Renal acidosis in mineralocorticoid deficiency is not dependent on NaCl depletion or hyperkalemia

Ammonia transport in the kidney by Rhesus glycoproteins

Renal ammonia metabolism is a fundamental element of acid-base homeostasis, comprising a major component of both basal and physiologically altered renal net acid excretion. Over the past several years, a fundamental change in our understanding of the mechanisms of renal epithelial cell ammonia transport has occurred, replacing the previous model which was based upon diffusion equilibrium for NH3 and trapping of NH4(+) with a new model in which specific and regulated transport of both NH3 and NH4(+) across renal epithelial cell membranes via specific membrane proteins is required for normal ammonia metabolism. A major advance has been the recognition that members of a recently recognized transporter family, the Rhesus glycoprotein family, mediate critical roles in renal and extrarenal ammonia transport. The erythroid-specific Rhesus glycoprotein, Rh A Glycoprotein (Rhag), was the first Rhesus glycoprotein recognized as an ammonia-specific transporter. Subsequently, the nonerythroid Rh glycoproteins, Rh B Glycoprotein (Rhbg) and Rh C Glycoprotein (Rhcg), were cloned and identified as ammonia transporters. They are expressed in specific cell populations and membrane domains in distal renal epithelial cells, where they facilitate ammonia secretion. In this review, we discuss the distribution of Rhbg and Rhcg in the kidney, the regulation of their expression and activity in physiological disturbances, the effects of genetic deletion on renal ammonia metabolism, and the molecular mechanisms of Rh glycoprotein-mediated ammonia transport.

Mineralocorticoid modulation of apical and basolateral membrane H+/OH-/HCO3- transport processes in the rabbit inner stripe of outer medullary collecting duct

To examine the mechanism by which mineralocorticoids regulate HCO3- absorption in the rabbit inner stripe of the outer medullary collecting duct, we microfluorometrically measured intracellular pH (pHi) in in vitro perfused tubules using 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) assaying the apical and basolateral membrane H+/OH-/HCO3- transport processes in three groups of animals: those receiving chronic in vivo DOCA treatment (5 mg/kg per d x 2 wk); those with surgical adrenalectomy (ADX, [chronic x 2 wk]) on glucocorticoid replacement; and controls. Baseline pHi was not different in the three groups. Cellular volume (vol/mm) was increased 38% in DOCA tubules versus controls, but unchanged in ADX tubules versus controls. Buffer capacities (BT) were not different in the three groups. Apical membrane H+ pump activity, assayed as the Na(+)-independent pHi recovery from an acid load (NH3/NH4+ prepulse) and expressed as JH (dpHi/dt.vol/mm.BT) was increased 76% in DOCA tubules versus controls, and decreased 56% in ADX tubules versus controls. Basolateral membrane Cl-/HCO3- exchange activity assayed as the pHi response to basolateral Cl- addition was increased 73% in DOCA tubules versus controls, and decreased 44% in ADX tubules versus controls. When examined as a function of varying [Cl-], the Vmax of Cl-/HCO3- exchange activity was significantly increased in DOCA tubules (control, 72.7 +/- 15.7 pmol.mm-1.min-1 vs DOCA, 132.3 +/- 22.5 pmol.mm-1.min-1, P less than 0.02), while the K1/2 for Cl- was unchanged. Basolateral membrane Na+/H+ antiporter activity assayed as the Na(+)-dependent pHi recovery from an acid load was not changed in chronic DOCA tubules versus controls. In conclusion, the apical membrane H+ pump and basolateral membrane Cl-/HCO3- exchanger of the rabbit OMCDi are regulated in parallel without chronic alterations in pHi under the conditions of mineralocorticoid excess and deficiency. The parallel changes in these transporters accounts for the alterations in OMCDi HCO3- absorption seen under these conditions.

Effect of selective aldosterone deficiency on acidification in nephron segments of the rat inner medulla

Mineralocorticoid plays a role in urinary acidification and acid-base balance, but the response of the inner medulla to aldosterone has not been elucidated. A model of selective aldosterone deficiency (SAD) with hyperkalemia and hyperchloremic metabolic acidosis was employed to assess segmental acidification by measuring in situ pH, titratable acidity (TA) and total ammonia (Am). Hydrogen ion secretion was also examined as a function of the increment in in situ PCO2 in the collecting duct during bicarbonate loading. SAD rats were compared to ADX controls that received adrenalectomy and chronic replacement of gluco- and mineralocorticoid and to rats with chronic metabolic acidosis induced by oral NH4Cl (CMA). Both fractional and absolute delivery of Am to the loop of Henle was lower in SAD vs. CMA rats (1.34 to 3.63 mM, P less than 0.01). Delivery of Am to the base and tip collecting duct (BCD and TCD) was also markedly lower in SAD (1.50 vs. 0.52 and 1.77 vs. 0.47 mM, respectively, P less than 0.01). Net addition of Am and net acid between BCD and TCD, observed in CMA rats, was not observed in SAD despite equivalent degrees of systemic metabolic acidosis. Similarly, the concentration gradient favoring transfer of NH3 between loop of Henle and CD was reduced in SAD. During bicarbonate loading the increment in PCO2 at BCD, TCD and in final urine was significantly lower in SAD rats than in adrenal intact bicarbonate-loaded rats. Therefore, the acidification defect in this model of SAD appears to be a result of a decrease in ammonia production and delivery to the loop of Henle, impaired transfer from loop to collecting duct and reduction in the rate of H+ secretion by the collecting duct.

