activity that was not stimulated by CO(2). CO(2) stimulation increased both total SFK activity and specific tyrosine phosphorylation of Src. The specific MEK1/2 inhibitor PD-98059 completely inhibited the CO(2) stimulation of NBC activity as well as the accompanying phosphorylation and activation of ERK1/2. Our data suggest the involvement of both SFKs, probably Src, and the "classic" MAPK pathway in mediating CO(2)-stimulated NBC activity in renal epithelial cells.

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.

Effect of glucocorticoids on renal dopamine production

This study assess the effects of glucocorticoids on dopamine excretion and evaluates the participation of renal dopamine in the effects of glucocorticoids on renal function and Na+ excretion. Dexamethasone (i.m.; 0.5 mg/kg) was administered to male Wistar rats on day 2 or on days 2 and 5. Daily urinary excretions of Na+, dihydroxyphenylalanine (DOPA), dopamine and dihydroxyphenylacetic acid were determined from day 1 to day 7. Renal function was evaluated 8 h after dexamethasone administration in a separate group. The first dose of dexamethasone increased about 100% diuresis and natriuresis, increased urinary DOPA and renal plasma flow, and did not affect urinary dopamine or the other parameters evaluated. These effects were not affected by previous administration of haloperidol. The second dexamethasone dose increased about 200% diuresis and natriuresis, increased urinary dopamine, DOPA, dihydroxyphenylacetic acid, Uosm x V and both glomerular filtration rate and renal plasma flow. Carbidopa administered before the second dexamethasone dose blunted both the diuretic and the natriuretic response whereas haloperidol abolished or blunted all the effects of the second dexamethasone dose. These results show that modifications in renal dopamine production produced by corticoids may contribute to the effects of these hormones on Na+ balance and diuresis and suggest that regardless the factor that promotes an increase in renal perfusion and glomerular filtration rate during long term administration of glucocorticoids, a dopaminergic mechanism is actively involved in the maintenance of these hemodynamic changes.

Regulation of the renal Na-HCO3 cotransporter: IX. Modulation by insulin, epidermal growth factor and carbachol

To examine the role of tyrosine kinase (TK) on basolateral membrane (BLM) transport, we looked for the presence of TK activity in these membranes and showed that the synthetic substrate for TK, poly [Glu80 Na, Tyr20] caused a three-fold increase in tyrosine phosphorylation. This effect was completely blocked by the TK inhibitors, 2-hydroxy-5(2,5-dihydroxybenzyl) aminobenzoic acid (HAC), 1 microM, and methyl 2,5-dihydroxycinnamate (DHC), 5 microM. We then examined the effect of agents that cause TK stimulation on tyrosine kinase immunocontent and on the Na-HCO3 cotransporter activity in BLM and in primary cultures of the proximal tubule. We utilized the cholinergic agent, carbachol (10(-4) M), epidermal growth factor (EGF 10(-8) M), and insulin (10(-8) M), well known activators of TK. Carbachol, insulin, and EGF caused a significant increase in TK immunoreactive protein content which was blocked by HAC and DHC. In BLM, carbachol significantly stimulated HCO3-dependent 22Na uptake and this effect was totally prevented by the monoclonal antibody against TK. In cultured proximal tubule cells, carbachol, EGF and insulin at physiologic concentration caused a significant stimulation of the cotransporter activity and this effect was completely blocked by the TK inhibitor, HAC. Increasing the dose of insulin 100-fold did not cause further stimulation of the cotransporter indicating that insulin plays a permissive role on the cotransporter. These results demonstrate the presence of TK in renal proximal tubule cells and show that activation of this kinase by dissimilar agents enhance the activity of the Na-HCO3 cotransporter.

