Role of Hemodynamic Changes in the Increased Cation Excretion after Acute Unilateral Nephrectomy in the Anesthetized Dog

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Abstract
Studies were carried out in anesthetized dogs to characterize the increase in cation excretion which occurs after acute unilateral nephrectomy (AUN). 60 min after AUN, cation excretion had increased from 31.5±2.7 to 66.3±12.0 μeq/min (P < 0.005) and fractional cation excretion had increased from 0.56±0.05 to 1.03±0.14% (P < 0.005), as the glomerular filtration rate was unchanged and renal blood flow fell. The increased cation excretion was accompanied by an increase in fractional phosphate excretion, no change in chloride excretion, and a fall in renin secretion. These alterations in renal function were associated with marked changes in systemic hemodynamics: cardiac output fell from 2.52±0.24 to 1.85±0.16 liters/min (P < 0.001), as diastolic pressure rose without an overall increase in mean arterial pressure, and heart rate fell. To assess the importance of these hemodynamic changes in the renal response, AUN in a separate group of dogs was accompanied by the simultaneous opening of a surgically created femoral artery-to-vein fistula at flow matching the blood flow to the removed kidney. When this was done, no alterations in systemic or renal hemodynamics were observed, and cation excretion did not differ from control. Subsequent closure of the fistula then caused a fall in cardiac output from 2.15±0.25 to 1.77±0.20 liters/min (P < 0.05), and an increase in cation excretion from 34.6±9.5 to 52.3±13.7 μeq/min (P < 0.01), thus mimicking the findings with AUN alone. These results demonstrate that AUN causes hemodynamic changes resembling those seen on closure of a chronic arteriovenous fistula. Prevention of these hemodynamic changes after AUN also prevents the functional adjustment of the remaining kidney, suggesting that they may be important in initiating the renal response. The increased electrolyte excretion after AUN may occur through mechanisms similar to that seen on closure of an arteriovenous fistula.