Factors influencing the handling of insulin by the isolated rat kidney

Tubular Secretory Clearance Is Associated With Whole-Body Insulin Clearance

CONTEXT

The kidneys eliminate insulin via glomerular and peritubular mechanisms; consequently, the kidney contribution to insulin clearance may be underestimated by the glomerular filtration rate (GFR) alone.

OBJECTIVE

To determine associations of tubular secretory clearance with whole-body insulin clearance and sensitivity in a dedicated study of glucose and insulin metabolism.

DESIGN, SETTING, AND PARTICIPANTS

We performed an ancillary, cross-sectional study of tubular secretion in the Study of Glucose and Insulin in Renal Disease (SUGAR). Hyperinsulinemic-euglycemic clamps were performed in 57 nondiabetic persons with chronic kidney disease and 38 persons without kidney disease.

INTERVENTION

We measured plasma and 24-hour urine concentrations of endogenous solutes primarily eliminated by tubular secretion. Kidney clearances of secretory solutes were calculated as the amount of blood fully cleared of that solute per minute.

MAIN OUTCOME MEASURES

Whole-body insulin clearance, insulin sensitivity.

RESULTS

Mean whole-body insulin clearance was 924 ± 228 mL/min. After adjustment for age, sex, Black race, fat and fat-free mass, each 20% lower estimated GFR was associated with a 13 mL/min lower insulin clearance (95% confidence interval [CI], 2-24 mL/min lower). Each 20% lower clearance of isovalerylglycine and xanthosine were associated with a 16 mL/min lower (95% CI, 5-26 mL/min lower) and 19 mL/min lower (95% CI, 7-31 mL/min lower) insulin clearance, respectively. Neither estimated GFR nor secretory solute clearances were associated with insulin sensitivity after adjustment.

CONCLUSIONS

These results highlight the importance of tubular secretory pathways to insulin elimination but suggest that kidney functions in aggregate contribute only modestly to systemic insulin clearance.

Properties of the Glomerular Barrier and Mechanisms of Proteinuria

This review focuses on the intricate properties of the glomerular barrier. Other reviews have focused on podocyte biology, mesangial cells, and the glomerular basement membrane (GBM). However, since all components of the glomerular membrane are important for its function, proteinuria will occur regardless of which layer is affected by disease. We review the properties of endothelial cells and their surface layer, the GBM, and podocytes, discuss various methods of studying glomerular permeability, and analyze data concerning the restriction of solutes by size, charge, and shape. We also review the physical principles of transport across biological or artificial membranes and various theoretical models used to predict the fluxes of solutes and water. The glomerular barrier is highly size and charge selective, in qualitative agreement with the classical studies performed 30 years ago. The small amounts of albumin filtered will be reabsorbed by the megalin-cubulin complex and degraded by the proximal tubular cells. At present, there is no unequivocal evidence for reuptake of intact albumin from urine. The cellular components are the key players in restricting solute transport, while the GBM is responsible for most of the resistance to water flow across the glomerular barrier.

Drug dosing in chronic kidney disease

Patients with chronic kidney disease (CKD) are at high risk for adverse drug reactions and drug-drug interactions. Drug dosing in these patients often proves to be a difficult task. Renal dysfunction-induced changes in human pathophysiology regularly results may alter medication pharmacodynamics and handling. Several pharmacokinetic parameters are adversely affected by CKD, secondary to a reduced oral absorption and glomerular filtration; altered tubular secretion; and reabsorption and changes in intestinal, hepatic, and renal metabolism. In general, drug dosing can be accomplished by multiple methods; however, the most common recommendations are often to reduce the dose or expand the dosing interval, or use both methods simultaneously. Some medications need to be avoided all together in CKD either because of lack of efficacy or increased risk of toxicity. Nevertheless, specific recommendations are available for dosing of certain medications and are an important resource, because most are based on clinical or pharmacokinetic trials.