Regulation of the renal Na-HCO3 cotransporter: VII. Mechanism of the cholinergic stimulation

Cholinergic agents regulate proximal tubule acidification but the mechanism responsible for this effect is unclear. We examined the effect of the cholinergic agent carbachol on the activity of the Na-HCO3 cotransporter in primary cultures of the proximal tubule of the rabbit. The activity of the cotransporter was assayed either as HCO3-dependent 22Na uptake or as the recovery of intracellular pH in cells perfused continuously with Cl-free physiologic solution containing amiloride to block the Na-H antiporter. Carbachol caused a dose-dependent stimulation of the cotransporter activity with a maximum increase of 90% above control values at 10(-5) M and half maximal stimulation at 10(-7) M. The stimulation was blocked by atropine and pirenzepine indicating an effect through the M1 muscarinic receptor. Carbachol increased intracellular calcium fourfold and the rise in cytosolic calcium was prevented by the intracellular calcium chelator, BAPTA. BAPTA also blocked the effect of carbachol on the cotransporter. Because carbachol activates phospholipase C and protein kinase C, we examined the effect of carbachol in the presence of the phospholipase C inhibitor, U73122, or the PKC inhibitor, calphostin C, or PKC depletion. The phospholipase C inhibitor prevented both the effect of carbachol on the cotransporter and on the intracellular Ca. Calphostin C and PKC depletion also prevented the stimulation of the cotransporter. Carbachol increased PKC activity and caused translocation of the PKC to the particulate fraction. We also examined the effect of the phosphatase inhibitor, calyculin A or the calmodulin kinase inhibitor, W-13 on carbachol stimulation. Calyculin A and W13 likewise prevented the carbachol-induced stimulation of the cotransporter. These results demonstrate that cholinergic stimulation modulated the activity of the cotransporter through multiple pathways including phospholipase C/PKC and phosphatase systems.

Regulation of the renal Na-HCO3 cotransporter: VI. Mechanism of the stimulatory effect of protein kinase C

We have previously shown that the activity of the Na-HCO3 cotransporter is stimulated by protein kinase C (PKC) activation, but the mechanism responsible for this effect is not clear. We have shown that cultured proximal tubule cells of the rabbit have DIDS-sensitive Na-HCO3 cotransporter activity as assessed by HCO3-dependent 22Na uptake or by measurement of intracellular pH. In cells loaded with BCECF and treated with the amiloride analogue, ethylisopropyl amiloride, removal of extracellular Na was associated with a rapid decrease in pH which returned to normal with re-addition of Na. This pH recovery was inhibited by DIDS and was used to quantify the activity of the Na-HCO3 cotransporter. In the present study, we utilized primary cultures of the proximal tubule of the rabbit to examine the effect of PKC activation on the activity of the Na-HCO3 cotransporter. Short term incubation (5 min) with the active phorbol ester, phorbol 12-myristate, 13-acetate (PMA), 10(-7) M, caused a significant stimulation of the Na-HCO3 cotransporter activity as compared to controls. Incubation for two hours also caused a significant stimulation of the Na-HCO3 cotransporter activity. The inactive analogue of PMA, 4-alpha phorbol, failed to alter the cotransporter. Similar results were observed when we examined the effect of PMA on HCO3-dependent 22Na uptake. The effect of PMA to stimulate the cotransporter was mediated by PKC activation since it could be prevented by the PKC inhibitors, calphostin C or sphingosine, or by prior PKC depletion. The long term but not the short term effect of PMA to stimulate the Na-HCO3 cotransporter activity was prevented by the protein synthesis inhibitors, actinomycin D or cycloheximide. The early effect of PKC to stimulate the cotransporter appeared to be associated with increased phosphorylation of a 56 kD protein band, while the late effect appeared to be associated with an increase in immunoreactive content of a 56 kD protein which is thought to be an active component of the cotransporter. Thus PKC stimulation activates the Na-HCO3 cotransporter by two distinct mechanisms: a long term effect which is protein synthesis-dependent and a short term effect which is protein synthesis-independent and is likely mediated by phosphorylation.

