Effect of Brief Hypercalcaemia on Free Water Reabsorption during Solute Diuresis: Evidence for Impairment of Sodium Transport in Henle's Loop

Abstract

1. The relationship between osmolal clearance (C_osm) and the reabsorption of solute free water (T^ch₂o) was examined during mannitol and hypertonic saline diureses in both normocalcaemic and hypercalcaemic rabbits. 2. At high values for C_osm (2–4 ml/min) in normocalcaemic rabbits T^ch₂o during saline diuresis exceeded that during mannitol diuresis as previously shown. 3. Prolonged hypercalcaemia (72–96 h) reduced T^ch₂o in both mannitol and saline diureses. 4. Brief hypercalcaemia (18–24 h) abolished the increment in T^ch₂o normally seen during saline, as compared with mannitol diuresis. This impairment of T^ch₂o during saline diuresis occurred at a time after the start of hypercalcaemia when T^ch₂o during mannitol diuresis was still unaffected. 5. The results cannot be attributed to an effect of hypercalcaemia on membrane permeability to water, and are best explained by a reversible limitation of sodium transport in Henle's loop. This interpretation is supported by the fact that the hypercalcaemic rabbits excreted a greater percentage of the filtered sodium load than did the controls.