Effect of furosemide on local and zonal glomerular filtration rate in the rat kidney.

Furosemide has been reported to produce disproportional changes in blood flow in cortical zones and to inhibit tubuloglomerular feedback (TGF), suggesting that furosemide might alter the intracortical distribution of glomerular filtrate. We have tested this hypothesis by a new method for measuring local and total glomerular filtration rate (GFR) based on proximal tubular accumulation of the basic polypeptide aprotinin (mol wt 6513). Local GFR was calculated in tissue samples dissected from outer cortex (OC), inner cortex (IC) and the corticomedullary border zone (CM) from the plasma clearances of two aprotinin tracers injected i.v. before and after a 3 min i.v. infusion of 25 mg kg-1 furosemide. The mean of five samples from each region was used to determine zonal GFR. Isotonic saline was infused at a rate corresponding to urine flow. Furosemide reduced whole kidney GFR from 1.17 to 1.00 mL min-1 and gave a similar reduction of renal artery blood flow. Urine flow increased from 0.6 to 17% of GFR. Haematocrit (approximately 0.48) and plasma protein concentration (approximately 55 mg mL-1) were maintained while the arterial blood pressure tended to decline (118 +/- 5 mmHg to 108 +/- 6 mmHg, P < 0.05). GFR in OC, IC and CM (1.58, 1.18, 0.42 mL min-1 g-1) fell to 87, 88 and 88% of control after furosemide infusion respectively. The furosemide/control ratio for each sample showed a coefficient of variation of about 3%. We conclude that furosemide produced a modest GFR reduction that was uniform throughout the renal cortex. The homogenous GFR response suggests a similar TGF constriction tone in preglomerular vessels of deep and superficial nephrons.

Glomerular filtration and tubular absorption of the basic polypeptide aprotinin.

The basic polypeptide aprotinin (Ap), mol. wt 6500, pI 10.5, is filtered in the glomeruli, virtually completely taken up by the proximal tubular cells and retained there for many hours. This process was studied in rats by determining the renal plasma clearance (CAp) as the amount of [125I]Ap accumulated in the kidney plus that excreted in the urine per unit of time divided by the integrated plasma concentration. In periods lasting 4-20 min after i.v. bolus injection or infusion to constant plasma concentration, CAp was 65% of glomerular filtration rate (GFR) estimated as kidney plus urinary clearance of [51Cr]EDTA (Ccr-EDTA). Less than 0.8% of the filtered Ap appeared in the urine. CAp varied inversely with plasma protein concentration in mg ml-1: CAp/Ccr-EDTA = 0.98-0.0058 x Ppr, corresponding to a glomerular Gibbs-Donnan distribution for a net molecular charge of +6, in agreement with the amino acid composition of Ap. CAp (kidney + urinary) was not altered by inhibiting tubular uptake of [125I]Ap by maleate or by exceeding the uptake capacity with large doses of unlabelled Ap. Neutralized Ap (malonylated) did not accumulate in the kidney, but showed a urinary clearance indistinguishable from that of [51Cr]EDTA. Both CAp and Ccr-EDTA were reduced to 0.04 ml min-1 when glomerular filtration pressure was lowered by ureteral stasis and increased Ppr (80-90 mg ml-1). These findings indicate: (1) no steric or charge restriction to filtration of Ap in the glomerular membrane, (2) the Gibbs-Donnan equilibrium should be considered when estimating glomerular sieving of charged polypeptides in intact animals (3) charge dependent tubular uptake, (4) little or no transtubular transport of intact Ap, (5) no appreciable tubular uptake of Ap from the peritubular side and (6) local renal accumulation of Ap in a period of up to 20 min may be used to estimate local glomerular filtration and/or local proximal tubular reabsorption rates. Model analysis based on the appearance of 125I in plasma, the time course of renal Ap content, and literature data on subcellular Ap distribution are consistent with two populations of endosomes, transporting Ap at widely different rates from the proximal tubular brush border to the lysosomes where breakdown occurs at a high rate.

