Jerry B. Hook

Lack of Correlation Between Natriuretic Activity and Inhibition of Renal Na-K-Activated ATPase.

Summary Ethacrynic acid, which is a potent natriuretic in man and dog but inactive in the rat, and furosemide, which is a potent natriuretic in man and dog as well as in the rat, were compared for their ability to inhibit microsomal Na-K-activated ATPase following in vivo administration in the rat. Both agents were found to inhibit the activity of this enzyme. Furosemide produced a significant natriuretic response whereas ethacrynic acid did not. Therefore, no correlation between natriuretic activity and enzyme inhibition could be made with these agents. This study does not rule out a correlation between Na-K-activated ATPase and natriuretic activity but it does question the practice of determining Na-K-activated ATPase in vitro and attempting to relate this to responses in vivo.

EFFECT OF FUROSEMIDE ON RENAL MEDULLARY SODIUM GRADIENT.

Summary Furosemide, 25 mg/kg/hr (25 mg/kg prime) abolished the sodium medullary gradient in the dog kidney. It is concluded that the natruresis produced by furosemide is due, at least in part, to inhibition of sodium reabsorption in the ascending limb of the loop of Henle.

Hepatic and renal conjugation (phase II) enzyme activities in young adult, middle-aged, and senescent male Sprague-Dawley rats

Abstract Acetaminophen (APAP)-induced nephrotoxicity is age dependent in male Sprague-Dawley rats: nephrotoxicity occurs at lower dosages of APAP in 12- to 14-month olds compared with 2- to 3-month olds. The mechanisms responsible for enhanced nephrotoxicity in 12-month-old Sprague-Dawley rats are not entirely clear, but may be related to age-dependent differences in APAP metabolism in liver and/or kidney. Major pathways of hepatic APAP metabolism include sulfation and glucuronidation; glutathione conjugation represents a pathway for detoxification of reactive oxidative APAP metabolites. The present studies were designed to quantify in vitro activity of three Phase II enzyme activities: glutathione S-transferase using 1-chloro-2,4-dinitrobenzene as substrate, UDP-glucuronyl transferase using APAP as substrate, and sulfo-transferase using APAP as substrate, in subcellular fractions of liver and kidney of 3-, 12-, 18-, and 30-month-old naive male Sprague-Dawley rats. In liver, glutathione S-transferase, UDP glucuronyl transferase, and sulfotransferase activities were not significantly different in rats from 3 through 30 months of age. Renal UDP glucuronyl transferase and sulfotransferase activities were similar in rats from 3 through 30 months of age. In contrast, renal glutathione S-transferase activity was characterized by a lower Km in 12- and 30-month olds when compared with 3-month olds. These data suggest that the reduced total systemic clearance of APAP in 12-month-old male Sprague-Dawley rats previously observed cannot be attributed to age-dependent differences in hepatic APAP metabolism. In addition, it is unlikely that differences in renal APAP metabolism contribute to age-dependent APAP nephrotoxicity.

Hemodynamic effects of furosemide in isolated perfused rat kidneys.

In anesthetized dogs, the diuretic furosemide has been shown to increase renal blood flow, an effect which can be blocked by the prostaglandin synthetase inhibitor, indomethacin (1, 2). Furosemide also caused an increase in PGE2 release into renal venous blood and this increase was inhibited by indomethacin (2). These data suggest a relationship between the hemodynamic effect of furosemide and the renal prostaglandins. PGE2 is a vasodilator in the dog kidney; however, Malik and MgGiff (3) demonstrated that this prostaglandin increases renal vascular resistance in the rat. This apparent species difference between the rat and other mammals (4) offers a potential tool for elucidating the relationship between furosemide and PGE2 on renal hemodynamics. If the action of furosemide to dilate the renal vasculature in dogs is dependent on release of PGE2, and if PGE2 is a vasoconstrictor in rats, then furosemide would be expected to decrease renal blood flow in the rat. Alternatively, if the primary effect of furosemide is to produce hemodynamic changes which secondarily influence prostaglandin synthesis, furosemide might increase renal blood flow in rats as it does in dogs. The purpose of this investigation was to determine the effect of furosemide on renal vascular resistance and to elucidate the interactions between this diuretic and PGE2 in isolated perfused rat kidneys. Since the renin-angiotensin system may also influence the renal vasculature, experiments were performed to evaluate interactions between this system and PGE2 and furosemide.

Lack of Effect of Actinomycin D on Aldosterone Induced Antinatriuresis When Administered After the Hormone.

Summary Actinomycin D was administered at varying times after the administration of aldosterone to rats in order to determine if this inhibitor would either enhance or prolong the antinatriuretic action of aldosterone. No such effects were found, thus it would appear that an actinomycin D sensitive “represser” is not involved in terminating the sodium retaining action of the adrenocortical steroid hormone, aldosterone.

Factors contributing to depressed renal transport of organic anions in the obese rat.

Summary The effect of obesity produced by feeding rats a 60% fat diet (HF) on accumulation of the organic anion p-amino-hippurate (PAH) by renal cortical slices was determined using an in vitro technique. The rate of PAH uptake was determined and analyzed kinetically using a Lineweaver-Burk plot. In control rats fed a grain ration (GR) the V max and Km were less in old (60 wk) than in young (12 wk) animals. The animals on high fat (HF) had V max and Km values less than the respective age controls. The decrease in apparent affinity and maximal velocity with age and diet could indicate noncompetitive inhibition. The differences observed in PAH transport in the HF animals were not the result of differences in oxygen consumption, histology (light microscopy) or composition of renal cortical slices. Kidneys from animals fed HF and switched to GR at least 2 days prior to sacrificing accumulated PAH as well as those from animals fed GR from weaning. Kidneys from animals fed GR and switched to HF for at least 2 days had depressed PAH accumulation. Organic cation transport as determined with N-methylnicotinamide (NMN) accumulation was unaffected by diet switching. These data indicate the presence of some factor inhibiting PAH accumulation, perhaps of dietary origin (as another organic acid), in the serum. However, stimulation, not inhibition, was demonstrated when serum from the HF or control animals was added to the incubation medium. The data are consistent with the hypothesis that in the HF animals there is some factor which is probably tightly bound in the kidney but is present in such low concentrations in the serum that no acute inhibitory effects on PAH accumulation are demonstrable in vitro.

The Role of Oxidative Stress in Cephaloridine Nephrotoxicity

The cephalosporin antibiotic, cephaloridine (CPH), is nephrotoxic when administered in large dosages to humans and laboratory animals (1,2) . In vivo, CPH nephrotoxicity is characterized histologically by acute proximal tubular necrosis and functionally by glycosuria, enzymuria, proteinuria and an impaired ability of renal cortical slices to accumulate organic ions (1-4). Previous studies have indicated that the nephrotoxicity of CPH is intimately related to its renal cortical accumulation and intracellular concentration (5). CPH is actively transported into the proximal tubule cell by an organic anion transport system (5,6). However, unlike many organic anions and cephalosporins, CPH undergoes only limited movement across the lumenal membrane into the tubular fluid. Consequently, high intracellular concentrations of CPH are attained in the proximal tubule which are critical to the development of nephrotoxicity (5).