Antilipolytic activity of insulin and proinsulin on ACTH and cyclic nucleotide-induced lipolysis in the isolated adipose cell of rat

Does C-peptide have a physiological role?

Short-term administration of physiological amounts of C-peptide to patients with insulin-dependent diabetes was found to reduce the glomerular hyperfiltration in these patients as well as augment whole body glucose utilization. It could also be shown that C-peptide administration increases blood flow, oxygen uptake and capillary diffusion capacity of exercising forearm muscle in IDDM patients, probably by increasing capillary recruitment in the working muscle. Studies under in vitro conditions have shown that C-peptide stimulates glucose transport in skeletal muscle with its maximal effect within the physiological concentration range. The findings in a clinical study in which IDDM patients were given C-peptide and insulin or insulin alone for 4 weeks in a double-blind randomized study design, indicate that C-peptide improves renal function by reducing urinary albumin excretion and glomerular filtration, decreases blood retinal barrier leakage and improves metabolic control. Preliminary findings suggest that C-peptide administration on a short-term basis (3h) may ameliorate autonomic neuropathy by restoring to near normal the heart rate variability in response to expiration and inspiration. Insight into a possible mechanism of action of C-peptide is provided by the finding that C-peptide stimulates Na+K(+)-ATPase activity in renal tubular segments. In conclusion, the present results suggest that, contrary to the prevailing view, C-peptide possesses important physiological effects.

Effects of C-peptide on blood flow, capillary diffusion capacity and glucose utilization in the exercising forearm of type 1 (insulin-dependent) diabetic patients

Microvascular dysfunction is frequently seen in patients with Type 1 (insulin-dependent) diabetes. The present study was undertaken to examine whether skeletal muscle microcirculation in Type 1 diabetic patients is influenced by C-peptide. Forearm blood flow, capillary diffusion capacity and substrate exchange were studied during strenuous rhythmic forearm exercise on a hand ergometer. Measurements were made before and during i.v. infusion for 60 min of C-peptide or 0.9% NaCl in Type 1 diabetic patients and healthy subjects. During infusion the C-peptide levels in the diabetic patients increased from less than 0.05 nmol/l to 1.32 +/- 0.08 nmol/l. Prior to infusion forearm blood flow and capillary diffusion capacity during exercise were lower in the diabetic patients than the control subjects. During C-peptide infusion both variables increased in the diabetic patients (blood flow +27 +/- 4%, capillary diffusion capacity +52 +/- 9%) to levels similar to those in the healthy subjects, while no significant change was seen in the healthy control subjects or the diabetic patients given NaCl. Forearm uptake of oxygen and glucose in the diabetic patients increased markedly after C-peptide administration but were unchanged after NaCl infusion. Significant uptake of C-peptide to the deep forearm tissues was observed in the resting state; approximately 7 +/- 2% of the arterial C-peptide concentration was extracted by forearm tissues in diabetic patients as well as in healthy control subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

The specific protein phosphatase inhibitor okadaic acid differentially modulates insulin action

The pleiotropic nature of insulin action suggests diverse mechanisms of signal transduction for the hormone. The specific protein phosphatase inhibitor, okadaic acid, is utilized to differentiate metabolic pathways that may be regulated by phosphorylation or dephosphorylation of key enzymes. In H-35 hepatoma cells, okadaic acid inhibits insulin-stimulated glycogen synthesis with an IC50 of 400 nM. In contrast, activation of lipogenesis by insulin is inhibited with an IC50 of 50 nM okadaic acid. The toxin also inhibits stimulation of lipogenesis in these cells by the insulin-sensitive inositol glycan enzyme modulator. In isolated rat adipocytes, insulin-stimulated lipogenesis is also inhibited by okadaic acid with an IC50 of approximately 1,700 nM. The antilipolytic effect of insulin in these cells is more sensitive to okadaic acid, exhibiting an IC50 of 150 nM. Maximal activation of lipogenesis by insulin is dramatically reduced by okadaic acid with no effect on the concentration required for half-maximal activation, whereas the sensitivity of insulin-induced antilipolysis is attenuated by okadaic acid, with no apparent reduction in the maximal effect of the hormone. Taken together, these data suggest that specific phosphatases may be differentially involved in some of the metabolic pathways regulated by insulin.