Handling of low-density lipoprotein by the renal tubule: release of fragments due to incomplete degradation

Because the mechanism by which lipoproteins are processed and modified in the renal tubule in patients with nephrosis is not completely understood, we studied the handling of low-density lipoprotein (LDL) in perfused rat kidneys made permeable by protamine. Protamine pretreatment increased the clearance of 125(I) LDL 25-fold compared to controls, thereby simulating a proteinuric kidney. Similar studies were also conducted in kidneys of rats made proteinuric by the induction of passive Heymann nephritis. Of the perfused iodinated LDL, 5% was localized in the cortex and lesser amounts in the medulla and urine. In the cortex and medulla, iodinated LDL was present mainly in the intact form (90%); just 10% was present in the degraded form. Using horseradish peroxidase conjugated to LDL, we demonstrated specific staining in the proximal tubules, suggesting that specific LDL receptors were present in that location. Although LDL in the tissue was present mostly in the intact form, it was 95% degraded in urine, and the degradation was inhibited by chloroquine, indicating that the lysosomes were the site of LDL metabolism. Gel chromatography and electrophoresis of iodinated LDL in the urine showed the presence of fragments in the range of 5 to 15 kD. We conclude that renal degradation of LDL is incomplete and that the incompletely degraded fragments released into the urine may be toxic to the kidney by virtue of their lipid side-chains.

Effects of filtration rate on the glomerular barrier and clearance of four differently shaped molecules

The effect of shape on the transglomerular passage of solutes has not been hitherto systematically studied. We perfused isolated rat kidneys to determine the fractional clearances (theta) at various filtration rates for four molecules of different shapes but with similar Stokes-Einstein radii (aSE = 34-36 A). The theta for hyaluronan, bikunin, and Ficoll36 A were 66, 16, and 11%, respectively, at a glomerular filtration rate (GFR) of 0.07 ml x min(-1) x g wet wt(-1) and decreased to 46, 14, and 7%, respectively, on a fivefold increase in GFR. Under the same conditions, theta for albumin increased from 0.15 to 0.74%, and similar behavior was observed for larger Ficolls (aSE >45 A). Pore analysis showed that the "apparent neutral" solute radii of Ficoll, albumin, bikunin, and hyaluronan were 35, 64, 33, and 24 A, respectively, despite similar aSE. In addition, the properties of the glomerular filter changed with increasing GFR and hydrostatic pressure. We conclude that elongated shape, irrespective of size and charge, drastically increases the transglomerular passage of a solute, an effect that is related to its frictional ratio.

The intrarenal site of calcitonin gene-related peptide degradation in the isolated perfused rat kidney

1. Calcitonin gene-related peptide (CGRP) is a potent vaso-active 37 amino acid peptide, typically elevated in plasma from patients with medullary thyroid cancer (MTC), but undetectable in the plasma of normal subjects. 2. The kidney is a major site for the clearance of exogenously infused CGRP but the intrarenal site of this clearance is unknown. Extra-organ clearance is also significant for CGRP, and whereas the site and mechanism of this degradation remain uncertain, the vasculature has been postulated as the most likely site. 3. The isolated perfused rat kidney (IPRK) was studied to (i) localize the intrarenal site of CGRP clearance and (ii) determine the contribution of the renal vasculature to the clearance of CGRP. The half-life of CGRP in the filtering IPRK was 63.9 +/- 4.5 min, whereas blocking of filtration by elevation of the perfusate osmolarity abolished the degradation. This suggests that (i) renal CGRP degradation occurs after glomerular filtration with intratubular metabolism and (ii) that there is no active CGRP degradation in the (glomerular) capillary endothelium. 4. These results do not support the theory that renal vascular endothelium plays a major active role in CGRP degradation.

In vivo imaging of insulin receptors in monkeys using 18F-labeled insulin and positron emission tomography

We previously described a prosthetic group methodology for incorporating 18F into peptides and showed that 18F-labeled insulin (18F-insulin) binds to insulin receptors on human cells (IM-9 lymphoblastoid cells) with affinity equal to that of native insulin (1). We now report studies using 18F-insulin with positron emission tomography to study binding to insulin receptors in vivo. Positron emission tomography scans were performed in six rhesus monkeys injected with 0.3-1.4 mCi of 18F-insulin (approximately 0.1 nmol, SA 4-11 Ci/mumol). Integrity of the tracer in blood, determined by immunoprecipitation, was 94% of control for the first 5 min and decreased to 31% by 30 min. Specific, saturable uptake of 18F was observed in the liver and kidney. Coinjection of unlabeled insulin (200 U, approximately 1 nmol) with the 18F-insulin reduced liver and increased kidney uptake of the labeled insulin. Liver radioactivity was decreased by administration of unlabeled insulin at 3 min, but not 5 min, after administration of the tracer, while some kidney radioactivity could be displaced 5 min after injection. Clearance of 18F was predominantly in bile and urine. 18F-insulin is a suitable analogue for studying insulin receptor-ligand interactions in vivo, especially in the liver and kidney.

