Plasma Volume Expansion and Proximal Tubular Reabsorption of Salt and Water by Rat Kidney

Venous infusion of both hypertonic and isotonic solutions has been known to lead to diuresis and natriuresis. In recent micropuncture studies, attempts have been made to identify the tubular locus at which renal functions may be altered and to identify the mechanism by which functional changes are mediated. In the rat, Giebisch et al. (1964) found suppressed fractional reabsorption of sodium at the level of the distal tubule when hypertonic saline was administered, whereas suppressed proximal fractional reabsorption was found only when excreted sodium was greater than 13% of the filtered load. On the other hand, Cortney et al. (1965) have observed reduced proximal fractional sodium reabsorption following infusion of isotonic sodium chloride. Similar results have been obtained in the dog (Dirks, Cirksena, and Berliner, 1965). To resolve this seeming discrepancy, Kamm and Levinsky (1965) have suggested that the distal tubule responds to increases in plasma sodium concentration, while the proximal tubule responds to volume expansion. It is our intent to make a detailed study of this response. The present report, however, presents preliminary data showing that the proximal tubule does, indeed, respond to volume expansion