Search PubMed⌕ Search

Biomedical subjects

A H Rubenstein

Publications and source records attributed to A H Rubenstein.

At least 73 records · Page 4Linked to original sources
PubMed1984

C-peptide and insulin secretion. Relationship between peripheral concentrations of C-peptide and insulin and their secretion rates in the dog.

Estimation of the insulin secretory rate from peripheral C-peptide concentrations depends upon the following characteristics of C-peptide kinetics: (a) equimolar secretion of insulin and C-peptide by pancreatic beta cells; (b) negligible hepatic extraction of C-peptide; (c) constant metabolic clearance rate (MCR) of C-peptide over a physiological and pathophysiological range of plasma levels; and (d) proportional changes in the secretion rate of C-peptide and its peripheral concentrations under varying physiological conditions. In the present experiments, the relationship between a variable intraportal infusion of C-peptide and its concentration in the femoral artery was explored in 12 pancreatectomized dogs. As the infusion of C-peptide was rapidly increased, the magnitude of its peripheral concentration initially increased less than the infusion rate by 20-30%. After an equilibration period of approximately 30 min, however, further increases and decreases in the intraportal infusion were accompanied by nearly proportional changes in its peripheral concentration. Estimates of the amount of C-peptide infused during the experiment based on the steady state C-peptide MCR and its peripheral concentration were within 20% of the amount of C-peptide actually infused. These experiments demonstrate that the portal delivery rate of C-peptide can be calculated from its MCR and peripheral concentration in the dog. They also provide a basis for testing the validity of more complicated models of insulin secretion based on peripheral C-peptide concentrations in the dog as well as other species, including man. Finally, we have shown that the hepatic extraction of endogenously secreted C-peptide is negligible in the basal state (3.1 +/- 6.1%), and does not change after oral glucose ingestion. The MCR of exogenous dog C-peptide was similar whether measured by constant peripheral intravenous infusion (12.3 +/- 0.7 ml/kg per min), constant intraportal infusion (13.4 +/- 0.6 ml/kg per min), or analysis of the decay curve after a bolus injection (13.5 +/- 0.7 ml/kg per min).

Animals↗
PubMed1984

Isolation and amino-acid sequence determination of monkey insulin and proinsulin.

Insulin has been isolated and purified from rhesus monkey pancreas by means of acid-ethanol extraction, gel filtration and ion exchange chromatography. The complete amino-acid sequence of the hormone has been determined by amino-acid analysis of the oxidized A- and B-chains, by end group determination, by the identification of the C-terminal residues (AsnA21 and ThrB30) by carboxypeptidase A digestion and by Edman degradation of the S-carboxymethylated A- and B-chains. The 51-residue monkey insulin was shown to be identical to human insulin. From the known insulin and C-peptide sequence the primary sequence of monkey proinsulin has been proposed.

Amino Acid Sequence↗
PubMed1984

C-peptide as a measure of the secretion and hepatic extraction of insulin. Pitfalls and limitations.

The large and variable hepatic extraction of insulin is a major obstacle to our ability to quantitate insulin secretion accurately in human subjects. The evidence that C-peptide is secreted from the beta cell in equimolar concentration with insulin, but not extracted by the liver to any significant degree, has provided a firm scientific basis for the use of peripheral C-peptide concentrations as a semiquantitative marker of beta cell secretory activity in a variety of clinical situations. Thus, plasma C-peptide has proved to be extremely valuable in the study of the natural history of type 1 diabetes, to monitor insulin secretion in patients with insulin antibodies, and as an adjunct in the investigation of patients with hypoglycemic disorders. The use of the peripheral C-peptide concentration to accurately quantitate the rate of insulin secretion is more controversial. This is mainly because understanding of the kinetics and metabolism of C-peptide under different conditions is incomplete. Unfortunately, sufficient quantities of human C-peptide are not available to allow the experimental validation of the mathematical formulae that have been proposed for the calculation of insulin secretion from peripheral C-peptide concentrations. Until it is possible to perform such experiments, the accuracy of studies that have derived insulin secretion rates from peripheral C-peptide levels will remain uncertain. The assumption that the peripheral C-peptide:insulin molar ratio can be used as a reflection of hepatic insulin extraction has not been experimentally validated. The marked difference in the plasma half-lives of insulin and C-peptide complicates the interpretation of changes in their ratios.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1984

Naloxone decreases centrally induced hyperglycemia in dogs. Evidence for an opioid role in glucose homeostasis.