Receptor-mediated endocytosis of A14-125I-insulin by the nonfiltering perfused rat kidney

The mechanism of insulin uptake and/or degradation in the peritubular circulation of the kidney was investigated using nonfiltering perfused rat kidneys, in which glomerular filtration was sufficiently reduced. After perfusion of A14-125I-insulin in the nonfiltering kidney for designated intervals, the acid-wash technique was employed to separately measure the acid-extractable and acid-resistant A14-125I-insulin, which were quantitated by HPLC and TCA-precipitability. HPLC profiles showed that the nonfiltering kidney metabolizes A14-125I-insulin only to a small extent during 1-h perfusion, suggesting that the peritubular clearance of A14-125I-insulin was not due to extracellular degradation but for the most part to uptake by the kidney. Acid-extractable A14-125I-insulin rapidly increased with time and reached pseudo-equilibrium with perfusate at approx. 10 min, whereas acid-resistant A14-125I-insulin increased continuously. An endocytosis inhibitor, phenylarsine oxide, inhibited significantly the acid-resistant A14-125I-insulin with no change in acid-extractable A14-125I-insulin, suggesting that the peritubular uptake of A14-125I-insulin largely represents endocytosis of the peptide into the intracellular space. Moreover, both the acid-extractable and acid-resistant A14-125I-insulin were significantly decreased in the presence of unlabeled insulin (1 microM). These lines of evidence suggest that insulin is taken up by the nonfiltering perfused kidney via receptor-mediated endocytosis (RME), which possibly occurs at the basolateral side of renal tubular cells, and that the peritubular clearance of insulin is largely accounted for by this mechanism.

Metabolism of neurotensin by isolated perfused rat kidney

Studies in humans and conscious animals have established that the kidney is a key organ involved in the clearance of the brain-gut peptide neurotensin (NT). However, these in vivo studies cannot determine the mechanisms involved in the renal elimination of NT. We have therefore used the isolated perfused rat kidney preparation, which enables renal elimination to be studied in isolation from other organs. NT was measured with both COOH-terminal (biologically active end) and NH2-terminal (biologically active end) directed antisera. NT was stable in control perfusions (no kidney) with a disappearance half-life of greater than 250 min. The disappearance half-life in filtering kidneys was 52 +/- 3 min (COOH terminal) and 67 +/- 3 min (NH2-terminal). High-performance liquid chromatography of perfusate revealed a pattern similar to that seen in vivo with the NT being metabolized to NH2-terminal fragments. When the kidneys were rendered nonfiltering, NH2-terminal clearance was abolished, whereas COOH-terminal clearance was reduced by 75%. There was no release of NT peptidases into the perfusate. These data demonstrate that NT is metabolized directly by the kidney, predominantly by a filtration and reabsorption mechanism and with only a minor role for peritubular metabolism.

Abnormal insulin metabolism by specific organs from rats with spontaneous hypertension

Spontaneously hypertensive rats (SHR) have been shown to be both insulin resistant and hyperinsulinemic after oral glucose administration or infusion of exogenous insulin during an insulin suppression test. To determine if this hyperinsulinemia may be due to decreased removal of insulin, the metabolic clearance (k) of insulin was measured in isolated perfused liver, kidney, and hindlimb skeletal muscle from SHR and Wistar-Kyoto (WKY) control rats. The data indicate that the k for insulin removal by liver was similar in SHR and WKY rats, averaging 287 +/- 18 and 271 +/- 10 microliters.min-1.g-1 liver, respectively. In contrast, the k for insulin removal by hindlimbs from SHR was decreased 37% (P less than 0.001) compared with WKY rats (8.6 +/- 0.5 vs. 13.7 +/- 0.7 microliters.min-1.g-1 muscle), and this decrease was not accompanied by decreased binding of insulin to its receptor in plantaris muscle. Although the removal of insulin by glomerular filtration was similar in SHR and WKY rats (653 +/- 64 microliters/min vs. 665 +/- 90 microliters.min-1.kidney-1), total insulin removal by kidney was significantly lower (P less than 0.05) in SHR (710 +/- 78 microliters/min) compared with WKY rats (962 +/- 67 microliters/min), due to decreased peritubular clearance of insulin in SHR (56 +/- 73 vs. 297 +/- 59 microliters/min, P less than 0.05). These findings suggest that the decreased clearance of insulin in SHR rats was possibly not due to impaired hepatic removal of insulin but rather to decreased removal by skeletal muscle and kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)

