THE EFFECT OF REDUCING PROXIMAL TUBULAR FLUID DELIVERY ON THE RATE OFFILTRATION OF SINGLE NEPHRONS

The rate of delivery of tubular fluid from the proximal tubule (PT) is thought to reset nephron filtration rate (SNGFR). In micropuncture ex periments in rats we tested this hypothesis by reducing the efflux fro m the PT by simultaneously ''double collecting'' (DC) tubular fluid fr om the early distal tubule (DT) and from the last convolution of the P T of the same nephrons. SNGFR measured by total collection of tubular fluid was 34+/-3 nl/min at the DT and 34+/-3 nl/min at the PT (p>0.97, n=42). The simultaneous collection from proximal and distal sampling site was performed between these two paired measurements. It yielded a n average SNGFR of 40+/-3 nl/min (p<0.02). This may be due to the coll ection, at the distal site, of the extra amount of inulin stored betwe en distal and proximal pipette, prior to starting the aspiration of tu bular fluid. Since this error would decrease in longer collections, th e difference in SNGFR between single and double collections was plotte d against the duration of collections. In fact it was negatively corre lated with the sampling time (p<0.01), indicating no difference in SNG FRs for collections >4 minutes. Reduction and complete interruption of the delivery of native proximal tubular fluid to the Macula Densa doe s not seem to influence the measurement of SNGFR. Filtration rate is n ot significantly different when measured within few minutes at the DT and PT of the same nephrons.