Renal handling of low molecular weight proteins

The renal filtration, absorption and final disposal of lysozyme (lysozyme—mol wt 14,000), insulin and growth hormone were studied to gain a better quantitative understanding of the fundamental variables involved in the renal handling of low molecular weight proteins. The glomerular barrier offers little hindrance to the filtration of lysozyme, the glomerular sieving coefficient being 0.8 ± 0.1 (SD). The intrarenal route by which injected lysozyme accumulates in the kidney is via filtration and subsequent absorption (uptake) by renal tubular cells. Uptake or adsorption from the peritubular side is negligible compared to luminal uptake. Renal clearance and renal titration experiments in the intact dog and in the isolated perfused rat kidney showed that the lysozyme absorption process can be best characterized as a high capacity, low affinity transport system which is directly or indirectly dependent on energy input. The final disposal of absorbed ¹²⁵I-lysozyme, ¹²⁵I-insulin and ¹²⁵I-growth hormone was studied in the isolated perfused rat kidney by measuring the radioactivity released from the kidney to the perfusate and analyzing the nature of the released product by gel chromatography. The rate of release of radioactivity as well as its nature was dependent on the molecular species of the absorbed protein. The rate of release was higher for ¹²⁵I-insulin and ¹²⁵I-growth hormone and lower for ¹²⁵I-lysozyme. Lysozyme absorbed from the luminal side was released to the perfusate both as intact protein molecules and as catabolic products, whereas absorbed ¹²⁵I-insulin was almost entirely released to the perfusate as catabolic products. It is concluded that low molecular weight proteins are extensively filtered by the kidney, absorbed from the luminal side by renal tubular cells and released back to the circulation either as intact molecules or as catabolic products (amino acids and polypeptides). This process contributes in an important way to the plasma turnover of low molecular weight proteins including peptides and protein hormones.     

Inhibition by diltiazem of pressure-induced afferent vasoconstriction in the isolated perfused rat kidney

The renal hemodynamic response to calcium entry blockade depends on the neural, hormonal and physiologic determinants influencing basal renal vascular tone. The effects of perfusion pressure per se on the renal vascular response of the rat kidney to diltiazem were evaluated using normal kidneys and hydronephrotic kidneys perfused extracorporally. In isolated perfused normal kidneys, diltiazem did not alter perfusate flow or glomerular filtration rate (GFR) when administered at a perfusion pressure of 100 mm Hg. In contrast, when diltiazem was administered at a perfusion pressure of 150 mm Hg, the calcium antagonists caused a striking increase in GFR, which was accompanied by an increase in renal perfusate flow. In the isolated perfused hydronephrotic rat kidney, elevation of perfusion pressure was associated with an increase in renal vascular resistance and a reduction in afferent arteriolar diameter. Diltiazem abolished the pressure-induced constriction of afferent arterioles and caused an increase in renal perfusate flow in hydronephrotic kidneys perfused at pressures above 100 mm Hg. These findings suggest that in the setting of increased renal perfusion pressure, diltiazem's effects on GFR are mediated in part by an inhibition of pressure-induced constriction of the afferent arteriole.     

Apolipoprotein AI and AII metabolism in patients with primary high-density lipoprotein deficiency associated with familial hypertriglyceridemia