Repeatable measurement of local and zonal GFR in the rat kidney with aprotinin.

The basic polypeptide aprotinin (Ap), mol. wt 6513, is freely filtered in glomeruli and completely reabsorbed by the proximal tubules. Cellular processing is slow with return to plasma of breakdown products beginning after 20-30 min. When corrected for Gibbs-Donnan distribution of Ap between glomerular filtrate and plasma (i.e. 0.65 at a plasma protein concentration of 50 mg ml-1), the renal clearance of [125I]Ap, estimated as the ratio of kidney uptake and integrated non-protein bound plasma 125I concentration, equals that of [51Cr]EDTA (urine + kidney content). Zonal GFR per gram tissue was obtained from uptake in three to six samples from outer and inner cortex (OC, IC) and the cortico-medullary border zone, 5-30 min after i.v. injection in rats. Control GFR in OC was 2.05 (SD 0.39) ml g-1 min-1 and the IC/OC ratio 0.66 (SD 0.14). Repeated local clearances (CI and CII) were obtained by injecting a second tracer (i.e. [131I]Ap) 15 min after the first injection (i.e. [125I]Ap), which by then had a low plasma concentration. The kidneys were removed at 30 min, frozen and dissected. During control conditions CII/CI averaged 1.06 in OC and IC, and the coefficient of variation (CV) between CII/CI ratios of individual tissue samples was 2% in both zones. Lowering left renal arterial pressure before the second injection reduced GFR proportionally in both zones (34 and 37%) with a CV of intersample CII/CI ratios of 5%. We conclude that the method allows precise and repeatable measurements of local and zonal GFR.

Comparison of depressant actions of orphenadrine and diazepam on hypertonic skeletal muscle activity.

The muscle relaxant activities of orphenadrine (Norflex) and diazepam (Valium) were compared in several animal models. In mice, tonic extensor seizures evoked by electroshock or pentylenetetrazol were inhibited by both agents. The protective index (ataxic dose divided by protective dose) was greater than 1 for orphenadrine, whereas for diazepam it was greater than 1 only in the case of the pentylenetetrazol-induced seizures. In strychnine-treated mice, diazepam protected against deaths following tonic extensor seizures, but orphenadrine did not. The protective index for diazepam, however, was less than 1. In cats, orphenadrine and diazepam both were capable of blocking decerebrate ridigity, but not in all animals. The protective index for orphenadrine in animals in which it was active, was greater than 1, while that for diazepam, when it was active, was less than 1. Orphenadrine and diazepam were tested in rabbits in the sciatic nerve-gastrocnemius muscle preparation. Neither drug directly affected nerve muscle stimulation. It would appear that both agents are acting via the central nervous system when they suppress hypertonic skeletal muscle activity, but only orphenadrine exhibits protective indices consistently greater than 1.

Interaction of cyclosporine and indomethacin in the rat.

The renal toxicity of cyclosporine has been reported to be due, at least in part, to a decrease in renal blood flow. Inasmuch as nonsteroidal antiinflammatory compounds also decrease renal blood flow, cyclosporine and such an agent (indomethacin) were given together to determine if there was an interaction in the kidney. Rats, 250-300 g, were divided into 4 groups: control, indomethacin (5 mg/kg), cyclosporine (25 mg/kg) and cyclosporine + indomethacin. At weekly intervals, renal excretion of creatinine/24 hrs and N-acetyl-beta-D-glucosaminidase activity in urine (NAG) were determined as well as body weights. At one week, values for creatinine, NAG and weight did not differ between the groups. At two weeks, the animals in the cyclosporine + indomethacin group showed decreased creatinine excretion and increased NAG excretion. At three weeks, 90% of the animals in the cyclosporine + indomethacin group were dead. Thus, when indomethacin and cyclosporine are given together, they interact to produce an increase in renal toxicity and lethality.

Interaction of furosemide and phenytoin in the rat.

