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Biomedical subjects

R Rabkin

Publications and source records attributed to R Rabkin.

At least 91 records · Page 5Linked to original sources
PubMed1988

Renal metabolism of calcitonin.

The kidneys account for approximately two-thirds of the metabolism of calcitonin, but relatively little is known regarding the details thereof. To further characterize this process, we examined the renal handling and metabolism of human calcitonin (hCT) by the isolated perfused rat kidney. We also studied the degradation of radiolabeled salmon calcitonin (sCT) by subcellular fractions prepared from isolated rabbit proximal tubules. The total renal (organ) clearance of immunoreactive hCT by the isolated kidney was 1.96 +/- 0.18 ml/min. This was independent of the perfusate total calcium concentration from 5.5 to 10.2 mg/dl. Total renal clearance exceeded the glomerular filtration rate (GFR, 0.68 +/- 0.05 ml/min), indicating filtration-independent removal. Urinary calcitonin clearance as a fraction of GFR averaged 2.6%. Gel filtration chromatography of medium from isolated kidneys perfused with 125I-labeled sCT showed the principal degradation products to be low molecular weight forms eluting with monoiodotyrosine. Intermediate size products were not detected. In the subcellular fractionation experiments, when carried out at pH 5.0, calcitonin hydrolysis exclusively followed the activities of the lysosomal enzyme N-acetyl-beta-glucosaminidase. Typically, at pH 7.5, 42% of total degradation occurred in the region of the brush-border enzyme alanyl aminopeptidase and 29% occurred in the region of the cytosolic enzyme phosphoglucomutase. Although 9% of the calcitonin-degrading activity was associated with basolateral membrane fractions, most of this activity could be accounted for by the presence of brush-border membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylglucosaminidase↗
PubMed1988

High performance liquid chromatographic analysis of insulin degradation products from a cultured kidney cell line.

The kidney is a major site for insulin removal and degradation, but the subcellular processes and enzymes involved have not been established. We have examined this process by analyzing insulin degradation products by HPLC. Monoiodoinsulin specifically labeled on either the A14 or B26 tyrosine residue was incubated with a cultured kidney epithelial cell line, and both intracellular and extracellular products were examined on HPLC. The products were then compared with products of known structure generated by hepatocytes and the enzyme insulin protease. Intracellular and extracellular products were different, suggesting two different degradative pathways, as previously shown in liver. The extracellular degradation products eluted from HPLC both before and after sulfitolysis similarly with hepatocyte products and products generated by insulin protease. The intracellular products also eluted identically with hepatocyte products. Based on comparisons with identified products, the kidney cell generates two fragments from the A chain of intact insulin, one with a cleavage at A13-A14 and the other at A14-A15. The B chain of intact insulin is cleaved in a number of different sites, resulting in peptides that elute identically with B chain peptides cleaved at B9-B10, B13-B14, B16-B17, B24-B25, and B25-B26. These similarities with hepatocytes and insulin protease suggest that liver and kidney have similar mechanisms for insulin degradation and that insulin protease or a very similar enzyme is involved in both tissues.

Animals↗
PubMed1988

Effect of insulin therapy on established diabetic nephropathy in rats.

The effectiveness of insulin therapy on early diabetic nephropathy has not been established. In this study we examined the influence of continuous subcutaneous insulin on the progression of established nephropathy in streptozocin-induced diabetic rats. Normal controls and diabetic rats were studied for 11 mo. During the first 6 mo, all the diabetic rats received 2 U protamine zinc insulin s.c. twice weekly. During the last 5 mo of study, diabetic rats either continued on the occasional subcutaneous insulin regimen or received regular insulin by continuous subcutaneous infusion. Six months after the initiation of the study, the diabetic rats were severely hyperglycemic, and their relative mesangial areas had increased. Continued poor glycemic control in the rats receiving occasional insulin was associated with relative increased mesangial area (25.2 +/- 1.0% of glomerular area) and significant proteinuria (148 +/- 17 mg/24 h) compared with normal controls. In contrast, the use of continuous subcutaneous insulin therapy with improved glycemic control arrested mesangial changes (19.5 +/- 1.4% of glomerular area) and prevented the excessive proteinuria (71 +/- 13 mg/24 h). Indeed, these parameters did not differ from age-matched controls. We conclude that in the rat, continuous subcutaneous insulin therapy instituted after the development of early glomerular pathology is effective in arresting the disease process.

Animals↗
PubMed1988

Effect of bacitracin on binding and processing of insulin by established renal cell line.

