Charge density of renal interstitium.

The charge density of renal interstitium was analysed from the volume of distribution of negative native albumin as compared with neutralized albumin, labelled with 125I and 131I, respectively. The experiments were conducted by infusing the two probes intravenously at a rate which kept the plasma concentrations stable. The concentration in renal hilar lymph, C(lymph)(t), will then obey the function C(lymph)(t) = C(lymph)(t infinity) (1-exp-Kt), where C(lymph)(t infinity) is the steady state concentration and K the time constant for passage of the tracer through the renal interstitium--the former is dependent on the permeability of the peritubular capillary membrane, whereas the time constant is inversely related to the interstitial distribution volume of the tracers. The lymph-to-plasma concentration ratio (L/P-ratio) of negative, native albumin was found to be lower than that of neutralized albumin, a finding suggesting that the peritubular capillary membrane is negatively charged. Regarding the interstitium, it was calculated from the respective time constants, K, that the interstitium/lymph concentration ratio of negative native albumin was 0.96 +/- 0.06 of that of neutralized albumin. This suggests the presence of negative fixed charges repelling negative native albumin. However, since the calculated charge density of -1.8 +/- 1.2 mEq l-1 was not significantly different from zero, it is concluded that the renal interstitium is uncharged. This does not, however, rule out the possibility that, for example, negative groups are fixed to the interstitial matrix, merely that the average fixed charge density of renal interstitial fluid is negligible.