The interaction of phenytoin and furosemide was examined in rats. After 2 or more weeks of pretreatment with phenytoin, the diuretic activity of furosemide given orally was decreased. This was due primarily to a generalized decrease in absorption from the gastrointestinal tract. Following intravenous administration of furosemide to phenytoin pretreated rats, a decrease in diuretic activity also occurred. Excretion of furosemide was not altered. Thus, phenytoin pretreatment would also appear to interfere directly with the renal action of furosemide.

Acute effects of high ceiling diuretics on pancreatic blood flow and function.

The possibility that a decrease in extracellular volume, induced by diuretics, would cause a decrease in pancreatic blood flow which, in turn, might compromise pancreatic function was examined. Employing fasted anesthetized mongrel dogs, the acute effects of furosemide, a typical high ceiling diuretic, on pancreatic blood flow and plasma levels of insulin and glucose were examined. Furosemide was found to induce a decline in pancreatic blood flow which was similar in all regions of the pancreas and the decrease was antagonized when extracellular volume depletion was prevented by infusing saline at the same rate as urine flow. The decrease in blood flow was significantly correlated with cumulative volume loss. Plasma levels of insulin and glucose were, however, not significantly altered during the studies. To increase the likelihood of determining significant decreases in plasma levels of insulin, acute studies were repeated in dogs in which plasma levels of insulin were increased by a continuous infusion of glucose. Both furosemide and the structurally unrelated high ceiling diuretic, ethacrynic acid, caused a decrease in pancreatic blood flow which was similar in all regions of the pancreas. The cumulative volume loss observed with administration of either furosemide or ethacrynic acid was significantly correlated with the level of pancreatic blood flow observed. Plasma levels of insulin and glucose were not significantly altered. It can be concluded that high ceiling diuretic drugs such as furosemide and ethacrynic acid do produce a loss in volume which is correlated with a decrease in pancreatic blood flow, but decreases in pancreatic blood flow alone do not appear to be sufficient to produce overt changes in pancreatic function in acute studies.

Effect of furosemide on canine inferior mesenteric blood flow.

The administration of diuretics to patients receiving cardiac glycosides has been reported to be a contributing factor in the incidence of nonocclusive mesenteric infarction. Presumably, the loss of fluid induced by the diuretics results in a decrease in blood flow to the intestines. This, combined with the vasoconstrictor action of the glycosides, can result in too great a decrease in blood flow and hence the development of an infarct. The ability of diuretics to alter intestinal circulation is supported by animal studies which have shown that blood flow in the superior mesenteric artery is decreased after the administration of furosemide. Since intestinal blood flow is also, in part, supplied by the inferior mesenteric artery, it was of interest to determine the effect of the administration of furosemide on blood flow in this vessel. Following intravenous administration of furosemide (2 mg/kg), inferior mesenteric flow was found to be decreased significantly at 10 minutes and it continued to decline until approximately 40 minutes after administration of the diuretic. Blood pressure was elevated slightly (less than 10 mm Hg) for the first 30 minutes after the administration of furosemide. It would seem likely that the reduction in inferior mesenteric blood flow following administration of furosemide is probably caused by the same factors which mediate the furosemide-induced decrease in superior mesenteric blood flow.

The role of volume depletion, antidiuretic hormone and angiotensin II in the furosemide-induced decrease in mesenteric conductance in the dog.

Furosemide caused a significant reduction in mesenteric blood flow and conductance as early as 10 min after administration. When fluid losses were not replaced, conductance continued to decline. In volume-repleted animals, conductance fell initially but failed to decrease further. Thus, furosemide decreases mesenteric conductance in two ways: a small early decrease which is not related to volume loss and a later more marked decrease which is related to volume loss. The initial decrease in conductance seen in furosemide-treated animals appears to be mediated via the renin-angiotensin system. In volume-repleted as well as volume-depleted animals, the plasma concentrations of renin and angiotensin II, but not antidiuretic hormone, were increased 10 min after furosemide administration. Also, inhibitors of the renin-angiotensin system abolished the response. The later decrease in mesenteric conductance induced by furosemide is more complex. When fluid losses were not replaced, plasma levels of angiotensin II and renin, as well as antidiuretic hormone, were increased 40 min after furosemide administration. Neither an infusion of Sar1-Ile8-angiotensin II nor hypophysectomy, alone, prevented the furosemide-induced decrease in conductance. The decrease in conductance was reversed when Sar1-Ile8-angiotensin II was infused into hypophysectomized dogs. Thus, the later more marked decrease in conductance induced by furosemide is related to three factors: volume loss, plasma concentration of angiotensin II and plasma concentration of antidiuretic hormone. Mesenteric conductance is decreased by furosemide if plasma concentrations of one or both vasoactive factor are elevated in the presence of a decrease in extracellular volume.