Plasma high-density lipoproteins (HDL) and their major proteins--apolipoprotein (apo) AI and apo AII--are subnormal in most patients with familial hypertriglyceridemia. However, the pathophysiology of low-plasma apo AI and apo AII is unclear. The kinetic parameters (turnover) of HDL apo AI and apo AII were studied in six lean patients with primary HDL deficiency associated with familial hypertriglyceridemia and five normolipidemic controls. Autologous 125I labeled HDL were injected intravenously (IV; 25 microCi) and blood samples drawn ten minutes after the injection and periodically thereafter for 12 days. Urine samples were collected daily and their radioactivity measured. Kinetic parameters were calculated from the area under the decay curve using three exponentials. Mean plasma apo AI and apo AII were significantly lower (P less than 0.001) in patients than normals (70.4 +/- 2.7 v 106.9 +/- 7.0; 24.2 +/- 1.6 v 39.2 +/- 0.9 mg/dL, respectively). The mean fractional catabolic rates (FCR) obtained from plasma 125I-HDL, apo AI, apo AII radioactivity decay curves and by Berson and Yalow's method (urine/plasma radioactivity ratios) were significantly greater (P less than 0.05) in patients than in controls (0.387 v 0.299; 0.391 v 0.309; 0.361 v 0.275; 0.272 v 0.207/d; respectively). The mean synthetic rates (SR) of apo AI and apo AII were significantly lower in patients than in controls (11.12 v 14.17 mg/kg body weight/d, P less than 0.05; 3.53 v 4.68 mg/kg body weight/d, P less than 0.05, respectively). In vitro lipolysis of triglyceride (TG) rich lipoproteins by bovine lipoprotein lipase, and measurement of hepatic TG lipase and lipoprotein lipase in postheparin plasma were similar in patients and controls, indicating no abnormality in these factors that are linked to HDL and TG catabolism. However, a significant positive correlation between hepatic TG lipase and the FCR of apo AI and apo AII was found. The data suggest that in this series of patients with HDL deficiency the low plasma HDL-cholesterol, apo AI, and apo AII levels resulted from decreased synthesis and an increased fractional catabolic rate of apo AI and apo AII, the major proteins of HDL.     

Furosemide augments the effects of captopril on nuclear studies in renovascular stenosis

Captopril facilitates detection of unilateral renovascular hypertension by selectively reducing glomerular filtration rate in affected kidneys. To determine if volume depletion augments this response, we compared the effects of captopril, furosemide, and combined furosemide plus captopril on individual kidney computer-derived clearances of 99mTc-diethylenetriamine pentaacetic acid (DTPA) and [131I]o-iodohippurate in two-kidney, one clip Goldblatt hypertensive rats and normal controls. In clipped kidneys, captopril reduced DTPA clearance significantly from baseline (from 0.31 +/- 0.02 to 0.19 +/- 0.04 ml/min/100 g; p less than 0.02) whereas furosemide alone had no effect (0.28 +/- 0.03 ml/min/100 g). Combined furosemide plus captopril further reduced clipped kidney DTPA clearance to a level significantly less than captopril alone (0.10 +/- 0.02 ml/min/100 g; p less than 0.02). Clipped kidney o-iodohippurate clearance was not changed from baseline by any treatment. In contralateral unclipped and normal kidneys, DTPA clearance did not decline from baseline following either captopril or furosemide plus captopril treatment. Since the dose of captopril used (3 mg/kg by intraperitoneal injection) did not reduce systolic blood pressure of hypertensive rats significantly, these changes probably reflect intrarenal rather than systemic hemodynamic effects of converting enzyme inhibition and are consistent with the hypothesis that captopril interferes with glomerular filtration in stenotic kidneys by reducing efferent arteriolar vascular resistance. Prior volume depletion accentuates the effect of captopril on stenotic kidney glomerular filtration rate, providing improved functional discrimination of stenotic kidneys from contralateral unclipped and normal kidneys. These results indicate that furosemide-induced volume depletion may increase the diagnostic sensitivity of captopril-enhanced 99mTc-DTPA renography in the detection of unilateral renovascular hypertension.     

Effect of nitric oxide inhibition on kidney function in experimental renovascular hypertension