Internalization and catabolism of insulin by an established renal cell line

Proximal renal tubules are a key site of insulin metabolism. To explore the kinetics and metabolic requirements of insulin internalization and catabolism, we used the opossum kidney cell line, which has proximal tubular-like features and possesses insulin-specific receptors. Internalization was determined by separating membrane-bound insulin from intracellular insulin by exposure to an acidified medium. Internalization of membrane-bound insulin was rapid, and half-maximal internalization occurred within 2.5 min. Degradation products did not accumulate in the cell but appeared in the medium after a delay of 5 min from the onset of internalization. In other experiments, addition of KCN (2 mM) or omission of glucose did not alter degradation, but KCN, combined with the omission of glucose, inhibited degradation by 64%. This was associated with a 240% increase in membrane-bound insulin and an 81% decrease in intracellular insulin. Accordingly, it appears that under these circumstances impaired degradation was a consequence of impaired internalization. In contrast, although 0.1 mM chloroquine, an endosomal-lysosomal inhibitor, also depressed degradation (by 57%), intracellular insulin increased fourfold, indicating failure of intracellular processing. We conclude that these cultured kidney cells rapidly internalize and degrade insulin and that internalization, a prerequisite for degradation, is dependent on energy that can be derived from anaerobic glycolysis or oxidative metabolism. Furthermore, the intracellular degradative processing of insulin by these cells involves a chloroquine-sensitive pathway.

The effect of streptozotocin diabetes on insulin binding by isolated rat kidney tubules

Preparations of kidney tubules were isolated from rat kidney cortex and were demonstrated to possess specific binding sites for insulin. The binding was time-and temperature-dependent and the label was displaced by bovine insulin, A1-B29 dodecoyl insulin, proinsulin and insulin A- and B-chains in proportion to their relative activity. Cell-associated degradation was studied by incubating tubules in the presence of fatty-acid-free albumin. The tubules showed high insulin-degrading activity, which was dependent on temperature, time and cell concentration. The number and affinity of insulin receptors on tubules isolated from kidneys taken from streptozotocin-diabetic rats was not significantly different from tubules isolated from untreated control or insulin-treated diabetic rats. Diabetes did not alter the kinetics of insulin degradation by the tubules. This lack of response by the tubules to changes in the concentration of circulating insulin supports the hypothesis that the kidneys do not play an active role in modulating the rate of insulin removal from the circulation.

Net release or uptake of histidine and carnosine in kidney of dogs

Previous studies are equivocal as to whether the dog kidney produces histidine. Because one possible source of renal histidine is carnosine (beta-alanyl-L-histidine), we investigated net renal production (release) or utilization (uptake) (Qmet) of histidine and carnosine in 19 female dogs after they were fed histidine-free (9 dogs) or histidine-containing diets (10 dogs). Diets were fed in short-(2-11 days) or long-term (52-57 days) studies. Dogs were infused with half-normal saline for 120 min followed by an infusion of half-normal saline containing carnosine, 50 mumol/min. Renal Qmet histidine, calculated from either plasma or whole blood values, was positive during infusion of half-normal saline. During carnosine infusion, Qmet histidine increased markedly, and there was net renal uptake of carnosine. The Qmet histidine and carnosine were not different in the dogs fed histidine-free vs. histidine-containing diets. Thus there is net renal release of histidine in female dogs that increases when carnosine is administered. Qmet histidine or carnosine do not change adaptively when dogs are fed histidine-free diets.