Lack of effect of high-dose biosynthetic human C-peptide on pancreatic hormone release in normal subjects

We studied the effect of high doses of biosynthetic human C-peptide on pancreatic hormone secretion in response to oral (75 g) and intravenous [( IV] 0.33 g/kg of D50%) glucose on normal volunteers. The infusion of human C-peptide at a rate of 360 ng/kg/min body weight, increased the plasma C-peptide concentration from a basal level of 0.32 +/- 0.04 pmol/mL to 38.5 +/- 1.8 pmol/ml. Overall, C-peptide had no significant effect on the serum levels of glucose, insulin, proinsulin, glucagon, and pancreatic polypeptide, either under basal conditions or following IV and oral glucose administration. However, small decreases in glucose and insulin concentrations that were not statistically significant were seen during the first hour after C-peptide infusion. The results of the present studies are therefore consistent with the conclusion that even supraphysiologic plasma concentrations of infused C-peptide do not affect basal insulin secretion or overall insulin secretory responses to oral or IV glucose. However, we cannot definitively exclude a small reduction in insulin secretion in the first hour after oral glucose ingestion.

The insulin-like effects of phorbol myristate acetate (PMA) in the isolated fat cell

Recent data from many laboratories suggest that insulin stimulates diacylglycerol formation. Data presented in this manuscript demonstrate an insulin-like effect of PMA, a tumor promoting agent that mimics the action of diacylglycerol, in isolated adipocytes on; (a) glucose oxidation using uniformly labelled, C-1-labelled and C-6-labelled glucose, (b) epinephrine-induced lipolysis and (c) low Km cAMP phosphodiesterase activity. Additionally, a lipolytic effect of PMA is identified when unopposed by epinephrine. These data not only demonstrate an insulin-like effect of phorbol esters in adipose tissue but they lend support to the concept of diacylglycerol involvement in the mechanism of insulin action.

Is EGF a physiological inhibitor of mouse mammary casein synthesis? Unphysiological responses to pharmacological levels of hormones

It has been observed that EGF inhibits the induction of casein synthesis by mouse mammary tissue in vitro in addition to acting as a promoter of mammary epithelial proliferation. However, since the circulating level of EGF increases during lactation, and since functional EGF receptors are retained by the lactating cells, it seemed unlikely that EGF is an inhibitor of mammary differentiation in vivo. The current studies demonstrate, in fact, that EGF inhibits the induction of casein synthesis in vitro only when insulin is present in the culture medium at unphysiologically high concentrations. Other artifactual responses to high levels of hormones are described.

Insulin stimulates the generation from hepatic plasma membranes of modulators derived from an inositol glycolipid

Insulin binding to plasma membrane receptors results in the generation of substances that acutely mimic the actions of the hormone on certain target enzymes. Two such substances, which modulate the activity of the high-affinity cAMP phosphodiesterase (EC 3.1.4.17), have been purified from hepatic plasma membranes. The two have similar properties and activities but can be resolved by ion-exchange chromatography and high-voltage electrophoresis. They exhibit a net negative charge, even at pH 1.9, and an apparent molecular weight of approximately 1400. The generation of these substances from membranes by insulin can be reproduced by addition of a phosphatidylinositol-specific phospholipase C purified from Staphylococcus aureus. This enzyme is known to selectively hydrolyze phosphatidylinositol and release from membranes several proteins that are covalently linked to phosphatidylinositol by a glycan anchor. Both enzyme-modulating substances appear to be generated by the phosphodiesterase cleavage of a phosphatidylinositol-containing glycolipid precursor that has been characterized by thin-layer chromatography. Some of the chemical properties of these substances have been examined. They appear to be related complex carbohydrate-phosphate substances containing glucosamine and inositol. These findings suggest that insulin may activate a selective phospholipase activity that hydrolyzes a membrane phospholipid, releasing a carbohydrate-containing molecule that regulates cAMP phosphodiesterase and perhaps other insulin-sensitive enzymes.

C-peptide measurement: methods and clinical utility

Proinsulin is the single chain precursor of insulin. It consists of insulin, plus a peptide which connects the A and B chains of insulin. This peptide is termed C-peptide. C-peptide an insulin are secreted in equimolar amounts from pancreatic beta-cells, Hence, circulating C-peptide levels provide a measure of beta-cell secretory activity. C-peptide measurements are preferable to insulin measurements because of lack of hepatic extraction, slower metabolic clearance rate, and lack of cross reactivity with antibodies to insulin. This article reviews the methods for determination of C-peptide levels in body fluids, and discusses the applications of C-peptide measurement. These include the investigation of hypoglycemia and the assessment of insulin secretory function in insulin-treated and non-insulin-dependent diabetics. The contribution of C-peptide measurement to the understanding of the interrelationships between insulin secretory function and age, sex, obesity, blood lipids, and blood glucose concentrations will also be evaluated.

