Theodore W. Kurtz

Potentiation of gentamicin nephrotoxicity by metabolic acidosis.

Summary Gentamicin nephrotoxicity was examined in normal rats and in rats chronically ingesting 1% NH4C1 solution. Metabolic acidosis significantly exacerbated gentamicin nephrotoxicity as manifested by elevation of SUN and depression of tissue uptake of PAH and TEA by renal cortical slices. In contrast normal rats given the same dose of gentamicin did not develop nephrotoxicity. Pathological examination confirmed that acute tubular necrosis occurred only in the acid-loaded rats injected with gentamicin.

Renal Hemodynamics in HgCl2-Induced Acute Renal Failure

Renal blood flow (RBF), outer cortical blood flow (OC-rbf) and inner cortical blood flow were determined by the microsphere method in water-drinking rats and chronic saline-drinking rats at 3, 12 and 24 h after injection of HgCl2, 4.7 mg/kh body weight. RFB and OC-rbf were decreased in both groups at 3 h post HgCl2 injection. Persistent reduction of OC-rbf was noted in water-drinking rats at 12 and 24 h post HgCl2 even though the total RBF returned to normal by 24 h. These parameters were normal in chronic saline-drinking rats. Despite normal RBF in water-drinking and saline-drinking rats, serum creatinines were still signigicantly elevated 24 h post HgCl2. Therefore, alterations in total renal perfusion do not entirely account for the decreased renal function that occurs under these circumstances.

In vitro uptake of gentamicin by rat renal cortical tissue.

The mechanism of gentamicin uptake in vitro by renal cortical slices of rat kidney was investigated. The cortical-slice-uptake ratio of gentamicin concentration in 1.0 g of tissue water to that of 1.0 ml of incubation medium (SW/M) was 1.44 ± 0.04. The uptake of gentamicin was inhibited by 2 × 10−5 M dinitrophenol (SW/M = 1.03 ± 0.04) and by anoxia (SW/M = 1.01 ± 0.04). The results indicate that aerobic phosphorylation is required to transport gentamicin into the cells. The uptake of p-aminohippurate and tetraethylammonium chloride by renal cortical slices was not affected by gentamicin.

Measurement of Renal Blood Flow in the Rat

A radioactive microsphere technique has been developed for the measurement of renal blood flow and intrarenal blood flow distribution in the dog (1, 2). Various techniques have been proposed (3-5) for the measurement of renal blood flow (RBF) in the rat in addition to the conventional PAH clearance method. This study presents a simple method for the assessment of RBF in rats using radioactive microspheres. Method. Eight male Sprague-Dawley rats weighing 200-240 g were fed with Purina Lab Chow and given tap water ad lib. All animals were lightly anesthetized with ether and cannulated with polyethylene tube No. 10 (i.d. 0.011 in., o.d. 0.024 in.) through the femoral artery for blood collection and through the carotid artery into the left ventricle for injection of microspheres. After awakening from anesthesia, animals were placed in restraining cages and allowed to recover for 45-60 min prior to injection. Radioactive microspheres, 15 μm ± 5 μm diameter, (3m Co., St. Paul, MN) were utilized to measure renal blood flow (RBF) and its intracortical distribution. Two different nuclides were used, 85Sr and 141Ce, for two separate measurements taken 1 hr apart. For each measurement, approximately 0.1 ml of 2 mg/ml concentration of microspheres in 10% dextran was injected through the carotid artery catheter within 5-7 sec. Just prior to injection, the spheres were sonicated for 5-10 min and then vigorously agitated with a vortex mixer for at least 3 min. Immediately upon injection of the spheres into the left ventricle, the femoral catheter was opened and blood was allowed to flow freely into a preweighed tube for exactly 1 min. Approximately 0.2 ml of blood was collected from the femoral artery in each minute. The adequacy of 1-min collection periods, vis à vis complete removal of microspheres from the circulation, was tested in a separate group of five animals.

Intrarenal Hemodynamics in Acute Myohemoglobinuric Renal Failure

Renal blood flow (RBF) and its distribution were measured in acute renal failure induced by glycerol injection to water-drinking rats and to rats chronically loaded with NaCl solution. Mean RBF and intracortical blood flow distribution of both water-drinking and saline-loaded rats at 24 h after glycerol injection were not different from those of control rats. Although chronic saline loading blunted the impairment of renal function caused by glycerol as evidenced by serum creatinine values, no differences in renal hemodynamics were noted. This suggests that changes in glomerular arteriolar resistance or glomerular permeability might be of greater importance in the reduction of glomerular filtration rate than are alterations of blood flow.

