Two-phase recollection micropuncture experiments were performed on female New Zealand rabbits to investigate the effect of flow rate (volume-expansion) compared to reabsorptive rate (furosemide) on calcium and sodium handling along the nephron. Group 1 (n = 6) rabbits represented nonvolume-expanded animals. Each experiment was conducted with a control phase followed by a second phase of furosemide administration (lmg/kg/min). Group 2 rabbits (n = 6) were initially volume-expanded to 3% body weight with modified Ringers. The fractional excretion of sodium and calcium in the control phase of group I and II was 3 ± 1 and 22 ± 6% and 4 ± 1 and 26 ± 2%, respectively. Fractional excretion of sodium, calcium and magnesium rose after furosemide administration. The effect of volume expansion on sodium, calcium and magnesium remaining in the proximal tubule was relatively modest and not affected by furosemide. Our distal micropuncture data reveal that volume expansion has a greater inhibitory effect on fluid reabsorption at a site beyond the proximal micropuncture site (group 1, 9 ± 2%, group 2, 22 ± 2%). After furosemide infusion, the amount of electrolytes remaining rose similarly in both groups; however, additional sodium and calcium reabsorption did not occur in the volume-expanded group in the final segment of the nephron. These results indicate that calcium reabsorption by the cortical terminal segment of the rabbit is passive similar to that suggested by the in vitro perfused study since no additional calcium reabsorption is seen in the volume-expanded rabbit.

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