| Abstract: | Henle's loops of surface nephrons in rat kidneys were perfused orthogradely with various solutions of different compositions, while the stop-flow pressure (SFP) was monitored in the early proximal tubulus as a measure of the tubuloglomerular feedback response. Modified Ringer solution or iso-osmotic solutions of NaCl, KC1, LiCl, RbCl, choline-Cl, and Na-acetate used to perfuse Henle's loop led to significant SFP decrease indicating an intact operation of a tubuloglomerular feedback mechanism. Increased rates with isoosmotic solutions of sodium sulphate, potassium sulphate and mannitol did not yield SFP alterations. In order to estimate intratubular sodium and chloride concentration at the macula densa during Ringer-, LiCl-, choline-Cl-, and Na-acetate perfusion, early distal tubular fluids were collected at low and high perfusion rates, corresponding to an absent or a maximal feedback response. Analysis was performed by microflame photometry and microchloride titration. The results showed that there does not exist a common threshold for the early distal sodium or chloride concentration at which the feedback starts to operate. We conclude that the intraluminal signal at the macula densa, initiating a tubuloglomerular feedback response is not necessarily coupled to a single ion species. Either an unspecific ion transfer or more passive events of ionic interaction with the macula densa cells might operate as the initiating step in the feedback loop. |