Intracerebroventricular (ICV) instillation of morphine and beta-endorphin causes centrally induced hyperglycemia. Locally active, endogenous opioids in the central nervous system may, therefore, also be involved in the elevation of blood sugar. This possibility was tested by examining the glucoregulatory response to central glucoprivation induced by ICV administration of 2-deoxy-D-glucose (2DG) in dogs. Administration of 2DG resulted in a rise in plasma glucose and immunoreactive glucagon (IRG) of 108 +/- 19 mg/dl and 70 +/- 20 pg/ml, respectively. These changes were attenuated by the simultaneous central infusion of the opiate antagonist naloxone: plasma glucose levels increased by 77 +/- 14 mg/dl and IRG by 43 +/- 3 pg/ml, both significantly different from the effect of 2DG alone (P less than 0.05-0.01). These findings suggest that opiate receptors participate in the counterregulatory response to central glucoprivation. They also provide a mechanism by which endogenous opioid peptides may play a role in the central regulation of glucose homeostasis.

Animals↗
PubMed1984

Characterization of lipoprotein produced by the perfused rhesus monkey liver.

Isolated livers from rhesus monkeys (Macaca mulatta) were perfused in order to asses the nature of newly synthesized hepatic lipoprotein. Perfusate containing [3H]leucine was recirculated for 1.5 hr, followed by an additional 2.5-hr perfusion with fresh perfusate. Equilibrium density gradient ultracentrifugation clearly separated VLDL from LDL. The apoprotein composition of VLDL secreted by the liver was similar to that of serum VLDL. The perfusate LDL contained some poorly radiolabeled, apoB-rich material, which appeared to be contaminating serum LDL. There was also some material of an LDL-like density, which was rich in radiolabeled apoE. Rate zonal density gradient ultracentrifugation fractionated HDL. All perfusate HDL fractions had a decreased cholesteryl ester/unesterified cholesterol ratio, compared to serum HDL. Serum HDL distributed in one symmetric peak near the middle of the gradient, with coincident peaks of apoA-I and apoA-II. The least dense fractions of the perfusate gradient were rich in radiolabeled apoE. The middle of the perfusate gradient contained particles rich in radiolabeled apoA-I and apoA-II. The peak of apoA-I was offset from the apoA-II peak towards the denser end of the gradient. The dense end of the HDL gradient contained lipoprotein-free apoA-I, apoE, and small amounts of apoA-II, probably resulting from the relative instability of nascent lipoprotein compared to serum lipoprotein. Perfusate HDL apoA-I isoforms were more basic than serum apoA-I isoforms. Preliminary experiments, using noncentrifugal methods, suggest that some hepatic apoA-I is secreted in a lipoprotein-free form. In conclusion, the isolated rhesus monkey liver produces VLDL similar to serum VLDL, but produces LDL and HDL which differ in several important aspects from serum LDL and HDL.

Animals↗
PubMed1983

Aldose reductase inhibition improves nerve conduction velocity in diabetic patients.

To assess the potential role of polyol-pathway activity in diabetic neuropathy, we measured the effects of sorbinil--a potent inhibitor of the key polyol-pathway enzyme aldose reductase--on nerve conduction velocity in 39 stable diabetics in a randomized, double-blind, cross-over trial. During nine weeks of treatment with sorbinil (250 mg per day), nerve conduction velocity was greater than during a nine-week placebo period for all three nerves tested: the peroneal motor nerve (mean increase [+/- S.E.M.], 0.70 +/- 0.24 m per second, P less than 0.008), the median motor nerve (mean increase, 0.66 +/- 0.27, P less than 0.005), and the median sensory nerve (mean increase, 1.16 +/- 0.50, P less than 0.035). Conduction velocity for all three nerves declined significantly within three weeks after cessation of the drug. These effects of sorbinil were not related to glycemic control, which was constant during the study. Although the effect of sorbinil in improving nerve conduction velocity in diabetics was small, the findings suggest that polyol-pathway activity contributes to slowed nerve conduction in diabetics. The clinical applicability of these observations remains to be determined, but they encourage further exploration of this approach to the treatment or prevention of diabetic neuropathy.

Adult↗
PubMed1983

Comparison of peripheral glucose uptake after oral glucose loading and a mixed meal.

