Effect of Changes in Renal Perfusion Pressure on the Suppression of Proximal Tubular Sodium Reabsorption due to Saline Loading

known to suppress reabsorption of sodium and water in the proximal tubule. It has previously been shown that this suppression is accompanied by two changes which in combination might account for the over-all decrease in reabsorption: a reduction in the intrinsic reabsorptive capacity of the tubular epithelium (C/rr') and a reduc-tion in the ratio between tubular volume and GFR (7rrd/Vo). The present micropuncture experiments were carried out in order to study the possible role of altered peritubular physical forces (hydrostatic and colloid on-cotic pressure) in mediating these two changes. Proxi-mal tubular reabsorptive capacity, transit time, fractional reabsorption of sodium and water, 7rrd/Vo, and intra-tubular hydrostatic pressure were measured in saline-loaded rats during acute changes in renal perfusion pressure induced by intermittent constriction of the ab-dominal aorta. We found that when renal perfusion pressure was low-ered to 70-90 mm Hg, the usual effects of saline loading on C/irr, 7rrd/Vo, and fractional reabsorption in the proximal tubule were greatly minimized. When the aortic clamp was released and renal perfusion pressure allowed to rise, C/irr', 7rrd/Vo, and fractional reabsorption fell markedly to levels characteristically seen in saline di-uresis. Reclamping of the aorta reversed all of these changes. In order to determine whether the changes in C/rr' accompanying changes in renal perfusion pressure were mediated by a circulating natriuretic hormone, we as-sayed in hydopenic rats the dialysate of plasma collected from saline-loaded rats during and after release of aortic constriction by the split oil drop method. N