Basolateral endocytosis of protein in isolated perfused proximal tubules

Luminal uptake and degradation of protein in proximal tubules is well documented. However, abluminal uptake has only been demonstrated in a few species and probably only amounts to a few percent of luminal absorption. To investigate this absorptive pathway, isolated perfused proximal tubules from rabbit kidney were exposed to either cationized ferritin or horseradish peroxidase in the bath for 30 min. The tubules were then fixed and processed for electron microscopy. Peroxidase and small amounts of ferritin were found in the intercellular spaces, in endocytic vesicles located in the abluminal part of the cells and in multivesicular bodies. No tracer was found in the lumina or in the apical part of the cells. The tubules were ultrastructurally intact thus excluding the possibility that the proteins were absorbed via the luminal endocytic pathway or as a result of damaged cell membranes. In conclusion, this study presents evidence that ferritin and peroxidase can be absorbed via the basolateral membranes in rabbit proximal tubules.

The renal metabolism of insulin

The kidney plays a pivotal role in the clearance and degradation of circulating insulin and is also an important site of insulin action. The kidney clears insulin via two distinct routes. The first route entails glomerular filtration and subsequent luminal reabsorption of insulin by proximal tubular cells by means of endocytosis. The second involves diffusion of insulin from peritubular capillaries and subsequent binding of insulin to the contraluminal membranes of tubular cells, especially those lining the distal half of the nephron. Insulin delivered to the latter sites stimulates several important processes, including reabsorption of sodium, phosphate, and glucose. In contrast, insulin delivered to proximal tubular cells is degraded to oligopeptides and amino-acids by one of two poorly delineated enzymatic pathways. One pathway probably involves the sequential action of insulin protease and either GIT or non-specific proteases; the other probably involves the sequential action of GIT and lysosomal proteases. The products of insulin degradation are reabsorbed into the peritubular capillaries, apparently via simple diffusion. Impairment of the renal clearance of insulin prolongs the half-life of circulating insulin by a number of mechanisms and often results in a decrease in the insulin requirement of diabetic patients. Much needs to be learned about these metabolic events at the subcellular level and how they are affected by disease states. Owing to the heterogeneity of cell types within the kidney and to their anatomical and functional polarity, investigation of these areas will be challenging indeed.

Renal handling of high-density lipoproteins by isolated perfused kidneys

Recent studies show that the normal and diseased kidney is an important organ in the catabolism of high-density lipoproteins (HDL). However, little is known about the renal handling of HDL. To investigate this aspect, kidneys were isolated from normal rats and rats made nephrotic with puromycin aminonucleoside. They were perfused in a chamber at 37 degrees C with a modified Krebs-Hensleit Bicarbonate Buffer containing 1%, 3%, 6%, and 10% Bovine Serum Albumin (BSA). Presence of 10% BSA in the perfusate prevented glomerular filtration and urine formation. Thus, the filtering and the nonfiltering kidney perfusion models distinguish the renal parenchymal function independently of luminal events that follow filtration. 125I-labeled rat HDL was injected into the perfusate and radioactivity in perfusate, urine, and kidney was examined. At the end of perfusion (30 minutes or four hours), each kidney was flushed with 125I-HDL-free perfusate and kidney radioactivity was measured. At four hours, 1.9% +/- 0.5% of injected radioactivity was present in urine from kidneys perfused with 6% BSA. Kidneys with intact glomerular filtration sequestered significantly more radioactivity (1.1% +/- 0.2% of injected radioactivity) than nonfiltering kidneys (0.7% +/- 0.2%); P less than 0.05. Radioactivity in filtering kidneys was significantly higher than in nonfiltering kidneys (33.9 +/- 7.8 v 15.6 +/- 2.6 cpm/mg kidney tissue protein, respectively; P less than 0.001). Nephrotic kidneys (filtering and nonfiltering) sequestered two to four times more 125I-HDL than normal kidneys. These data support the hypothesis that prior to urinary excretion, partial reabsorption of filtered HDL (or subfractions) occurs in the normal kidney.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by insulin of cellular autophagy in proximal tubular cells of rat kidney

Adult male Sprague-Dawley rats were injected intraperitoneally with 5 U insulin/kg body wt (45 animals). As determined by quantitative electron microscopy, the volume fraction and the numerical density of autophagic vacuoles (AV) in proximal tubular cells decreased within 10 min by 46 and 26%, respectively. A partial recovery of the AV volume fraction was observed 20 and 30 min after the injection contrary to our previous findings with liver (J. Cell Biol. 78: 152-167, 1978). In an additional experiment (12 animals) it was shown that an insulin dose of 0.5 U but not of 0.05 U/kg body wt reduced the AV volume fraction to an extent similar to that of 5 U. To eliminate possible secondary effects, Ringer solution containing 0.8 microM insulin was dropped intravitally for 15 min to one pole of the decapsulated kidney and Ringer solution without additions to the other pole (8 animals). After intravital fixation, the AV volume fraction and numerical density in proximal tubular cells was found to be reduced under the influence of insulin by 22 and 36%, respectively. This data shows that insulin inhibits the process of cellular autophagy in proximal tubular cells of the kidney.