Regulation of the renal Na-HCO3 cotransporter: V. mechanism of the inhibitory effect of parathyroid hormone

PTH administration decreases proximal HCO3 reabsorption and inhibits the brush border Na-H antiporter. We studied the effect of PTH on the renal Na-HCO3 cotransporter and examined whether this effect is mediated through the adenylate cyclase/cyclic AMP system or through the phospholipase A pathway. We studied the effect of PTH [1-34] on the Na-HCO3 cotransporter activity in rabbit renal basolateral membranes incubated with 50 microM ATP by measuring the 22Na uptake in the presence of HCO3 and gluconate. Na-HCO3 cotransporter activity (expressed in nmol/mg protein/3 seconds) was taken as the difference in 22Na uptake in the presence of HCO3 and gluconate. PTH (10(-10) M) completely inhibited Na-HCO3 cotransporter activity from 1.23 +/- 0.14 to -0.58 +/- 0.23, P < 0.001. This effect of PTH to inhibit the Na-HCO3 cotransporter was prevented by the polyclonal antibody against G alpha s indicating that PTH acts through G alpha s protein. Because G alpha s stimulates adenylate cyclase/cyclic AMP system, we examined the effect of PTH in the presence and in the absence of the adenylate cyclase inhibitor, dideoxyadenosine (DDA). DDA alone (10(-4) M) stimulated the Na-HCO3 cotransporter activity. In the presence of DDA, the net inhibitory effect of PTH was the same magnitude as that of control, suggesting the existence of other pathways for the effect of PTH on the cotransporter. Calmodulin inhibition also partially prevented the effect of PTH. To determine whether the inhibitory effect of PTH is mediated at least in part, through phospholipase A, we first examined the effect of PTH on arachidonic acid release and then measured the Na-HCO3 cotransporter activity in presence and in absence of arachidonic acid or eicosatetraynoic acid (ETA), an inhibitor of arachidonic acid metabolism. PTH significantly increased the release of arachidonic acid by isolated proximal tubule cells and arachidonic acid inhibited the Na-HCO3 cotransporter in basolateral membranes. ETA (3 microM) partially prevented the inhibitory effect of PTH. In cultured proximal tubule cells, PTH inhibited the HCO3-dependent 22Na uptake and ethoxyresorufin, an inhibitor of cytochrome P-450, blocked the inhibitory effect of PTH on the cotransporter. These results demonstrate that PTH inhibits the renal Na-HCO3 cotransporter through multiple mechanisms, that are mediated through G proteins, G alpha s and GP, and CaM-KII.

Regulation of renal Na-HCO3 cotransporter: III. Presence and modulation by glucocorticoids in primary cultures of the proximal tubule

We looked for the presence of the Na-HCO3 cotransporter in primary cultures of the proximal tubule and examined the modulation of this cotransporter by glucocorticoid hormones. Primary cultures of the proximal tubule of the rabbit have Cl-independent, HCO3-dependent 22Na uptake which is DIDS-sensitive. In addition, in cells loaded with BCECF and perfused with Cl-free solution, removal of Na was associated with a decrease in intracellular pH which returned to normal with re-addition of Na. The pH recovery was not inhibited by EIPA but was sensitive to DIDS. These findings are compatible with existence of Na-HCO3 cotransporter in these cells. We examined the role of glucocorticoids on the activity of the Na-HCO3 cotransporter by culturing proximal tubule cells in the presence of hydrocortisone and when confluence was reached, hydrocortisone was deleted from the medium. In the absence of hydrocortisone, the activity of the cotransporter, measured either isotopically or fluorometrically, was significantly decreased, whereas re-addition of hydrocortisone 10(-8) M, restored the activity of the cotransporter to normal levels. The effect of hydrocortisone could not be duplicated by aldosterone, suggesting a glucocorticoid-dependent effect. Dexamethasone, a glucocorticoid without mineralocorticoid activity, stimulated the activity of the cotransporter within physiologic concentrations and this effect was blocked by progesterone. The effect of dexamethasone was time-dependent and was prevented by cycloheximide, a protein synthesis inhibitor. These results demonstrate that primary cultures of the proximal tubule have Na-HCO3 cotransporter activity which is modulated by physiological concentrations of glucocorticoids through a protein synthesis-dependent mechanism.