Renal actions of oxyphenbutazone.

Oxyphenbutazone decreased the renal excretion of sodium and water in anesthetized dogs. As these excretions decreased, the drug also produced a decrease in renal blood flow and in the glomerular filtration rate. Blood pressure increased slightly. These changes are consistent with an inhibition of renal prostaglandin synthesis and could explain why oxyphenbutazone is reported to produce weight gain and edema when used clinically.

Effect of furosemide on peripheral venous compliance following ureteral ligation in the adult dog.

The effect of furosemide on peripheral venous compliance in the absence of the diuretic effect of the drug was examined. Following bilateral ligation of the ureters, adult dogs were given furosemide, 2 mg/kg. A significant increase in peripheral venous compliance was observed as early as ten minutes after the drug was given (120% of control), and an additional increase was seen at 60 minutes after administration of the drug (136% of control). No change in peripheral venous compliance was observed in animals receiving only the furosemide vehicle, nor was there any change in animals in which bilateral nephrectomy had been performed prior to the administration of furosemide. These results indicate that furosemide produces a significant increase in peripheral venous compliance, which is independent of its diuretic action. However, the presence of the kidneys appears to be necessary for this effect.

Effect of indomethacin and meclofenamate on canine mesenteric and celiac blood flow.

Two nonsteriodal antiinflammatory agents, indomethacin and meclofenamate, were found to produce a marked decrease in mesenteric blood flow, an increase in blood presure and no significant change in celiac blood flow. The initial effect of indomethacin on mesenteric blood flow differed from that seen with meclofenamate. Whereas meclofenamate induced a fall in mesenteric blood flow that was gradual in onset, indomethacin induced a marked vasoconstriction of the mesenteric bed that was immediate in onset and of short duration, which was followed by a decrease of slower onset similar to that seen with meclofenamate.

Decrease in hepatic blood flow during furosemide-induced diuresis.

The effect of furosemide on hepatic hemodynamics was investigated using electromagnetic flow probes positioned around the hepatic-portal vein and common hepatic artery of anesthetized dogs. Furosemide administration induced a vigorous diuresis and concomitantly decreased total hepatic blood flow. Hepatic arterial blood remained relatively stable. Thus, the decrease in total hepatic blood flow was due primarily to a decrease in portal blood flow. When furosemide was given to animals with ureters ligated to prevent extracellular volume contraction, the drug did not reduce total hepatic blood flow, portal blood flow or hepatic arterial blood flow. Thus, furosemide induces a decrease in total hepatic blood flow that appears to involve a mechanism dependent upon the volume contraction produced by the diuretic. The contraction causes a reduction in venous blood flow but not arterial blood flow to the liver.

Effect of furosemide on canine splenic arterial blood flow.

Furosemide has been reported to cause pathological changes in the intestines as a consequence of the decrease in splanchnic blood flow that it produces. In view of the pancreatic toxicities of this diuretic, it was of interest to examine the effect of the agent on blood flow to this organ. An electromagnetic flow probe, was placed around the splenic artery, a vessel which supplies a major fraction of pancreatic blood flow. Within 60 min after furosemide (1 mg/kg, iv) administration, splenic blood flow (SBF) decreased by 20% and the decrease paralleled the drug-induced diuresis. When the diuresis was prevented, the agent failed to affect SBF. Thus, furosemide causes a reduction in SBF, which appears to involve a mechanism dependent upon the volume reduction produced by the drug-induced diuresis.