Enalaprilat handling by the kidney: barrier-limited cell entry

The angiotensin-converting enzyme inhibitor enalaprilat is formed in vivo in liver and kidney by esterolysis of the antihypertensive drug enalapril. To gain insight into the renal elimination of enalaprilat, we carried out multiple-indicator dilution experiments in the isolated perfused rat kidney. Kidneys were perfused single pass with an amino acid-supplemented Krebs-Henseleit buffer containing 20% bovine red blood cells and 4% bovine serum albumin, at a flow rate of 0.11 +/- 0.02 (SD) ml.s-1 x g-1. A bolus of 51Cr-labeled red blood cells (vascular red blood cell indicator), 125I-labeled albumin (vascular plasma indicator), L-[14C]glucose (interstitial space indicator), and [3H]-enalaprilat was injected into the renal artery, and timed samples of venous blood (up to 1 min) and urine (up to 10 min) were collected. The data were analyzed using a variable-transit-time, space-distributed model with modifications accounting for glomerular filtration and the observed 14% protein binding of enalaprilat; the glomerular filtration rate (GFR) estimated from L-glucose clearance was 9.0 +/- 2.9% of total plasma flow. The ratio of renal clearance of unbound enalaprilat to GFR was 1.56 +/- 0.29, indicating both glomerular filtration and net tubular secretion of enalaprilat. Unidirectional influx from plasma to tubular cells exceeded tubular secretion by a factor of 2.2 +/- 0.5. Thus only about one-half of the enalaprilat taken up by the tubular cells was excreted into urine, with the remainder refluxing into the capillary blood stream, indicating bidirectional permeation of enalaprilat across the basolateral tubular membrane.