A B S T R A C Ttubule anid late proximal tubule yielded urinary recoverv rates of 85+3% and 101+2%, respectively, suggesting that oxalate absorption does occur in the mlid-portionis of the proximal tubule. Droplet precessioni studies confirmed a secretorv flux for oxalate. In contrast to oxalate, para-aminlohippurate (PAH), the more traditional marker for organic acid transport, was secreted in the late portions of the proximal tubule and in large measure at a site between the late proximal and distal tubules, presumably the pars recta. Probenecid inhibited PAH secretion but was w%ithout effect on net oxalate transport, oxalate absorption, or oxalate secretion.These studies demiionistrate that net oxalate secretion ocecurs in the earlx portions of the proximlal convoltuted tubule, uinidergoes bidirectional trainsport of approxim-atel e(qulal milagnittude in later segmiienits of the proximual tubtule, aind problalyl is not transported in imiore distal nephroni sites. The secretory mechaniism for oxalate differs fromn that of PAH in that it is located in a differenit segment of the nephroni and is not inhibited by probenecid. These differences These studies wvere performiied wvhile Dr.
The ability to excrete a volume of isotonic saline equal to 10% of body weight infused over 60 min, was examined in awake rats and in rats anesthetized with 1 of the 2 agents most commonly used in renal clearance studies, Inactin or Nembutal. Rats anesthetized with Inactin excreted significantly less of the infused sodium during the period of infusion and in the 120-min post-infusion periods as compared to Nembutal-anesthetized rats or awake rats. Following saline infusion, there was a significantly greater decrease in serum protein concentration (25.5 +/- 4.7%) in rats anesthetized with Inactin, compared to that observed in the awake or Nembutal-treated rats. In a separate group of saline-infused awake rats, induction of anesthesia with Inactin resulted in a significant increase in hematocrit and a decrease in serum protein concentration. These studies suggest that Inactin anesthesia decreases the ability of the kidney to excrete a saline load and that, in studies of sodium excretion in the rat, especially if volume expansion is to be part of the experimental protocol, Nembutal rather than Inactin may be the anesthetic of choice.
The decrease in urinary excretion of urate following the administration of pyrazinamide or its active metabolite, pyrazinoic acid (PZA), has been extensively utilized as a pharmacologic aid in dissecting out the contribution of secreted urate to the urinary excretion of uric acid (1, 2). As originally proposed, the use of the "Pyrazinamide Suppression Test" was based upon the assumptions that this compound was a specific and perhaps complete inhibitor of urate secretion and was without effect on the urate reabsorptive processes (3, 4). Indirect evidence has been presented, however, that neither of these assumptions is totally valid (5-8). Published studies on the separate effects of PZA on urate reabsorption and secretion, however, have been limited and somewhat conflicting (8-1 1). The current studies were designed to examine the effect of PZA, in varying dosages, on net urate transport and on the urate reabsorptive and secretory mechanisms in the rat.Methods. Male Sprague-Dawley rats with free access to food and water until the time of study were used in all experiments. Anesthesia was induced with Inactin (Promonta, Hamburg, Germany), 0.5-0.6 mM/kg body wt injected intraperitoneally. After a tracheostomy, the right and left jugular veins were cannulated and the urinary bladder catheterized. In the clearance experiments, the left femoral artery was cannulated for collection of blood samples. In the microinjection and precession studies, the left kidney was prepared for micropuncture as previously described (12, 13). The ureter of the left kidney was catheterized with PE-50 tubing to permit separate urine collections from each kidney. Only animals in which the urine flow rate of the left kidney was at least 85% of that from the contralateral kidney were included for study. In all animals, surgical losses of fluid were replaced with a volume of isotonic saline equal to 1% of body wt. Body temperature was maintained at 37". Pyrazinoic acid was dissolved in a solution of sodium hydroxide (0.1 M); the pH was then adjusted to 7.4 with either hydrochloric acid or sodium bicarbonate. In all control periods, the diluent alone was infused to control for the effect, if any, of diluent infusion.Clearance studies. Clearance studies were performed in diuretic rats receiving 5% mannitol in isotonic saline at a rate of 12.0 ml/hr so as to reproduce the protocol of the microinjection studies which require high urine flow rates. A priming dose of 50 ,uCi of [methoxy-jH] inulin in one ml of isotonic saline was infused followed by a sustaining infusion of isotonic saline containing 25 ,uCi/ml of [methoxy-"H] inulin at a rate of 1.2 ml/hr. After a 90-min equilibration period, two 20-min urine collections were obtained. 1.5 ml of arterial blood was obtained at the midpoint of each clearance period and was replaced with the same volume of blood from a donor rat.After collection of samples in the control periods, pyrazinoic acid in a dose of either 0.40, 0.80, or 1.6 mM/kg body wt (50, 100, or 200 mg/kg body wt respectively) was infus...