We examined the effect of acute systemic blockade of nitric oxide (NO) synthesis on blood pressure and renal function in rats with angiotensin II dependent two-kidney, one-clip Goldblatt hypertension. Hypertensive animals had significantly higher blood pressures, plasma NO metabolite concentrations and urinary NO metabolite excretion rates than control rats. Intravenous administration of N(G)-nitro-L-arginine methylester (L-NAME) (10 mg/kg) increased mean arterial pressure in both hypertensive and control animals with the magnitude of increase being greater in hypertensive than control rats (32 +/- 3 vs. 20 +/- 2 mmHg, p < 0.05). L-NAME did not affect glomerular filtration rates of normal and clipped kidneys but significantly decreased non-clipped kidney glomerular filtration rate (1.1 +/- 0.1 vs. 0.7 +/- 0.1 ml/min per g kidney wt, p < 0.05). Blood flow to normal and non-clipped kidneys fell in response to L-NAME. Percent reduction in renal blood flow produced by L-NAME was significantly greater in non-clipped than normal kidneys (38 +/- 3 vs. 24 +/- 2%, p < 0.05). In contrast, clipped kidney blood flow increased after L-NAME (3.3 +/- 0.2 vs. 4.0 +/- 0.2 ml/min per g kidney wt, p < 0.05). An identical improvement in clipped kidney blood flow occurred when arterial pressure was raised with aortic constriction indicating that the systemic pressor effect of L-NAME was responsible for this finding. These results indicate that NO plays an important role in systemic and non-clipped kidney hemodynamics in renovascular hypertension. Because NO has little influence on stenotic kidney function, the stimulus for increased NO system activity in this disease appears to be vascular shear stress rather than elevated circulating or intrarenal angiotensin II concentrations.     

Parathyroid hormone and calcium: interactions in the control of renin secretion in the isolated, nonfiltering rat kidney.

The present study was designed to characterize the interaction of calcium and PTH in the control of renin release in isolated rat kidneys perfused in a closed circuit at constant flow. Kidneys were rendered nonfiltering using low perfusion pressures (70 mm Hg) and a hyperoncotic perfusate (100 g/liter BSA). Under these conditions, differences in perfusion pressure were less than 9 mm Hg between control and PTH-treated kidneys over the 50 min of perfusion. In the absence of PTH, renin release was inversely correlated with ionized calcium (Ca2+) concentration, with the highest release of renin noted with 1 mM EGTA and no added calcium. Also, verapamil treatment markedly elevated renin release, even in the presence of 2 mM Ca2+. In contrast, renin secretion was strongly depressed by 20 nM BAY-K8644 in the perfusate. In medium containing normal calcium concentrations (1 mM Ca2+), rat PTH(1-34) induced a 2-fold greater renin accumulation than in the control, non-PTH-treated kidneys. Isoproterenol induced a 5-fold stimulation under the same conditions. In the 0 Ca2+/1 mM EGTA perfusion, PTH did not elevate renin secretion. Renin release in response to PTH in 2 mM Ca2+ was similar to that observed in the 1 mM Ca2+ perfusion. PTH also reversed the effects of BAY-K8644 to suppress renin release. In verapamil-treated kidneys, PTH failed to stimulate renin release. These results indicate that PTH stimulates renin release by a process independent of the baroreceptors and macula densa. The Ca2+ modulation of PTH-induced renin release is consistent with the reported ability of PTH to block calcium channels and relax vascular smooth muscle.     

Triiodothyronine production by the perfused rat kidney is reduced by diabetes mellitus but not by fasting

The effects of fasting and streptozotocin-induced diabetes on renal T3 production were studied in the isolated perfused rat kidney. Kidneys were perfused for 1 h at 37 C and pH 7.4 with two perfusion media, containing different oncotic agents (BSA and a modified collagen product, Haemaccel) and with widely varying free T4 concentrations [15.6 ng/dl (200 pM) in BSA, 132 ng/dl (1703 pM) in Haemaccel]. Basal kidney T3 concentrations fell by 65.3 +/- (SE) 5.2 and 56.4 +/- 3.4% of control in fasted and diabetic rats, respectively, in parallel with similar decreases in serum T3 concentrations. Fasting did not alter T3 production, T4 uptake, or the conversion of T4 to T3 in the perfused kidney. In contrast, diabetes decreased renal T3 production by 43.7 +/- 5.4% (P less than 0.001) and 31.2 +/- 3.5% (P less than 0.005) from control when perfused with BSA and Haemaccel, respectively. This decline in T3 production was the result of an insulin-reversible decrease in the conversion of T4 to T3, whereas T4 uptake was unchanged. Kidneys from diabetic rats retained more of the T3 produced than did kidneys from control rats, and this was also reversed with insulin treatment. These studies demonstrate significant differences in the metabolism of T4 and disposition of the T3 generated in the kidneys of fasting and diabetic rats. In both fasting and diabetes, T4 metabolism in the kidney was qualitatively and quantitatively different than that previously observed in the perfused liver. We postulate that decreased sensitivity or exposure to intraportal hormones and/or metabolites may explain these differences.     

