FEEDBACK REGULATION OF NEPHRON FILTRATION-RATE DURING PHARMACOLOGIC INTERFERENCE WITH THE RENIN-ANGIOTENSIN AND ADRENERGIC SYSTEMS IN RATS

Tubuloglomerular feedback has been defined as a mechanism in which changes in distal tubular chloride delivery induce changes in glomerular arteriolar resistance. Experiments were performed in rats to test the hypothesis that the alterations in vasomotor activity are controlled by local hormonal mechanisms. Early proximal flow rate (EPFR), used as an index of filtration rate, was assessed at loop perfusion rates of 10 and 40 nl/min and during zero loop flow before and during intravenous administration of agents which interfere with the renin-angiotensin or adrenergic systems. During infusion of the angiotensin (A) antagonists [Sar1, Ile8]-AII or [Me2, Gly1, Ile8]-AII at doses ranging from 4.8 to 30.6 μg/kg . min, feedback response, expressed as percent change of EPFR during loop flow elevation from 0 to 40 nl/min, fell from a mean of 47.6 ± 3.3% to 33.2 ± 2.9% (P<0.05). Likewise, after administration of the converting enzyme inhibitor SQ 20881 in a dose ranging between 5.5 and 34.0 mg/kg, feedback response decreased from 48.5 ± 2.1% to 25.9 ± 1.9% (P<0.001) and returned to 43.1 ± 5.1% after the inhibitory effect of SQ 20881 on the pressure response to angiotensin I had disappeared. Luminal application of [Sar1, Thr8]-AII (5 mM) or of SQ 20881 (5 or 10 mM) had no effect on the feedback response. A significant reduction in the feedback response was noted also during intravenous infusion of propranolol (46.4 ± 3.2% vs. 29.0 ± 2.8%, P<0.001), whereas 6-OH-dopamine, reserpine, or phenoxybenzamine had no detectable effect. Our results are in agreement with the concept that the reninangiotensin system may mediate feedback-induced resistance changes. In addition, circulating catecholamines may, in some unknown manner, act as modulators of the feedback response.