p(Dipropylsulfamy1)benzoic acid (probenecid) has been utilized in clinical practice and in experimental studies to decrease the tubular reabsorption of uric acid (1). Probenecid is also known to inhibit the secretion of organic acids in the kidney (2). Despite the extensive use of this agent, its separate effects on the urate reabsorptive and secretory processes have not been examined by direct techniques. The current investigations, therefore, were designed to examine the effects of probenecid on net urate transport, urate reabsorption, and urate secretion using clearance, microinjection, and precession techniques in the rat.Methods. Male Sprague-Dawley rats anesthetized with Inactin (Promonta, Hamburg, Germany), 100 mg/kg body wt intraperitoneally, were used for all studies. Probenecid was dissolved in a solution of dilute NaOH to a final concentration of 10-20 mg/ml and was administered in a dose of 100 mg/kg body wt/hr intravenously. In control animals and in control periods of the clearance experiments, diluent alone was infused.Clearance studies. After a tracheotomy, cannulae were placed in two jugular veins, in a femoral artery, and in the urinary bladder. Body temperature was maintained at 37". Five percent mannitol in isotonic saline was infused at a rate of 12 ml/hr to match the protocol of the microinjection studies. Through the other venous line, normal saline containing [rnetho~y-~HIinulin (25 pCi/ ml) was infused at a rate of 1.2 ml/hr. After 60-90 min of equilibration, two control urine samples, 20 min each, were collected. One milliliter of blood was obtained from the femoral artery at the midpoint of each period and was replaced with the same vol-To whom all correspondence should be addressed at Veterans Administration Hospital, Houston, Texas 77211.ume of blood from a donor rat. Following collection of control samples, probenecid (100 mg/kg body wt/hr) was infused. After an additional 60 min, repeat blood and urine samples were obtained.Microinjection studies. Animals were prepared for study as above, except that inulin was not infused. Animals were prepared for micropuncture as previously described (3-5 ) . The ureter of the left kidney was cannulated with PE-50 tubing to permit separate urine collections from each kidney. The urine flow rate of the micropunctured kidney was at least 85% of that from the contralat-era1 kidney. Intratubular microinjections were performed with a solution containing [2-14C]urate (50 pCi/ml) and [methoxy-3H]inulin (100 pCi/ml), adjusted to a pH of 7 . 4 with a solution of NaHCO, (30 mequiv/ liter). Three samples of equal volume (12-20 nl) were prepared, one of which was used for the microinjection, while the other two were counted directly for total radioactivity. Microinjections were performed into early or late proximal tubule sites over a 60to 90-sec interval, and total urine collections were obtained sequentially in 1-min intervals from both right and left kidneys over a 1 0-min period. The procedures for microinjection, the localization of injection sites, and...
Introduction. The current studies were designed to examine the effect of variations in urine flow rate, urine osmolality, and antidiuretic hormone (ADH) on urate excretion in the rat. Alterations in the extracellular fluid volume and the rate of solute excretion have been demonstrated to affect the urinary excretion of urate (1, 2). The influence of changes in urine flow rate, however, has not been clearly defined, and species variations may exist (3-5). Urine flow rate and urine osmolality are a function of the collecting tubule and duct cells, given a constant glomerular filtration rate (GFR) and rate of solute excretion. The results of the present investigations advance evidence that these nephron sites are not important determinants of the rates of urate excretion.Materials and methods. Male Sprague-Dawley rats were used in all studies. Catheters were placed in both femoral veins, femoral artery, and the urinary bladder, under light ether anesthesia. The animals were placed in restraining cages and allowed to awaken. Through one venous catheter, [metho~y-~H]inulin in isotonic saline (25 pCi/ml) was infused at a rate of 1-2 ml/hr for the duration of the study. Through the other venous catheter, in five animals, a solution of 0.21% saline was infused at a rate of 12 ml/hr. After 90 min of equilibration and after the urine flow rate and osmolality had stabilized, a timed urine collection of 10 to 20 min was obtained. A blood sample (1 ml) was obtained from the femoral artery and replaced with the same volume of blood from a donor rat. The infusion was then sequentially changed to 0.425% and 0.85% saline at infusion rates of 6 and 3 ml/hr, respectively. At each infusion rate, an equilibration period was permitted to elapse until urine flow rate and osmolality stabilized. In five animals, the order of infusions was reversed. In these animals, following the infusion of 0.21% saline at a
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