The existence of a tubulo-glomerular feedback mechanism in the Amphiuma nephron

A single nephron tubulo-glomerular feedback control of the glomerular filtration rate, which is known in mammalian animals, could be one way by which amphibians regulate the glomerular filtration rate (GFR). To investigate whether the Amphiuma means shows any sign of a tubulo-glomerular feedback control, micropuncture experiments were carried out. Six different series of experiments were performed. In the first series, tubular stop-flow pressure (SFP) was measured during distal tubular perfusion with amphibian Ringer solution at a rate of 10, 25 and 50 nl/min. A significant decrease of SFP was found at the three perfusion rates compared to the controls. In the second group, single nephron glomerular filtration rate (SNGFR) was measured, while the distal tubule was perfused at 10, 25 and 50 nl/min. At a perfusion rate of 10 nl/min the SNGFR did not decrease, whereas at 25 and 50 nl/min it decreased significantly. In the third group the perfusion pipette was located in the proximal tubule and the nephron was perfused at 10, 25 and 50 nl/min, while at the same time the proximal tubular stop-flow pressure was measured. No reduction of SFP was found at a perfusion rate of 10 nl/min, while significant reductions were noted at rates of 25 and 50 nl/min. In the fourth group the SNGFR was measured in the distal tubule beyond the macula densa and in Bowman's space of the same nephron. No significant difference was found. In the fifth group, the glomerular capillary pressure (GCP) was measured before and after blockade of the tubular fluid flow. No significant difference was found between these two measurements. The sixth series deals with the changes occurring at the single nephron level by the tubulo-glomerular feedback control. The single nephron filtration fraction (FF) was determined from efferent arteriolar protein concentration with and without a feedback-induced reduction of the SNGFR. The FF values were not significantly different from one another. From these results and data from the other series, the afferent (Raff) and efferent (Reff) arteriolar resistances were calculated. Reff did not change, while Raff increased significantly when a feedback stimulus was applied. These experiments indicate the existence of a tubulo-glomerular feedback control which depresses the SNGFR and SFP by contracting the afferent arteriole.