Cardiac output, GFR, and renal excretion rates during maintained volume load in rats

The correlation among cardiac output (CO), glomerular filtration rate (GFR), fractional tubular sodium rejection (TFRNa), and renal excretion rates of water and salt was investigated during ischemic blood volume expansion in rats. Initially circulating blood volume was equilibrated isovolemically with a reservoir volume of 6% albumin solution equal to one-third the estimated blood volume. Later the equilibrated reservoir contents were infused intravenously. CO was measured by thermodilution, GFR by inulin clearance. Significant linear correlations existed between GFR and the rates of urine flow (r = 0.90), sodium excretion (r = 0.75) and potassium excretion (r = 0.76) that prevailed 5--10 min after a given GFR change. The increased GFR was highly correlated with CO (r = 0.94), probably correlated with mean central venous pressure (r = 0.45), but not correlated with mean abdominal aortic blood pressure. The correlation between CO and time-delayed (5--10 min) TRFNa was also highly significant (r = 0.98). The saluresis appears to have been caused initially by increased tubular load and subsequently by decreased absolute tubular reabsorption.