The effect of bacitracin on the binding and processing of 125I-labeled insulin was studied in a proximal tubular epithelium-like opossum kidney cell line. This cultured cell line handles insulin in a manner comparable to the in vivo situation, which requires membrane binding, internalization, and intracellular degradation. The addition of bacitracin inhibited insulin degradation significantly and delayed the time of appearance of products in the medium (22 min) compared with control cells (14 min). Maximum total cell-associated radioactivity increased from 1.5 +/- 0.19% in the control cells to 2.5 +/- 0.17% in the treated cells. Separation of cell membrane from internalized radioactivity was achieved by acid washing and showed no change in membrane-bound radioactivity or rate of internalization, but a significant increase in intracellular radioactivity was noted. Gel-filtration chromatography revealed that this was due to an accumulation of chromatographically intact insulin. Accordingly, we conclude that bacitracin inhibits insulin degradation in cultured kidney cells by perturbing the intracellular processing of insulin, not by altering the binding or internalization of the hormone or by inhibiting the release of small degradation products. Because of the multiple actions of this agent, the exact site in these kidney cells at which intracellular degradation is inhibited remains to be established. However, in contrast to studies with lysosomes isolated from cells of other tissues, this study showed that when lysosomes isolated from rat renal cortex were exposed to bacitracin, insulin degradation was inhibited markedly (81%).

Animals↗
PubMed1986

Effect of experimental diabetes on insulin binding by renal basolateral membranes.

Removal of insulin from the peritubular vessels involves binding of insulin to specific receptors in the basolateral membranes (BLM); this is followed by phosphorylation of the receptor which may mediate the actions of the hormone. In most tissues receptor number is regulated by plasma insulin levels and is increased in insulinopenic diabetics. To determine whether cortical BLM insulin receptors are similarly regulated, we studied insulin binding to receptors in BLM from normal control rats and rats with streptozotocin diabetes of varying severity. Specific binding of insulin did not differ between control and modestly insulinopenic diabetics but was increased significantly in the severely insulinopenic diabetics. Insulin treatment returned binding to normal. Scatchard analysis suggested an increase in the binding capacity of the severe diabetic BLM rather than an increase in affinity for insulin. This latter was confirmed by competitive experiments in which similar displacement curves were obtained with control and diabetic membranes. Insulin removed by glomerular filtration binds to specific receptors in the luminal membranes but unlike BLM receptors, phosphorylation of these luminal receptors has not been observed. To determine whether luminal and BLM receptors differ structurally, binding sites in both membranes were affinity labelled with 125I-insulin and the cross linking agent, disuccinimidyl suberate, and subjected to SDS-polyacrylamide gel electrophoresis in the presence of a reducing agent. Autoradiograms revealed that the major specifically labelled subunit in both membranes is a 135,000 Mr species which is more abundant in the BLM. We conclude that insulin receptors in cortical BLM respond to severe insulinopenic diabetes as do receptors in most other tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Affinity Labels↗
PubMed1986

Insulin metabolism by liver, muscle, and kidneys from spontaneously diabetic rats.

The in vivo metabolism of insulin is a complex process in which liver, kidney, and muscle are major participants. In this study we evaluated the effect of spontaneous hyperglycemic nonketoacidotic diabetes (DH) and ketoacidotic diabetes (DKA) on insulin clearance and degradation by these organs. Livers, hindlimbs, and kidneys from nondiabetic controls and DH and DKA Bio-Breed rats were isolated and perfused with artificial media. Liver clearance of immunoreactive insulin (ml/min) was significantly higher in DH rats, 6.0 +/- 0.2, but significantly lower in DKA rats, 3.4 +/- 0.5, compared with controls, 4.6 +/- 0.2. Acidosis alone induced by ammonium chloride loading, did not impair liver insulin clearance (4.8 +/- 0.4 ml/min). Muscle responded differently to the diabetic state in that insulin clearance was not altered by DH and DKA. Renal (organ) clearance of insulin was significantly depressed in the DKA state when compared with controls (0.52 +/- 0.04 and 0.75 +/- 0.07 ml X min-1 X g-1, respectively). This could largely be explained by a lower glomerular filtration rate. Fractional urinary insulin clearance was increased twofold above control values in DH kidneys and fourfold in DKA kidneys, indicating that tubular luminal absorption of insulin was impaired in both states. By contrast contraluminal uptake (peritubular clearance) did not differ significantly from controls. 125I-insulin degrading activity of the 100,000 g supernate fraction from muscle homogenates was similar in the diabetic and control groups. However in liver and kidney, degrading activity did not correspond to whole organ insulin clearance in a consistent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

Mechanisms and consequences of proteinuria.

