Effect of intraluminal pressure on proximal tubular sodium reabsorptin. A shrinking drop micropuncture study.

1mt-larecial oilcotic lmld hiydrostatic w-rcsst-c as e vbeen slhowvn to play an impoortant role in the regulatioin of sodiutm transport across the proximal ttb)ular epitlhelium. Wlhile peritubuilar changes in onicotic pressure in part control tlle rate of tubular so(liUm reabsorption(l), similar modifications of protein concentration at the intraluminal level are of muclh lesser significance(2). Tlle decrease of net sodlium reabsorption rate inducedl by a fall in peritubtular protein concentration lhas been slhown to be associated with an increase in ionic conductance of the intercellular slhtunt patlhways, resulting in increased backflow of sodium to the lutmen(3,4). Regarding lhydrostatic pressure clhanges, peritubular hydrostatic pressure clhanges lhave been suggested to affect tubular sodium reabsorption(5,6). At present, evidence is lacking that modifications of the transepitlhelial hydrostatic gradient, wlhen induced by intraluminal pressure changes, exert a reverse(d btut symmetrical effect on proximal tubular sodium and fluid transfer. In the present study a significant effect of intraluminal pressure on tlle rate of sotlium reabsorption of Necturus proximal tubule has been clearly demonstrated. An intraluminal pressure clamp technique is described in this paper, and its application to the slhrinking drop micropuincture teclhnique is illustrated. MIoreover, the pressure clharacteristics of the shrinking drops have been determined and are compared to the pressures occurring in various tubular and extratubular structures of the Nectuirus kidney.