EFFECT OF NATRIURETIC AGENTS, VASOACTIVE AGENTS AND OF THE INHIBITION OF METABOLISM ON SODIUM HANDLING IN THE ISOLATED PERFUSED KIDNEY OF THE NEPHROTIC RAT

The kidney taken from a rat rendered nephrotic by exposure to puromycin aminonucleoside retains sodium abnormally when perfused in isolation and has an abnormally low vascular resistance (J.D. Firth et al., Clin. Sci. 1989; 76, 387-95). In this study the relation of oxygen consumption to sodium reabsorption has been examined in the isolated nephrotic organ, which has also been exposed to a variety of natriuretic agents and to the effect of inhibition of metabolism by cooling, in an attempt to discern the transport process, or processes, responsible for abnormal tubular handling of sodium. In addition, the effects of three endogenous vasoconstrictors, noradrenaline, angiotensin II and endothelin, on the function of the isolated nephrotic kidney have been examined. The ratio of mol of sodium reabsorbed by the tubules of the isolated nephrotic kidney to mol of oxygen consumed was reduced in comparison with the control kidney (means ± SEM): 9.22 ± 0.97 versus 15.43 ± 1.55 (P < 0.002). In the presence of ouabain (1 mmol/l), acetazolamide (1 mmol/l), frusemide (200 μmol/l), the combination of these three agents, together, hydroflumethiazide (100 μmol/l), benzamil (100 nmol/l) or atrial natriuretic peptide (1000 pmol/l), a lesser increment in sodium excretion was induced in the isolated nephrotic kidney than in the control kidney and the nephrotic organ continued to excrete less sodium in both absolute and fractional terms. This suggests that enhanced tubular sodium reabsorption in the isolated nephrotic kidney does not depend upon abnormally increased activity of the Na+/K+-adenosine triphosphatase, bicarbonate-dependent sodium transport, Na+/K+/2Cl- co-transport, electrically neutral proportionate reabsorption of sodium and chloride (distal tubule), epithelial sodium channel (distal tubule) or atrial natriuretic peptide-sensitive sodium transport processes. When isolated nephrotic kidneys and normal kidneys were cooled to 8-10°C the handling of sodium became virtually identical in the two groups. On re-warming to 37°C, the original differences in sodium handling between nephrotic and control kidneys were restored. This implies that the mechanism responsible for the abnormal tendency to retain sodium is temperature-sensitive; as yet it remains otherwise undefined. The sensitivity of the renal vessels to noradrenaline, angiotensin II and endothelin, as judged by the percentage reduction in perfusate flow rate produced by a given concentration of any of these agents, was not substantially altered in the nephrotic kidney compared with the control kidney. Increase in vascular tone was not associated with amelioration of the tendency of the isolated nephrotic organ to retain sodium. Increasing concentrations of angiotensin II caused the filtration rate to increase in the nephrotic kidney. This effect was unexpected: in the control preparation, as anticipated, angiotensin II caused the filtration rate to decrease.