Forearm glucose uptake (FGU) and other metabolic responses were studied in six normal men for three hours after a 75-g oral glucose load and a mixed meal containing 75 g carbohydrate. After the meal the rise in arterial glucose levels was considerably less than that following the oral glucose load but the overall insulin responses from 0 to 180 minutes were not statistically different. Although the initial rise in FGU was more gradual after the meal, the subsequent elevation was more sustained and, at the termination of the study, exceeded significantly that seen after the oral glucose load. The rise in GIP levels during the first hour was similar after the meal and the oral glucose load, but thereafter concentrations following the oral glucose load fell while those after the meal continued to rise. When the incremental area (delta) is used as the index of response, the results show that while the glucose response (delta G) after the meal (19.1 +/- 5.3 units) was only 26% of that after oral glucose loading (72.7 +/- 7.0 units), the corresponding increase in FGU (delta FGU) reached 62% (55.0 +/- 12.8 units after the meal, 89.2 +/- 20.0 units after the oral glucose load). Thus, the increase in peripheral glucose uptake relative to the glycemic response (delta FGU/delta G) was significantly greater after the meal than following the oral glucose load alone (P less than 0.05). In conclusion, relative to the rise in arterial glucose levels, peripheral glucose uptake is greater after a meal than after glucose loading with an equivalent carbohydrate challenge. Furthermore, the present data support previous studies emphasizing the failure of GIP alone to explain the entero insular axis.

Administration, Oral↗
PubMed1983

Peripheral glucose uptake in young men with myocardial infarction.

Forearm glucose uptake (FGU) was studied during 100 g oral glucose tolerance tests (GTT) in nonobese, nondiabetic men who had suffered a myocardial infarction (MI) at or before the age of 40, and the results compared with the response in age-matched normal men. In the MI group the rise in both glucose and insulin concentrations after glucose loading was similar to that in normal subjects, although in the former, peak levels tended to be slightly higher. Concomitant FGU, however, was significantly greater in the MI group than in control subjects in the period 0-90 min and in the test as a whole (0-180 min). The results show that at least in some nondiabetics suffering MI at an early age hyperinsulinism is not a feature and peripheral tissue sensitivity is increased.

Adult↗
PubMed1983

Studies on mutant human insulin genes: identification and sequence analysis of a gene encoding [SerB24]insulin.

Both alleles of the insulin gene of a patient with mild diabetes [maturity-onset-diabetes-of-the-young (MODY)-type syndrome] associated with hyperinsulinemia have been cloned, and the sequences have been determined. One allele contained a mutation (single nucleotide transition) in the coding sequence for the B chain at position 24 (TTC leads to TCC), resulting in the loss of a restriction enzyme (Mbo II) cleavage site in the gene. This mutation results in the substitution of serine for phenylalanine in a critically important region of the insulin molecule that is intimately involved in receptor binding. Both insulin alleles were of the alpha type and, aside from a single nucleotide deletion in the 5' region of the normal allele, their sequences were identical to those previously determined.

Amino Acid Sequence↗
PubMed1983

Identification of a mutant human insulin predicted to contain a serine-for-phenylalanine substitution.

Using information gained from (i) the relative HPLC retention of an abnormal insulin present in the serum of a hyperinsulinemic diabetic patient and (ii) the loss of an Mbo II restriction site in one of the patient's insulin gene alleles, it was recently predicted that the mutant insulin contained a serine-for-phenylalanine substitution at position B24 or B25. We have now prepared human [SerB24]insulin and [SerB25]insulin by solid-phase peptide synthesis and semisynthesis using an improved approach whereby the protecting groups can be removed from the final product in a single step. During reversed-phase HPLC analysis, the two semisynthetic insulins were clearly separated from normal insulin and from each other. Analysis of the patient's immunoaffinity-purified serum insulin by HPLC and radioimmunoassay showed that the insulin eluted at the position of [SerB24]insulin and coeluted with the analog when the two were studied in admixture. Additional studies showed that [SerB24]insulin and [SerB25]insulin have about 16% and 0.5% of the activity of normal insulin, respectively, in stimulating glucose oxidation by isolated rat adipocytes. We conclude that the patient's abnormal insulin (insulin Los Angeles) is human [SerB24]insulin and that abnormal insulins with amino acid replacements at both positions B24 and B25 can be associated with human diabetes.

