Reabsorption and metabolism of quinapril and quinaprilat in rat kidney: in vivo micropuncture studies

Effects of a therapy with losartan and quinaprilat on the progression of chronic renal failure in rats after a single dose of uranyl nitrate or 5/6 nephrectomy

The renoprotective effect of losartan and quinaprilat was tested in two different animal models of renal failure [female Wistar rats, single administration of 0.5 mg uranyl nitrate (UN)/100 g body wt. or 5/6 nephrectomy (5/6NX)]. Losartan (1 mg/100 g body weight [wt.]) and quinaprilat (1 mg/100 g body wt.) were administered intraperitoneally, once daily, starting 10 days after UN and one week after 5/6NX till the end of 10 weeks experimental period. Parameters characterizing the therapeutic effect were blood pressure, urinary protein excretion 4 and 10 weeks after the injury, and p-aminohippurate accumulation in renal cortical slices in vitro, hydroxy-proline concentration in renal tissue and morphology at the end of the experiment. Summarizing our results we state: (1) the angiotensin 1 receptor blocker losartan is more effective in UN-treated than in 5/6 NX rats, and (2) the angiotensin converting enzyme inhibitor quinaprilat is more effective than losartan because of the amelioration of blood pressure and OH-proline concentration in renal tissue of UN-treated rats.

Organ clearance concepts: new perspectives on old principles

The removal capacity of an eliminating organ by metabolism and/or excretion is often expressed as its clearance. Metabolic and excretory clearances are considered to be mutually independent, and the sum of these constitute the whole organ clearance. The influence of metabolism on estimates of the excretory clearance and vice versa was examined for the liver and kidney with physiologically based models. Mass transfer first-order rate equations describing transport and removal were derived. Upon inversion of the matrices originating from the coefficients of these equations, the area under the curve (AUC) and clearance (dose/AUC) were obtained with the liver or kidney as the eliminating organ. A more complex solution was found to exist for the kidney since glomerular filtration, secretion; reabsorption, and intrarenal metabolism were present. To ascertain the effect of excretion on estimates of the metabolic clearance as well as the effect of metabolism on estimates of the excretory clearance, intrinsic clearances for excretion or metabolism were set to zero. Clearance values were found to be altered when alternate pathways were present. Whereas excretory clearance estimates were consistently reduced in the presence of metabolism, metabolic clearance estimates were affected differentially by excretion and varied according to the site of metabolism. Excretion reduced metabolic clearance estimates when metabolism occurred intracellularly. If metabolism occurred intraluminally (e.g., on the renal brush border or luminal membrane), the metabolic clearance estimate could become higher since the substrate was made available to the enzymes following its excretion. As expected, these changes depended on the relative magnitudes of the intrinsic clearances for metabolism and excretion. The above theory was applied to the elimination of enalapril which is both metabolized and excreted by the perfused rat liver and kidney preparations. Data obtained in these studies were consistent with a set of published physiologic parameters denoting transfer and intrinsic clearances. Perturbations on clearance estimates were studied by setting the metabolic/excretory intrinsic clearance to zero, then to some finite value. In liver, the avid hepatocellular metabolism of enalapril reduced biliary clearance by 73%. For the kidney, the fractional excretion (FE or unbound excretory clearance/glomerular filtration rate) was decreased modestly (from 0.64 to 0.44) with intracellular esterolysis, whereas if metabolism had occurred intraluminally, FE would have been significantly decreased (from 1.8 to 0.45). Simulation results show clearly that clearance estimates are affected by the presence of alternate removal pathways, and question the well-established principle the metabolic and excretory clearance estimates are independent of each other.

Tubular transport mechanisms of quinapril and quinaprilat in the isolated perfused rat kidney: effect of organic anions and cations

The clearance mechanisms of quinapril and quinaprilat were probed using an isolated perfused rat kidney model. Sixty-four experiments were performed with drug in the absence and presence of classic inhibitors of the organic acid (i.e., probenecid and p-aminohippurate) and organic base (i.e., tetraethylammonium and quinine) transport systems of the proximal tubule. Initial perfusate concentrations of quinapril and quinaprilat were approximately 2.36 microM (or 1000 ng/ml), and transport inhibitors were coperfused at 100-10,000 times the drugs' initial microM concentrations. Quinapril and quinaprilat concentrations were determined in perfusate, urine, and perfusate ultrafiltrate using a reversed-phase HPLC procedure with radiochemical detection, coupled to liquid scintillation spectrometry. Perfusate protein binding was determined using an ultrafiltration method at 37 degrees C. Overall, the clearance ratios of quinapril (total renal clearance divided by fu x GFR) and quinaprilat (urinary clearance divided by fu. GFR) were significantly reduced, and in a dose-dependent manner, by the coperfusion of organic acids but not organic bases. The data demonstrate that the organic anionic secretory system is the primary mechanism by which quinapril and quinaprilat are transported into and across renal proximal cells.