Heterogeneous acinar localization of the asialoglycoprotein internalization system in rat hepatocytes

Desialylated glycoprotein is rapidly cleared from plasma by a receptor-mediated endocytic mechanism located on hepatocytes. We studied the hepatic acinar distribution of this asialoglycoprotein transport system with the ligand 125I-asialoorosomucoid using rat liver perfused in either antegrade or retrograde direction in combination with quantitative light microscopic autoradiography. Grain distribution along the acinus appeared dependent on the perfusion direction. A rather shallow zone 1 to zone 3 gradient was observed if livers were perfused in the normal direction. However, a statistically significantly steeper zone 3 to zone 1 gradient was detected in retrograde perfusions. Kinetic analysis of perfusate clearance profiles yielded a hepatic clearance of 21.6 +/- 1.3 ml per min in antegradely perfused liver. Hepatic extraction was calculated to be 60.1 +/- 7.4%. Biliary secretion of radioactivity amounted to 1.89 +/- 0.18% of the dose within 1 hr after injection and consisted of intact material (1.39 +/- 0.25%) and radioactive low-molecular-weight degradation products (0.52 +/- 0.08%), of which more than 90% could be accounted for by 125I-. Apart from a minor difference regarding biliary secretion of an unidentified glycopeptide (less than 0.1% of the injected dose), transport data for the retrogradely perfused livers were identical to those obtained with livers perfused in antegrade direction, emphasizing the functional equivalence of both groups of livers. The autoradiographic data indicate that zone 3 hepatocytes take up 125I-asialoorosomucoid more avidly than zone 1 cells. The kinetic and biochemical data indicate that further processing in the hepatocytes is virtually similar in the two zones.(ABSTRACT TRUNCATED AT 250 WORDS)     

Impaired renal mevalonate metabolism in nephrotic syndrome: A stimulus for increased hepatic cholesterogenesis independent of GFR and hypoalbuminemia

Increased hepatic lipogenesis in the nephrotic syndrome is not adequately explained by hypoalbuminemia. In this disorder an enhanced delivery of the cholesterol precursor mevalonic acid (MVA) to the liver may be an unidentified stimulus to cholesterogenesis. Since the kidneys are the major site of mevalonate excretion and metabolism by either the sterol or nonsterol shunt pathways, an impairment of any of these metabolic alternatives could result in redistribution of mevalonate to the liver. Male Sprague-Dawley rats rendered nephrotic by puromycin aminonucleoside had their kidneys perfused with Krebs-Henseleit-bicarbonate buffer containing albumin, glucose and 5-¹⁴C-MVA. The number five carbon label was utilized so that any ¹⁴CO₂ produced would represent mevalonate shunt pathway activity. The isolated perfused kidney was used to eliminate confounding variables. In eight control kidneys perfused for 2 hr 62 ± 2% of the MVA was removed from the perfusate compared to 50 ± 2% in five nephrotic kidneys (p < .006). Urinary MVA recovery was 22 ± 2% in controls, 15 ± 1% in nephrotics (p < .05). The incorportion of ¹⁴C into renal tissue lipids was not different in the two groups. Recovery of ¹⁴CO₂ was two times greater in controls than in nephrotics (p < .006). Inulin clearance per gram of kidney and sodium reabsorption were similar for the two groups. Isolated perfused kidneys from nephrotic rats metabolize MVA abnormally such that less is excreted, less is oxidized, and more is available for recirculation to the liver. This occurs independently of hypoalbuminemia, a change in glomerular filtration rate, or an overt histo-pathologic lesion. These events create an environment for increased hepatic cholesterol synthesis.     