Amino Acid Sequence↗
PubMed1983

Metabolism of C-peptide in the dog. In vivo demonstration of the absence of hepatic extraction.

The in vivo hepatic metabolism of connecting peptide (C-peptide) in relation to that of insulin has not been adequately characterized. A radioimmunoassay for dog C-peptide was therefore developed and its metabolism studied in conscious mongrel dogs, with sampling catheters chronically implanted in their portal and hepatic veins and femoral artery. The hepatic extraction of endogenous C-peptide under basal conditions was negligible (4.3 +/- 4.5%) and was similar to the hepatic extraction of C-peptide measured during the constant exogenous infusion of C-peptide isolated from dog pancreas. Simultaneously measured hepatic extraction of endogenous and exogenously infused insulin were 43.8 +/- 7.6 and 47.5 +/- 4.4%, respectively. The metabolic clearance rate of infused C-peptide was 11.5 +/- 0.8 ml/kg per min and was constant over the concentration range usually encountered under physiological conditions. In additional experiments, the effect of parenteral glucose administration on the hepatic extraction of C-peptide and insulin was investigated. The hepatic extraction of C-peptide (6.2 +/- 4.0%) was again negligible in comparison with that of insulin (46.7 +/- 3.4%). Parenteral glucose administration did not affect the hepatic extraction of either peptide irrespective of whether it was infused peripherally, intraportally, or together with an intraportal infusion of gastrointestinal inhibitory polypeptide. The fasting C-peptide insulin molar ratio in both the portal vein (1.2 +/- 0.1) and femoral artery (2.1 +/- 0.3) was also unaffected by the glucose stimulus. These results therefore indicate that, since the hepatic extraction of C-peptide is negligible and its clearance kinetics linear, the peripheral C-peptide concentration should accurately reflect the rate of insulin secretion. New approaches to the quantitation of hepatic extraction and secretion of insulin by noninvasive techniques are now feasible.

Animals↗
PubMed1983

The production and characterization of monoclonal antibodies specific for human proinsulin using a sensitive microdot assay procedure.

The development of a simple microscale solid phase screening RIA and improved methods for cell cloning has lead to the establishment of 2 prohormone- and species-specific mouse monoclonal antibodies against human proinsulin (HPI). These antibodies react to determinant(s) only expressed on the HPI molecule. In addition, 11 rat monoclonal antibodies were generated which react with both human C-peptide (HCP) and HPI. All 11 rat antibodies recognize a very similar antigenic determinant in the C-peptide that appears to be made up of residues 40-45 and 57-63, which are probably brought into close proximity by a beta-turn near the center of the connecting segment. The identical behavior of both HPI-specific mouse antibodies in competition experiments indicates that the antigenic structure recognized in proinsulin might be the same for both antibodies. This structure could not be regenerated by mixing equimolar amounts of human insulin and C-peptide, including the chemically synthesized complete proinsulin connecting segment (Arg X Arg X HCP X Lys X Arg), which contains the entire sequence removed from proinsulin in the conversion to mature insulin. Indirect evidence is provided that a HPI molecule simultaneously can bind a C-peptide-directed rat antibody and a HPI-specific mouse antibody when the first antibody is presented to HPI in solution phase. However, when HPI is immobilized on a plastic surface, the binding of 1 type of antibody completely blocks the binding of the other. These antibodies provide useful new tools for studying the biosynthesis and 3-dimensional structure of HPI and HCP.

Animals↗
PubMed1983

Lack of glucagon response to hypoglycemia in type I diabetics after long-term optimal therapy with a continuous subcutaneous insulin infusion pump.

Counterregulatory hormonal responses were studied in six patients after 4-18 mo treatment with a continuous subcutaneous insulin infusion pump. In response to insulin-induced hypoglycemia, significant increases in epinephrine, norepinephrine, cortisol, and growth hormone were measured in all subjects, while in five of the six patients glucagon levels did not increase at all. The persistence of these abnormal glucagon responses despite long-term optimal glucose control suggests that they are not due to hyperglycemia per se, but are due rather to a specific alpha cell abnormality. The high incidence of asymptomatic hypoglycemia in these patients emphasizes that caution is necessary to avoid serious hypoglycemia when striving for near-normal glucose control with insulin infusion pump therapy.