Renal Hemodynamics in Experimental Acute Renal Failure

Results from many laboratories, including our own, support the following view of the status of renal blood flow (RBF) in acute renal failure (ARF). During the initiation phase of virtually all forms of experimental ARF, RBF appears to be substantially decreased. The mechanisms for the decrease in RBF vary depending on the model employed. However, we have shown that changes in cardiac output are involved in both HgCl2 and glycerol models of ARF. The degree to which the decreased RBF contributes to the impaired glomerular filtration rate (GFR) characterizing the initial phase of ARF also depends on the particular model that is studied. In terms of the maintenance phase of ARF, out studies show that total RBF is essentially normal in both glycerol and HgCl2 models of ARF. A general consensus exists that RBF is not related to the decreased GFR in the maintenance phase of ARF, regardless of the model of ARF employed. Results from this laboratory suggest, however, that a hemodynamic mechanism may still contribute to the decreased filtration in ARF despite the dissociation between total RBF and GFR. This mechanism may involve an increase in preglomerular resistance, either alone or in association with a decrease in postglomerular resistance. An extensive amount of research has been performed on the renal circulation in ARF over the past two decades. It appears that this research has basically confirmed Ole Muncks impression of the role of renal blood flow in the pathophysiology of ARF.

Impaired distal nephron acidification in chronically phosphate depleted rats.

Renal tubular bicarbonate reabsorption and acidification were evaluated in phosphate depleted rats (PD) and controls. After 33 days of phosphate depletion, urine pH of PD rats (N=5, 6.36±0.15) was significantly higher than control (N=5, 5.64±0.09,P<0.005) following an NH4Cl load. Urinary titratable acid of PD rats (9.6±1.8) was significantly reduced compared to control (117.2±19.7 μEq/3 h,P<0.001), whereas NH4+ excretion was not different. The plasma HCO3− thresholds at which bicarbonaturia occurred (approximately 25 mEq/l) were identical in controls and phosphate depleted rats during isotonic bicarbonate infusion. The higher urine pH of phosphate depleted rats following NH4Cl administration was not due to low urinary phosphate as 3-day phosphate depleted rats could normally acidify urine after NH4Cl (pH=5.86±0.09,N=6 vs. control 5.87±0.08,N=6,P=N.S.) despite urinary phosphate excretion as low as in 33-day PD rats. These data indicate the presence of impaired distal tubular acidification in chronically phosphate depleted rats.

Systemic Hemodynamics in Nephrotoxic Acute Renal Failure

Cardiac output (CO) and renal blood flow (RBF) were simultaneously evaluated (microsphere method) in awake rats, 3, 6, and 24 h after induction of acute renal failure by mercuric chloride (HgCl2; 4.7 mg/kg body weight). 3 h after injection of HgCl2, CO and RBF decreased to 77 and 72% of respective control values of 32.0 +/- 2.4 and 4.65 +/- 0.44 ml/min/100 g. Renal vascular resistance (RVR) and total peripheral resistance (TPR) were significantly increased compared to control at this time. Similar results were observed 6 h after administration of HgCl2. Volume expansion with plasma (2% of body weight) restored CO, RBF, TPR, and RVR to normal 3 h after injection of HgCl2. Despite significantly elevated blood urea nitrogen 24h after injection of HgCl2 (103.7 mg%), all hemodynamic parameters were within control range. Plasma volume was normal 3 h after HgCl2 but was significantly elevated compared to control 24 h after HgCl2 (4.73 vs. 3.92 ml/100 g, p less than 0.01). These findings indicate that factors other than preferential renal vasoconstriction may be involved in the transient renal ischemia of HgCl2-induced acute renal failure.

Renal Tubular Sodium and Water Excretion in Antibiotic-Induced Nephrotoxicity

Clearance techniques were used to evaluate renal tubular sodium and water excretion in 4 patients with antibiotic-induced acute renal failure (ARF). Creatinine clearances and maximal urine flow rates of patients with ARF (22.6 and 5.23 ml/min, respectively) were significantly lower than control values during hypotonic volume expansion (125.5 and 13.71 ml/min, respectively, both p less than 0.01). During the period of maximal hydration, fractional sodium excretion (CNa/Ccr) and maximal urine osmolality (11.4% and 171 mosm/kg H2O, respectively) were increased compared to controls (1.04% and 53 mosm/kg H2O, respectively, both p less than 0.05). The increased CNa/Ccr observed in patients with ARF was consistent with reduced proximal sodium reabsorption as reflected by increased (CH2O + CNa)/Ccr and reduced fractional distal sodium reabsorption as indicated by decreased CH2O/(CH2O + CNa). The reduction in proximal and distal sodium reabsorption cannot be explained on the basis of an osmotic effect of urea as fractional clearances of BUN (CBUN/Ccr) were similar in patients with ARF and controls.

The Role of Tubular Necrosis in the Pathophysiology of Acute Renal Failure

Renal tubular function was evaluated in vitro by kidney slice uptakes of p-aminohippurate (PAH) and tetraethylammonium (TEA) at 24 and 48 h in water-drinking rats and at 24 h in chronic saline-loaded rats after induction of acute tubular necrosis (ATN) by HgCl2 and glycerol injection. Significant correlations between decrease tubular uptake of PAH and TEA and elevated serum creatinine levels were noted in both models of ATN in water- and saline-drinking rats. However, with the same degree of impairment of PAH and TEA uptakes the creatinine was significantly lower in saline-loaded rats than in water-drinking rats in both forms of ATN. The correlation between impaired tubular function and elevated creatinine suggests that tubular damage and glomerular filtration reduction might be pathophysiologically related in ATN.