Renal structural properties in prehypertensive Dahl salt-sensitive rats

In 10- to 12-week-old Dahl salt-sensitive (DS) and salt-resistant (DR) rats fed a 0.3% salt diet (n=10 in each group), flow-pressure and pressure-glomerular filtration rate (F-P and P-GFR, respectively) relationships were established for maximally vasodilated perfused kidneys. From these relationships, 3 indices of vascular structural properties were estimated: slope of F-P (minimal renal vascular resistance reflecting overall luminal dimensions of preglomerular and postglomerular vasculature), slope of P-GFR (glomerular filtration capability against pressure), and threshold pressure for beginning filtration at P-GFR (preglomerular-to-postglomerular vascular resistance ratio). Thereafter, maximal renal vascular resistance was determined to assess wall-to-lumen ratios of the resistance vessels in half of each group. In the remainder, the kidneys were perfusion-fixed for histological analysis. Mean arterial pressure did not differ between the DS and DR rats. There were no significant differences in the slopes of F-P between the 2 groups. In contrast, the slope of P-GFR was significantly lower (33%) in DS rats than in DR rats, although the DS kidneys began filtering at a threshold pressure similar to that of the DR kidneys. Thus, in DS rats, there were no abnormalities in luminal dimensions at preglomerular and postglomerular vascular segments, but the kidney filtration capacity decreased at any given increase in pressure. Maximal vascular resistance was greater in DS than in DR rats, a finding compatible with the histological appearance, which showed vascular hypertrophy with little, if any, vascular narrowing in the interlobular arteries of DS rats. In conclusion, hypertrophic remodeling without vascular narrowing at preglomerular resistance vessels and structural defects in filtering at the glomeruli could occur in prehypertensive DS rats.     

Hemodynamic effects of platelet activating factor in the dog kidney in vivo

The effect of platelet activating factor (PAF) on renal hemodynamics and function was examined in anesthetized dogs. The infusion of PAF into the renal artery at 5, 10, and 20 ng X min-1 X kg-1 body weight resulted in dose-dependent reductions in renal blood flow, glomerular filtration rate, urine volume, and urinary sodium excretion, whereas the infusion of vehicle alone in the contralateral kidney did not result in significant changes in these parameters. The maximum decrease expressed as the percent change from baseline was 22.2 +/- 1.7% for renal blood flow, 50.8 +/- 11% for glomerular filtration rate, 67.3 +/- 4.2% for urine volume, and 69.0 +/- 8.5% for urinary sodium excretion, respectively. These renal effects were not accompanied by significant alterations in systemic arterial blood pressure and heart rate. Pretreatment with indomethacin to block prostaglandin synthesis enhanced the effect of PAF on kidney function. Our data demonstrate that, unlike the rat kidney, intrarenal PAF infusion into the intact dog results in vasoconstriction and serve reduction in glomerular filtration rate.     

Effects of amlodipine on renal hemodynamics

Recently, attention has focused on the effects of calcium antagonists on renal function. When administered in vitro to the isolated perfused kidney, calcium antagonists exhibit consistent actions permitting characterization of their renal effects. Calcium antagonists do not affect the vasodilated isolated perfused kidney, but they do dramatically alter the response of the kidney to vasoconstrictor agents. This study examined the effects of the novel dihydropyridine amlodipine on the hemodynamic response of the isolated perfused kidney to angiotensin II. Amlodipine completely reversed the angiotensin II-induced decrement in glomerular filtration rate of this model (0.72 +/- 0.15, 0.26 +/- 0.10 and 0.73 +/- 0.12 ml/min/g for control, angiotensin II and angiotensin II plus 0.1 microM amlodipine respectively). In contrast, amlodipine only partially restored renal perfusate flow (35.8 +/- 2.7, 14.7 +/- 1.9 and 23.7 +/- 2.5 ml/min/g for control, angiotensin II and angiotensin II plus amlodipine), thereby increasing filtration fraction. These findings are consistent with previous observations from this laboratory indicating that dihydropyridines predominantly vasodilate preglomerular renal resistance vessels and through this mechanism exert a preferential augmentation of glomerular filtration rate.     