Adult↗
PubMed1983

Quantitative determination of islet cell surface antibodies using 125I-protein A.

A quantitative method to measure islet cell surface antibodies in human patients has been developed using 125I-protein A. Isolated, dispersed, viable rat islet cells prepared by collagenase digestion were fixed in 4% paraformaldehyde to allow storage for up to 7 wk at 4 degrees C. Human sera, heat inactivated and adsorbed with rat liver and kidney powder (100 mg/ml), were incubated with the fixed cells (50 x 10(3)) for 60 min at 37 degrees C. Thereafter the cells were washed and exposed to 5 x 10(5) cpm 125I-protein A, which binds to IgG attached to the cell surface. Assay precision (14%) and reproducibility (16%) were established by repeated analysis of pooled sera from healthy individuals and IDDM patients using pooled batches of islet cells. Using this method, islet cell surface antibodies were detected in 35% of insulin-dependent diabetic patients.

Adult↗
PubMed1983

Identification of a point mutation in the human insulin gene giving rise to a structurally abnormal insulin (insulin Chicago).

Both insulin gene alleles of a diabetic patient with a mutant insulin were cloned in a lambda vector and their nucleotide sequences were determined. Nucleotide sequence analysis revealed, in one allele, a C (cytidylate) to G (guanylate) transversion in the codon for phenylalanine at position 25 of the insulin B-chain. This point mutation leads to the substitution of a leucine for phenylalanine accompanied by the loss of a restriction endonuclease Mboll recognition site and the creation of a new Rsal cleavage site at this position.

Amino Acid Sequence↗
PubMed1982

Feedback inhibition of insulin secretion by insulin: relation to the hyperinsulinemia of obesity.

We investigated the possible existence of a negative short-loop feedback of circulating insulin on the parent beta cell in 10 lean Caucasians, 10 obese Caucasians, and 10 obese Pima Indians. Plasma insulin levels were raised acutely by 100 microunits per milliliter for 90 minutes, and plasma glucose was maintained by the "clamp" technique. C-peptide levels were suppressed in all groups to approximately 50 per cent of basal values. However, the obese groups had absolute C-peptide levels much higher than those of the lean group. During the hour after infusion, the rate and magnitude of C-peptide recovery in the obese groups were higher than in the lean group. Thus, negative short-loop insulin-beta-cell feedback was operative in both the lean and obese states. Despite this suppression, the insulin-secretion rate in obese subjects was still greater than that in non-obese subjects. Inadequate feedback suppression may account in part for the prevailing hyperinsulinemia of the obese.

Adult↗
PubMed1982

Absence of exercise-induced hypoglycaemia in type i (insulin-dependent) diabetic patients during maintenance of normoglycaemia by short-term, open-loop insulin infusion.

To assess the risk and possible mechanisms of hypoglycaemia during moderate exercise in Type I (insulin-dependent) diabetic patients receiving constant insulin infusion, five insulin-dependent male diabetic patients were exercised 18 h after their last meal and 30 h after their last injection of intermediate acting insulin. Intravenous insulin was initially delivered via a closed-loop infusion system programmed to lower mean blood glucose from 11.3 +/- 1.8 to 4.8 +/- 0.4 mmol/l over approximately 3.5 h. Blood glucose was then maintained at this level for 4 h. At this time, the closed-loop infusion was discontinued and replaced by an open-loop system. The average amount of insulin infused per min during the 4 h normoglycaemic closed-loop period was calculated and this amount was infused at a constant rate during both a 30 min period of exercise on a bicycle ergometer (approximately 65% maximum oxygen uptake) and a 30 min rest period which followed. Five nondiabetic males served as control subjects. Despite significantly higher free insulin concentrations (p less than 0.05) and identical pre-exercise blood glucose concentrations, blood glucose rose during exercise only in the diabetic group (0.5 +/- 0.2 mmol/l; p less than 0.01). Changes in the serum concentrations of lactate, glycerol, glucagon, cortisol, non-esterified fatty acids and growth hormone were similar in the two groups and did not account for the increment of blood glucose in the diabetic patients. Beta-hydroxybutyrate concentrations were, however, higher in the diabetic patients at the onset of exercise (p less than 0.01) and decreased significantly more than the control subjects during exercise. We conclude that exercise under these conditions in diabetic patients is not attended by hypoglycaemia.

3-Hydroxybutyric Acid↗