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In dogs with chronic hypochloremic metabolic alkalosis associated with ECFV contraction, plasma [HCO-3] ([HCO-3]p) normalizes during expansion of ECFV with a solution containing Cl- and HCO-3 in concentrations duplicating those in the plasma before expansion (isometric expansion). The kidney selectively rejects administered HCO-3 and retains Cl-. If this preferential Cl- less than HCO-3 reabsorptive selectivity were a characteristic renal response to ECFV expansion, isometric expansion during hyperchloremic acidosis would exacerbate the acid-base disturbance rather than correct it as it does in alkalosis. We examined the effect of isometric expansion in dogs with chronic hyperchloremic metabolic acidosis induced by HCl feeding or mineralocorticoid hormone deficiency. During expansion, as the expected decrease occurred in the fractional reabsorption of Na+, a lesser decrease occurred in fractional reabsorption of HCO-3, whereas a greater decrease occurred in fractional reabsorption of Cl-. The kidney selectively retained administered HCO3 and rejected Cl-. [HCO-3]p normalized. The shift to bicarbonate-selective from chloride-selective anion reabsorption during ECFV expansion in metabolic acidosis vs. metabolic alkalosis indicates that in response to ECFV expansion- the kidney selectively alters the ratio of bicarbonate to chloride concentration in the tubular reabsorbate in the direction that tends to normalize plasma acid-base composition, irrespective of the direction of deviation of the initial plasma bicarbonate concentration. The signal that initiates the shift in anion reabsorptive selectivity remains to be identified.

The in vivo mechanisms whereby systemic alpha- and beta-adrenergic stimulation exert opposing effects on renal water excretion are reviewed. An extrarenal mechanism is suggested since the effect of intravenous infusion of norepinephrine or isoproterenol on water excretion cannot be mimicked by the intrarenal administration of these agents. A ROLE OF VASOPRESSIN IS IMPLICATED SINCE NEITHER MAN NOR DOG WITHOUT A PITUITARY SOURCE OF VASOPRESSIN DEMONSTRATE THE SAME EFFECT OF CATECHOLAMINES ON WATER EXCRETION AS OBSERVED IN INTACT MAN AND DOG. Evidence also is presented that systemic alpha- and beta-adrenergic stimulation affect vasopressin release primarily by altering baroreceptor tone. The potential role of the autonomic nervous system in mediating other nonosmotic stimuli for vasopressin is discussed.

When the administration of exogenous mineralocorticoid hormones was discontinued in adrenalectomized dogs maintained on glucocorticoid, net acid excretion decreased due largely to a reduction in urinary ammonium excretion (UNH4+V), and hyperchloremic hyperkalemic metabolic acidosis occurred and persisted. The reduction in UNH4+V was not associated with an increase in urine pH (UpH) or a decrease in urine flow, but correlated with the severity of hyperkalemia and was mitigated by dietary potassium restriction. UpH decreased to values as low as 5.3. During acidosis, UpH varied directly with UNH4+V, but in relation to UNH4+V, UpH exceeded that in acid-fed mineralocorticoid-replete dogs. Extrapolated to UNH4+V=0, however, UpH was not significantly different in the two groups (5.27 vs. 5.44). When distal delivery of sodium was increased by infusion of sodium phosphate, titratable acid excretion increased in both groups but pateaued at lower rates in the mineralocorticoid-deficient dogs. These results suggest that in mineralocorticoid-deficient dogs, renal ammonia production is diminished, in part due to potassium retention and hyperkalemia; renal hydrogen ion secretory capacity is reduced even when sodium and buffer delivery to the distal nephron is not reduced; and the ability of the kidney to generate normally steep urine-to-blood hydrogen ion concentration gradients is unimpaired.