Hyperkalemic forms of renal tubular acidosis: clinical and pathophysiological aspects

In contrast to distal type 1 or classic RTA which is associated with hypokalemia, hyperkalemic forms of RTA also occur usually in the setting of mild to moderate CKD. Two pathogenic types of hyperkalemic metabolic acidosis are frequently encountered in adults with underlying CKD One type, which corresponds to some extent to the animal model of selective aldosterone deficiency (SAD) created experimentally by adrenalectomy and glucocorticoid replacement, is manifested in humans by low plasma and urinary aldosterone levels, reduced ammonium excretion, and preserved ability to lower urine pH below 5.5. This type of hyperkalemic RTA is also referred to as type IV RTA. It should be noted that the mere deficiency of aldosterone when GFR is completely normal only causes a modest decline in plasma bicarbonate which emphasizes the importance of reduced GFR in the development of the hyperchloremic metabolic acidosis associated with SAD. Another type of hyperkalemic RTA distinctive from SAD in that plasma aldosterone is not reduced is referred to hyperkalemic DRTA because urine pH cannot be reduced despite acidemia or after provocative tests aimed at increasing sodium dependent distal acidification such as the administration of sodium sulfate or loop diuretics with or without concurrent mineralocorticoid administration. This type of hyperkalemic RTA (also referred to as voltage dependent DRTA) has been best described in patients with obstructive uropathy and resembles the impairment in both hydrogen ion and potassium secretion induced experimentally by urinary tract obstruction and when sodium transport in the cortical collecting tubule is blocked by amiloride.