Control of renin secretion in the dog. Effects of furosemide on the vascular and macula densa receptors.

Experiments were undertaken to investigate further the effect of furosemide on renin secretion in the anesthetized dog. To separate the effects of the macula densa and the baroreceptor mechanisms, experiments were conducted in kidneys made nonfiltering by combining 2.5 hours of renal ischemia with ureteral ligation. Furosemide, in a dose of 5 mg/kg, increased renin secretion and decreased renal resistance in dogs with a nonfiltering kidney. Prior dilation of the nonfiltering kidney with either acetylcholine or papaverine prevented changes in both resistance and renin secretion. However, following dilation of the intact filtering kidney with acetylcholine, furosemide caused an increase in renin secretion. Infusion of d,l-propranolol decreased renin secretion in both the filtering and the nonfiltering kidneys. Following propranolol treatment, furosemide increased renin secretion in the filtering kidney but had no effect on renal resistance. These experiments indicate that furosemide stimulates renin secretion by both the macula densa and the baroreceptor mechanisms. The data suggest that stimulation of the sympathetic nervous system may alter renin secretion by modulating the renal baroreceptor, but sympathetic innervation does not appear to be involved in the macula densa mechanism.     

Secretion of medullipin I by the kidney requires oxygen.

OBJECTIVE: To test the hypothesis that the secretion of medullipin I by the kidney involves an oxidative step. DESIGN: Medullipin I is secreted by kidney renomedullary interstitial cells and is converted to medullipin II by the liver. Medullipin I can be derived from the kidney in the renal venous effluent by perfusing normal rat kidneys with 95% O2- 5% CO2 at an elevated pressure (180 mmHg). To evaluate whether the secretion of medullipin I involves an oxidative step normal rat kidneys were perfused at an elevated pressure in the presence of O2, in the absence of O2 and after treatment of the kidneys with a powerful antioxidant. METHODS: Normal rat kidneys were perfused with 5% albumin bubbled with O2-CO2 at 180 mmHg. This was the control procedure for each of the three approaches. In approach (1), the kidneys were perfused with 5% albumin bubbled with N2. In approach (2), the kidneys were perfused with 'blood' treated with carbon monoxide. In approach (3), the kidneys were treated with the antioxidant butylated hydroxytoluene then perfused with 5% albumin bubbled with O2-CO2. Each perfusate was tested for medullipin I activity by rapid intravenous injection into the SHR. RESULTS: All three approaches, which exclude the action of molecular O2, prevented the secretion of medullipin I by the kidneys. CONCLUSION: The secretion of medullipin I by the kidneys involves an oxidative step.     

Clearance and metabolism of 125I-labeled tonin in the rat

125I-Labeled rat tonin was injected iv into male Wistar rats weighing 303 +/- 17 g (mean +/- SE), and the disappearance from the circulation, plasma interaction, tissue uptake, and metabolism of radioactivity was studied. After injection, 125I-labeled tonin was immediately bound by alpha 1-macroglobulin, the major circulating protease inhibitor of rats, and the complex was cleared in a biexponential manner: t1/2 of the fast component = 1.8 +2- 0.1 min; t1/2 of the slow component = 84 +/- 10 min; MCR = 2.1 +/- 0.2 ml/min (mean +/- SE, n = 8). The complex was taken up intact by tissues, the major ones being the liver, spleen, adrenal, thyroid, and kidney. Analysis of tissue extracts by gel filtration on Sephadex G-100 showed that the kidney was the major site of metabolism, and radioactivity similar in size to 125I appeared rapidly in urine. Therefore tonin circulates wholly as a complex with alpha 1-macroglobulin (the rat equivalent of human alpha 2-macroglobulin), which once formed is quickly eliminated from the bloodstream by tissue uptake and metabolized.     

High density lipoproteinuria in nephrotic syndrome.

