Stimulation of tubular reabsorption of magnesium and calcium by antidiuretic hormone in conscious rats. Study in Brattleboro rats with hereditary hypothalamic diabetes insipidus.

The effect of antidiuretic hormone on urinary electrolyte excretion was investigated by clearance techniques in conscious rats in metabolic cages. Brattleboro rats with hereditary diabetes insipidus (DI) (no ADH) were studied in the absence of exogenous ADH (control group = C, n = 4), and after several weeks of continuous dDAVP infusion (period A) followed by discontinuation of dDAVP (period B) (experimental group = E, n = 6). dDAVP, a non-pressor antidiuretic analogue to ADH, induced 1) a high urine concentration (2,645 +/- 44 (SEM) in group E vs 131 +/- 6 mosmol/kg H2O in group C), P less than 0.001; 2) no significant change in plasma osmolality (288 +/- 2 vs 297 +/- mosmol/kg H2O respectively) and in plasma concentration of major electrolytes, Na, K, Cl, Mg, and Ca; 3) a large decrease in urinary excretion of calcium and magnesium and no change in other electrolyte or total osmolar excretion. Fractional excretions in rats of groups C and E during period A were, respectively, for Na: 0.59 +/- 0.03 (SEM) and 0.51 +/- 0.33% (NS), for Ca: 2.92 +/- 0.62 and 0.34 +/- 0.05% (P less than 0.001) and for Mg: 7.75 +/- 0.83 and 1.38 +/- 0.28% (P less than 0.001). After treatment discontinuation, plasma osmolality in group E rose to 304 +/- 2 mosmol/kg H2O (P less than 0.01 compared to period A) with slight increases in plasma Na and Cl concentrations. Urine osmolality fell below, and urine flow rate rose above values observed in the control group.(ABSTRACT TRUNCATED AT 250 WORDS)