Insulin binding and degradation by luminal and basolateral tubular membranes from rabbit kidney

Insulin influences certain metabolic and transport renal functions and is avidly degraded by the kidney, but the relative contribution of the luminal and basolateral tubular membranes to these events remains controversial. We studied (125)I-insulin degradation [TCA and immunoprecipitation (IP) methods] and the specific binding of the hormone by purified luminal (L) and basolateral (BL) tubular membranes. These were prepared from rabbit kidney cortical homogenates by differential and gradient centrifugation and ionic precipitation steps in sequence, which resulted in enrichment vs. homogenate of marker enzymes' activities (sodium-potassium-activated adenosine triphosphatase for BL and maltase for L) of 8- and 12-fold, respectively. Both fractions degraded insulin avidly and bound the hormone specifically without saturation even at pharmacologic concentrations (10 muM). At physiologic insulin concentrations (0.157 nM) BL membranes degraded substantial amounts of insulin (44.2+/-2.6 and 40.7+/-2.2 pg/mg protein per min by the TCA and IP methods, respectively), even though at lesser rates (P < 0.001) than the luminal fraction (67.2+/-2.3 and 75+/-6.2 pg/mg protein per min, respectively); the rate of insulin catabolism by BL membranes was significantly higher (P < 0.001) than that which could be attributed to their contamination by luminal components [12.2+/-1.9 pg/mg per min (TCA method), or 13.7+/-1.9 pg/mg per min (IP method)]. Competition experiments suggested that insulin-degrading activity in both fractions includes both specific and nonspecific components. In contrast to degradation, insulin binding by both membranes was highly specific for native insulin and was severalfold higher in BL than L membranes [17.5+/-1.3 vs. 4.5+/-0.4 fmol/mg protein (P < 0.001) at physiologic insulin concentrations]. Despite the marked difference in the binding capacity for insulin by the two membranes, the patterns of labeled insulin displacement by increasing amounts of unlabeled hormone were superimposable (50% displacement required approximately 3 nM), suggesting that their receptors' affinity for insulin was similar. These observations provide direct evidence that interaction of insulin with the kidney involves binding and degradation of the hormone at the peritubular cell membrane.

Renal handling of homologous and heterologous insulin in the isolated perfused rat kidney

1. Renal handling of pig- and rat-insulin was studied in the isolated perfused rat kidney. 2. Metabolic clearance rates of both pig- and rat-insulin exceeded GFR. 3. Peritubular uptake of pig-insulin accounted for 13% of rat-insulin for 31% of the total metabolic clearance. 4. The nonfiltering kidney does not remove insulin from the peritubular circulation. 5. Metabolic clearance rates of pig- and rat-insulin are directly related to GFR. 6. The filtration process seems to be necessary for the uptake of insulin at the peritubular site.

Autoregulation of rat pituitary prolactin secretion demonstrated by a new perfusion method

Regulation of prolactin secretion was investigated by perfusing rat pituitaries in vitro. Two pituitary glands from inbred rats were transplanted beneath the renal capsule of a third recipient rat. Three weeks later, the transplanted kidney was removed and perfused in vitro with a defined cell-free medium. Normal renal function was maintained during perfusion, and cell morphology of the transplants remained unchanged as assessed by electron microscopy. Pituitary prolactin content did not change after 120 min of perfusion despite release of approximately 10 micrograms of hormone. Thyrotropin-releasing hormone (10 ng/ml) did not stimulate prolactin release; dopamine (20 ng/ml) rapidly, but transiently inhibited prolactin release; bromocriptine (20 ng/ml) rapidly and persistently inhibited prolactin release; haloperidol (100 ng/ml) blocked the inhibition by dopamine or bromocriptine, but when given alone inhibited prolactin release. Finally, prolactin release was also inhibited by the presence of 100 and 200 ng/ml, but not 50 ng/ml of NIAMDD RP-1 rat prolactin. It is concluded that in vitro perfusion of transplanted rat pituitaries provides a new model for studying the direct effect of agents on the secretion of prolactin from the pituitary and that rat prolactin and/or its metabolites directly inhibit pituitary prolactin secretion.