Intravenous administration of the aminonucleoside of puromycin produces the nephrotic syndrome (proteinuria, hypercholesterolemia, hypoproteinemia and edema) in rats. This model is very similar to human nephrotic syndrome caused by various disease states. The current study was designed to assess the nature of urinary lipoproteins in the urine of nephrotic rats, including studies related to the urinary loss of the "activator" apolipoproteins for the lipoprotein lipase-triglyceride interaction. Sprague-Dawley rats were given a single intravenous injection (10 mg/100 g) of puromycin aminonucleoside. Plasma and urine were collected before and 7, 18, 29, 36, and 53 days after injection of puromycin. Urine was fractionated in the preparative ultracentrifuge into density (d) fractions less than 1.006 (very low-density lipoproteins), d = 1.006-1.063 (low-density lipoproteins), and d = 1.063-1.210 (high-density lipoproteins--HDL). The cholesterol, triglyceride, phospholipid, and protein content of these fractions was analyzed. Lipoprotein electrophoresis was performed in agarose agar. Urine from normal and nephrotic rats was added to an in vitro system containing lipoprotein lipase and triglyceride. The free fatty acids (FFA) liberated were then measured as an index of urinary activator property on this system. Measurable urinary lipoproteins were present only on days 7 and 18 after induction of the nephrotic syndrome. Coelectrophoresis of these urinary lipoproteins with rat plasma revealed a single band having alpha- (HDL) electrophoretic mobility. The total mean protein content of day-7 urinary lipoproteins (64.3%) was greater than the content of plasma HDL (52.9%). The protein content of urinary lipoproteins also increased with time. When day-7 and day-18 postinjection urine at nephrotic rats was added to the lipoprotein lipase system, the hydrolysis of triglyceride yielded a mean of 0.320 and 0.235 muEq FFA/ml/20 min, respectively. Control rat urine yielded 0.030 muEq FFA/ml/20 min and 0.000 muEq FFA/ml/20 min 7 and 18 days after injection of normal saline, respectively. It is inferred that in this experimental model (1) high-density lipoproteins are probably excreted in the glomerular filtrate, (2) alterations in the composition of the excreted lipoproteins may occur during their passage through the nephron. The possibility that only a selective portion of the HDL spectrum is excreted into the glomerular filtrate cannot be excluded. It is suggested that the urinary or renal loss of this functionally important lipoprotein may contribute to the pathophysiology of hyperlipoproteinemia in the nephrotic syndrome.     

Efficiency of canine renal blood flow autoregulation in kidneys with or without glomerular filtration

In order to evaluate vascular (myogenic) and tubular (tubuloglomerular feedback) mechanisms involved in renal blood flow (RBF) autoregulation, canine kidneys with filtration (normal) and without filtration (nonfiltering, NFK; ureteral-obstructed, UO) were studied. RBF was monitored in response to stepwise reductions in renal perfusion pressure of -20, -40, and -60 mm Hg from control pressure. None of the three groups demonstrated significant changes in RBF from their respective control values (normal, 380 ml/min/100 g; NFK, 179 ml/min/100 g; UO, 153 ml/min/100 g) until the lowest pressure (-60 mm Hg from control pressure). All three groups responded to the pressure reductions with significant decreases in renal vascular resistance. However, the calculated efficiency of the autoregulatory response for the NFK and UO groups was significantly less than for the normal group. Elimination of tubuloglomerular feedback (NFK and UO) did not necessarily eliminate renal vascular autoregulation, but did reduce the efficiency of autoregulation. It is suggested that both vascular (myogenic) and tubular (tubuloglomerular) mechanisms may coexist to efficiently autoregulate blood flow in normal filtering kidneys. However, a reduction in metabolic activity as a contributor to the reduced ability to autoregulate in these kidneys could be an additional possibility.     