The glomerulus is a complex structure containing a remarkable capillary bed which is freely permeable to water and solutes up to the size of inulin. Many small proteins are filtered, reabsorbed, and catabolized by the kidney; but most large proteins, such as albumin or immunoglobulins, are almost entirely excluded from the glomerular ultrafiltrate due to the charge-size permselectivity of the glomerular capillary basement membrane. These large proteins appear in the urine when diseases reduce the charge selectivity or result in the development of large pores in this membrane. The reabsorptive capacity of the renal tubules for these proteins is overwhelmed. Hypoalbuminemia results when increased synthetic and decreased catabolic rates of albumin fail to compensate for the urinary loss of the protein. The resulting decrease in serum oncotic pressure increases the flux of fluid out of systemic capillaries into the interstitial space, a process that increases lymphatic flow and returns the relatively protein-poor ultrafiltrate to the plasma compartment. Interstitial proteins are swept into the plasma by the increased lymphatic flow, leading to a depletion of the extravascular pool of albumin even more severe than the depletion of albumin in the plasma compartment. The rate of albumin synthesis is increased but not sufficiently to replace losses and restore the serum concentration to normal. The rate of albumin catabolism is decreased. This decrease from the normal catabolic rate is as important as the increased rate of albumin synthesis in maintenance of albumin homeostasis in nephrosis. Whereas the reduced serum oncotic pressure certainly contributes to edema formation, sodium retention may result from processes intrinsic to the kidney itself; and plasma volume may actually be expanded despite hypoalbuminemia. The hyperlipemia that occurs in nephrosis is due to a combined defect in lipoprotein metabolism: increased hepatic synthesis of VLDL and decreased removal of TG and highly atherogenic remnants of incompletely metabolized CMs. The defects in lipoprotein metabolism may in part be the end result of the urinary loss of highly negative-charged macromolecules of the mucopolysaccharide called orosomucoid, which carries with it heparan sulfate, and important cofactor for LPL.

Albumins↗
PubMed1985

Hepatorenal syndrome managed with hemodialysis, then reversed by peritoneovenous shunting.

A patient with acute decompensated chronic liver disease developed acute tubular necrosis after an episode of hypotension. Renal failure was managed by hemodialysis for 11 weeks during which period hepatic function improved. Despite persistently severe oliguria, tubular function recovered as judged by a fall in urine sodium content and a rise in specific gravity, suggesting the development of the hepato-renal syndrome. Therefore, a peritoneovenous shunt was inserted. This was followed by a prompt diuresis; further dialysis was not required. This case suggests potential roles for hemodialysis and peritoneovenous shunting in patients with advanced, but potentially reversible hepatic and renal failure and draws attention to the need for formal evaluation of such a possibility.

Acute Kidney Injury↗
PubMed1984

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.

Animals↗
PubMed1982

Amino acids enhance renal tubular absorption of the los-molecular-weight proteins insulin and growth hormone.

The effect of amino acids(AA) on the tubular absorption of low-molecular-weight (LMW) proteins was studied in isolated rat kidneys. Kidneys were perfused with an albumin-electrolyte solution that contained insulin or human growth hormone (hGH) and, unless otherwise stated, the following L-amino acids: glycine, isoleucine, serine, alanine, methionine, proline, arginine, and aspartic acid. In kidneys perfused without AA, fractional urinary insulin clearance (FCi) averaged 7.4 +/- 1.54%, whereas in the presence of multiple AA the FCi was significantly lower (0.68 +/- 0.2%, P less than 0.01). Addition of glycine or alpha-aminoisobutyric acid (AIB) alone also reduced the FCi significantly (1.79 +/- 0.66 and 1.59 +/-1.06%, respectively). By contrast, perfusion with the other AA individually did not alter the FCi. The fractional urinary hGH clearance was also significantly lower in kidneys perfused with multiple AA (0.94 +/- 0.47%) than in those perfused without AA (9.07 +/- 1.2%). We conclude that tubular absorption of filtered insulin and hGH is enhanced by the presence of AA. The mechanism is unclear, but enhancement of insulin absorption can be produced by glycine and AIB alone. This raises the possibility of a link between the absorption of insulin and the glycine and AIB shared transport system, but excludes a primary metabolic effect because AIB is nonmetabolizable.