Reversible hyperinsulinuria in diabetic ketoacidosis in man

Urinary clearance and fractional urinary clearance of immunoreactive insulin (IRI) and beta 2-microglobulin (I beta 2M) were studied in patients with diabetic ketoacidosis (DKA) before, during, and after treatment. Our results indicate that in DKA in man a) there is an approximate 250-fold increase in urinary and fractional urinary clearance of IRI and a 600-fold increase in urinary and fractional urinary I beta 2M clearance, which suggests that the hyperinsulinuria is secondary to a nonspecific defect in tubular luminal uptake of low-molecular-weight proteins, although decreased IRI degradation cannot be excluded; b) because increased IRI clearance is not changed by the pharmacologic plasma IRI levels achieved, the residual tubular absorptive capacity is not saturable; c) I beta 2M clearance but not IRI clearance is significantly improved by the time metabolic control is attained, suggesting separate tubular transport systems; d) a small, therapeutically insignificant fraction of the infused insulin is lost in the urine during therapy of DKA; and e) defective renal tubular luminal uptake (and possibly degradation) of IRI is reversible.

Hepatic and renal metabolism of somatostatin-like immunoreactivity. Simultaneous assessment in the dog

The hepatic and renal metabolism of somatostatin-like immunoreactivity (SLI) was assessed simultaneously in an in vivo dog model. The hepatic extraction of this peptide was 29.4 +/- 2.3% and was similar for endogenous and infused exogenous SLI. The renal extraction was 62.3 +/- 5%. The renal clearance of SLI was significantly greater than that of inulin indicating that the peptide is handled by peritubular uptake from postglomerular blood in addition to glomerular filtration. In both organs SLI extraction was not saturable even at arterial concentrations in excess of 100 times physiological range. The overall metabolic clearance rate of SLI was 19.7 +/- 1.6 ml/kg per minute of which 32.7 +/- 4.6% was contributed by hepatic and 37 +/- 4.9% by renal uptake mechanisms. The plasma half disappearance time of exogenously infused SLI was 1.9 +/- 0.3 min. The studies indicate that in the dog, the liver and kidney are both major sites of SLI metabolism, together accounting for 70.0 +/- 8.7% of the metabolic clearance of the peptide.

The handling of immunoreactive vasopressin by the isolated perfused rat kidney

Using the isolated rat kidney perfused with an artificial medium containing glucose as the sole fuel, we studied the renal handling of immunoreactive arginine vasopressin (AVP) and determined the effect of various factors on the ability of the kidney to remove AVP. In control kidneys perfused with AVP at concentrations below 116 muU/ml, the organ clearance of AVP (OC(AVP)) was 1,145+/-47 (SE) mul/min, whereas glomerular filtration rate (GFR) averaged 515+/-37 mul/min. Filtration could thus account for up to 45% of the OC(AVP), the balance presumably being cleared from the peritubular circulation. Of the AVP filtered, only 38% could be recovered in the urine (urinary clearance AVP averaged 205+/-12 mul/min) suggesting that the balance was taken up by the tubular epithelium and degraded. Fractional excretion of filtered AVP rose significantly in the presence of anoxia and cold (10 degrees C) to 49 and 59%, respectively, but was not affected by ouabain or high levels of AVP (458+/-58 muU/ml). The OC(AVP) was not significantly altered by the absence of glucose in the perfusate, anoxia, or ureteral ligation, maneuvers that were associated with significant reductions in GFR. In these and the control experiments, there was a significant inverse correlation between GFR and peritubular clearance emphasizing the importance of the latter (r = -0.749; P < 0.001). Cold, ouabain, and high concentrations of AVP reduced the clearance of AVP by the kidneys. On the basis of these studies we conclude that the kidney clears AVP from the circulation via two pathways, glomerular clearance and peritubular clearance. This exposes both the luminal and contraluminal surfaces of the tubular cells to the hormone, allowing these cells to remove AVP from the filtrate and the peritubular compartment. Noteworthy is the observation that under several conditions when GFR falls reducing the glomerular clearance of AVP, peritubular clearance increases and the total clearance of AVP by the kidney remains constant.