Effects of atrial natriuretic peptide on renal function and renin release in the isolated perfused rat kidney

The effects of synthetic atrial natriuretic peptide (ANP) on the renal hemodynamics, glomerular filtration rate (GFR), fluid and electrolyte excretion and renin release were studied in the isolated perfused rat kidney (IPK). When 10(-9) mol of ANP was administered in 75 ml of perfusate, the renal vascular resistance (RVR) was transiently decreased for 3 to 5 min, thereafter increased for 30 min and then tended to return to the control level. ANP increased the GFR (0.55 +/- 0.08 to 0.71 +/- 0.07 ml/min), urine flow (UV) (0.018 +/- 0.002 to 0.194 +/- 0.028 ml/min), absolute Na excretion (UNaV) (1.83 +/- 0.03 to 17.93 +/- 2.71 microEq/min) and absolute K excretion (UKV) (0.67 +/- 0.13 to 2.33 +/- 0.18 microEq/min). The addition of indomethacin or mefenamic acid to the perfusate before the administration of ANP exerted no influence on any of the effects of ANP. Renin release was inhibited by approximately 50% compared to the ANP-free control group. With the administration of ANP, UV and UNaV reached a peak 15-20 min after the GFR reached a peak and remained elevated after the GFR fell below the control level. These findings suggest that 10(-9) mol of ANP causes natriuresis and renin suppression in the IPK, and that the natriuresis is prostaglandin-independent and cannot be explained only by an increase in GFR.     

Renal functional reserve in humans. Effect of protein intake on glomerular filtration rate

This study was designed to investigate the effect of protein intake on glomerular filtration rate, and to demonstrate and evaluate the functional reserve of the kidney. Normal subjects ingesting a protein diet had a significantly higher creatinine clearance than a comparable group of normal subjects ingesting a vegetarian diet. A progressive increment in protein intake in normal volunteers resulted in a significant increase in creatinine clearance. Diurnal variations in creatinine clearance were found. These daily variations correlated well with the periods of food intake. The capacity of the kidney to increase its level of function with protein intake suggests a renal function reserve. In short-term studies, the effect of a protein load on glomerular filtration rate was evaluated. Normal subjects showed an increase in glomerular filtration rate two and a half hours after protein load to a maximal glomerular filtration rate of 171.0 +/- 7.7 ml per minute. In patients with a reduced number of nephrons, renal functional reserve may be diminished or absent.     

Role of neutral endopeptidase in the metabolism of endothelin.

Endothelin is a potent vasoconstrictor produced by endothelial cells. Although endothelin has been studied extensively, little is known about its metabolism in vivo. Neutral endopeptidase EC.3.4.24.11 is reported to degrade endothelin in vitro. Therefore, we studied the effect of neutral endopeptidase inhibition by SQ29,072 on plasma levels and urinary excretion of endogenous and exogenous endothelin. Injection of 30 or 60 mg/kg SQ29,072 into anesthetized rats increased the urinary excretion of endothelin nearly 14-fold. The response was maximal during the first 30 minutes of collection and lasted for 90 minutes. The larger dose of inhibitor caused a 37-43% increase (p less than or equal to 0.05) in the plasma concentration of endothelin. Only 0.20 +/- 0.04% of the total radioactivity injected as 125I-endothelin (1 microCi; 1,308 pg) into normal rats was recovered in the urine within 30 minutes. Urinary radioactivity increased to 0.54-0.63% (p less than or equal to 0.05) of the total infused in rats pretreated with SQ29,072. Chromatographic analysis of radioactivity in the urine revealed that intact endothelin accounted for only 6-9% of the total counts in control rats but 50-56% in rats pretreated with the inhibitor. We also studied the effects of another inhibitor of neutral endopeptidase, SQ28,063, on the distribution of radioactivity in the urine, kidney, and lung of rats injected with 125I-endothelin. SQ28,063 increased urinary excretion of labeled endothelin and increased total radioactivity accumulated in the lung and kidney from 157 and 105 pg to 234 and 157 pg, respectively. Intact endothelin accounted for 90% or more of the accumulated counts in both tissues. These results indicate that 1) little circulating endothelin is cleared into the urine, 2) endothelin in the urine is likely of renal origin, and 3) neutral endopeptidase EC.3.4.24.11 plays a major role in the inactivation of endothelin.