Absorption↗
PubMed1982

Fate of [125I]insulin removed from the peritubular circulation of isolated perfused rat kidney.

Although there is considerable evidence that insulin is removed from the peritubular circulation of the mammalian kidney, it is unclear whether binding to insulin-specific receptors is involved in this process, whether after peritubular removal the hormone is degraded to small fragments with release into the circulation, or whether it merely undergoes a minor modification with loss of immunoreactivity. We examined the metabolism of [125I]insulin removed from the peritubular circulation of the nonfiltering isolated perfused rat kidney and compared it to that of [125I]insulin metabolized by filtering isolated kidneys and kidney homogenates. The results indicate that after peritubular removal, a small amount of insulin is degraded to form low-molecular-weight products similar to those seen with filtering kidneys and kidney homogenates. However, most of the insulin removed from the peritubular circulation is processed either to nonimmunoreactive products of molecular weight similar to that of insulin or, to a lesser extent, to products of larger molecular weight. Both these products are also formed by filtering kidneys. In the filtering kidney, the products having molecular weight similar to that of insulin probably originate from the peritubular process, because it is unlikely that material of this size could be derived from the filtration-absorption pathway. Of particular note was the finding that [125I]insulin trapped in the peritubular compartment of nonfiltering kidneys was displaced severalfold more effectively by unlabeled insulin than by several peptide hormones (P less than 0.01); the latter were no more effective than vehicle alone. The findings suggest the presence of peritubular insulin-specific receptors.

Animals↗
PubMed1982

Binding and degradation of insulin by isolated renal brush border membranes.

Filtered proteins including insulin are absorbed in the proximal tubule by means of pinocytosis. The first step in this process is binding of the protein to brush border membrane. As it is not known whether absorption exhibits specificity, we set out to determine whether specific binding sites for insulin are present in brush border membranes. Rabbit-isolated brush border membranes were incubated with 125I-insulin and varying concentrations of cold insulin or other peptide hormones. Binding and degradation of 125I-insulin occurred in a time- and temperature-dependent manner. Native insulin competitively inhibited 125I-insulin binding, but calcitonin, arginine vasopressin, glucagon, and growth hormone (10(-6) M) were relatively ineffective. Nonspecific binding averaged one-third of the total radioactivity bound. Scatchard analysis of binding data revealed two classes of insulin receptors: high affinity, low capacity receptors and low affinity, high capacity receptors. Gel filtration analysis of 125I-insulin exposed to brush border membrane revealed the formation of low-molecular-weight products similar to that produced by intact kidneys. The degrading process exhibited some specificity, for cold insulin (10(-6) M) was more effective than calcitonin, vasopressin, glucagon, or growth hormone in inhibiting degradation (32% versus less than 13% inhibition; P less than 0.01). Whether this reflects inhibition of insulin specific binding before exposure to degradation or inhibition of specific enzymes is unclear. In summary, it appears that renal brush border membranes have a major insulin-specific receptor component that could potentially mediate tubular insulin absorption. In addition, there is a smaller nonspecific component that may also have the potential to mediate insulin absorption. Finally, it appears that brush border membranes have the ability to degrade insulin to low-molecular-weight products by a process that exhibits some specificity for insulin.

Absorption↗
PubMed1981

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.

Adolescent↗
PubMed1981

Removal and excretion of immunoreactive rat growth hormone by the isolated kidney.

The renal uptake of immunoreactive rat growth hormone (rGH), molecular weight 21,500 daltons, was examined in the isolated perfused rat kidney to determine whether peritubular removal of a protein greater than 12,000 daltons occurs and to assess the functional characteristics of renal GH uptake. Organ clearance of rGH (OCGH) in control kidneys was 1,039 +/- 99 microliters/min and was unaffected by an excess of insulin but markedly depressed by col (10 degrees C( and KCN. Although glomerular filtration rate (GFR) did not differ significantly from OCGH in the control rats, we suspected that filtration could not account for all the rGH removed because of glomerular protein sieving. However, GFR was significantly less than OCGH with cold and KCN treatment, indicating the occurrence of peritubular removal. In nonfiltering kidneys, rGH removal exceeded that of [14C]inulin (P less than 0.05), demonstrating peritubular rGH removal. Tubular absorption of rGH was unaffected by insulin but markedly depressed by cold and KCN. We conclude that rGH is removed from the renal circulation mainly by the glomerular filtration-tubular absorptive pathway, but, in addition, as with smaller proteins, that peritubular removal occurs.

Animals↗