Long-term control of plasma calcitriol concentration in dogs and humans. Dominant role of plasma calcium concentration in experimental hyperparathyroidism

Despite great interest in the elevated circulating levels of calcitriol (1,25-[OH]2D) associated with the clinical syndrome of human primary hyperparathyroidism, the relative potencies of known and potential stimuli/suppressors of long-term calcitriol levels have not been evaluated in either clinical or experimentally induced hyperparathyroid states. Based on reports that aparathyroid animals exhibit suppressed plasma calcitriol concentration and that acute administration of parathyroid hormone (PTH) to both humans and experimental animals or to renal slices in vitro results in increased plasma calcitriol concentration/production rate, it might be predicted that a chronic experimental model of either hypercalcemic primary hyperparathyroidism or hypocalcemic secondary hyperparathyroidism would show increased plasma calcitriol concentration. Chronic alterations in plasma calcium concentration have not been implicated as modulating calcitriol levels in any species. Accordingly, we investigated the long-term response of plasma calcitriol concentration in states of sustained experimental primary and secondary hyperparathyroidism. Intact dogs (group I) undergoing continuous intravenous PTH infusion for 12 d developed sustained hypercalcemia and hypophosphatemia, and plasma calcitriol concentration decreased from 23 +/- 3 to 14 +/- 3 pg/ml (P less than 0.01). Subsequent chelator (EGTA)-induced chronic normalization of hypercalcemia during ongoing PTH infusion resulted in a large and sustained increase in plasma calcitriol concentration to supernormal levels, reversible during subsequent cessation of chelator infusion. In additional intact dogs (group II), chronic chelator-induced hypocalcemic secondary hyperparathyroidism resulted in a sustained increase in plasma calcitriol concentration despite hyperphosphatemia. In normal human subjects undergoing a 12-13-d continuous intravenous PTH infusion to result in sustained moderate hypercalcemia (12.0 +/- 0.2 mg/100 ml) and hypophosphatemia, plasma calcitriol concentration decreased significantly (P less than 0.01) as in group I dogs and was followed by reversal to normal levels in a recovery period. The present results provide strong evidence in both humans and dogs that during experimentally induced chronic PTH excess, alterations in plasma calcium concentration dictate the directional response of circulating calcitriol concentrations. The long-term potency of plasma calcium concentration as a modulator of calcitriol metabolism is sufficient to override opposing modulation by plasma phosphorus concentration and PTH.

Mineralocorticoid modulation of rabbit medullary collecting duct acidification. A sodium-independent effect

Rabbit medullary collecting duct (MCD) from inner stripe of outer medulla has been identified as a major distal nephron acidification site. The isolated, perfused tubule technique was used to examine the roles of mineralocorticoid and glucocorticoid in regulation of MCD acidification. Surgical adrenalectomy reduced bicarbonate reabsorptive rate (JHCO3, pmol X mm-1 X min-1) from the normal of 9.79 +/- 1.21 to 0.67 +/- 1.1. Chronic administration of deoxycorticosterone acetate (DOCA) increased JHCO3 of MCD significantly to 18.02 +/- 1.62 whereas chronic dexamethasone administration did not affect JHCO3. The direct effects of aldosterone and dexamethasone upon MCD acidification were examined by perfusing tubules harvested from adrenalectomized rabbits in the presence of aldosterone or dexamethasone. Aldosterone, at 5 X 10(-8) M, increased JHCO3 significantly from 1.27 +/- 0.28 to 3.09 +/- 0.34. At 10(-6) M, aldosterone produced a greater increase in JHCO3 from 0.67 +/- 1.1 to 9.39 +/- 1.59. In vitro dexamethasone treatment had no effect on JHCO3. Studies examining the sodium dependence of aldosterone-stimulated acidification demonstrated that JHCO3 in tubules harvested from normal and deoxycorticosterone acetate-treated animals was unaffected by total replacement of sodium with tetramethylammonium. Likewise, luminal amiloride (5 X 10(-5) M) had no effect on JHCO3 in tubules harvested from adrenalectomized and normal animals. Moreover, the acute, in vitro stimulatory effect of aldosterone was seen to occur in the presence of luminal amiloride. These studies define a mammalian distal nephron segment that possesses major acidifying capacity, which is modulated by mineralocorticoid but independent of luminal sodium.