Putative mediators of insulin action: regulation of pyruvate dehydrogenase and adenylate cyclase activities

Recent evidence suggests that certain actions of insulin may be mediated by the selective generation of chemically undefined intracellular substances. Incubation of rat liver particulate fraction with low concentrations of insulin enhances the release into the supernatant of a substance that stimulates mitochondrial pyruvate dehydrogenase. Higher concentrations of insulin release less stimulating activity. It is possible to resolve activities that stimulate and inhibit pyruvate dehydrogenase by differential ethanol extraction of the supernatant solutions. The elaboration of both factors is dependent upon the presence of insulin in a dose-dependent manner. Moreover, fractions that contain the pyruvate dehydrogenase-inhibiting activity also inhibit adipocyte basal and hormonally stimulated adenylate cyclase. The production of this adenylate cyclase inhibitory activity is also stimulated by insulin. Cyclase inhibition is virtually abolished when the nonhydrolyzable ATP analog, 5'-adenylyl imidodiphosphate, is included in the assay. These results indicate that the bimodal effects of insulin on certain functions may be ascribed to the generation of at least two distinct chemical substances that show opposing activities, which may operate by regulating phosphorylation reactions.

Purification to homogeneity of an insulin-degrading enzyme from human erythrocytes

The purification of an enzyme is described, a protease, from human erythrocytes which degrades insulin with a high specificity at physiological hormone concentrations. Since the enzyme contains free sulfhydryl groups, affinity chromatography on organomercuri-Sepharose proved to be applicable as a valuable step in the isolation procedure. The purification factor amounted to approx. 6000, the yield to 8%. 1mg of purified enzyme was capable of degrading 50 pmol of insulin/min into trichloroacetic acid-soluble split products. The purified insulin-degrading enzyme was shown to be homogeneous, as demonstrated by gel chromatography, gel electrophoresis and isoelectric focusing. The isoelectric points was at pH 5.8. The molecular weight of nativ enzyme was estimated by gel chromatography and gel electrophoresis and found to be about 150 000-160 000, consisting of 4 subunits. Degradation products of insulin eluted from a Biogel P 30 column are smaller than the A-chain of the hormone, suggesting the activity of a protease. The enzyme appears to be specific for insulin in that it does not degrade other peptide hormones such as growth hormone, prolactin, or thyroid-stimulating hormone. Furthermore, the enzyme does not inactivate enzymes such as lactate dehydrogenase, aldolase, fructose 1,6-bisphosphatase, hexosephosphate isomerase or hexokinase.

Proinsulin and C-peptide: a review

The recent work on proinsulin and C-peptide has been reviewed with major emphasis on the most significant findings since 1972. Proinsulin has now been established as the biosynthetic precursor of insulin in all species examined, including man, with a preproinsulin as a possible precursor of the prohormone. The conversion of proinsulin which appears to occur exclusively in the pancreas leads to equimolar production of insulin and C-peptide. Although proinsulin has a direct biologic effect which is one-tenth as much as that of insulin, C-peptide has no biologic activity on homologous or heterologous tissue and no ability to modify the action of insulin and/or proinsulin. Previous work on proinsulin immunoassay suggested that this prohormone, but not C-peptide, cross-reacts with insulin antiserum. On the other hand, in the C-peptide immunoassay, proinsulin but not insulin cross-reacts with the antiserum. Up to this time, therefore, it has not been possible to immunoassay human proinsulin or C-peptide specifically. The very recent work from the laboratory of Heding, however, has brought about major advances in this area in which human C-peptide and proinsulin can be separated in the plasma by the use of Sepharose particles. With this recent major advancement, it is now possible to measure human C-peptide specifically. This measurement has been shown to be a useful tool for the assessment of beta-cell function in diabetic patients treated with insulin and in insulinoma patients in whom endogenous C-peptide secretion is not suppressed with exogenous insulin-induced hypoglycemia. With the use of a specific enzyme which degrades insulin but not proinsulin, postprandial plasma proinsulin values have been measured in a large number of subjects under a variety of physiologic and pathologic conditions. These results, which are comparable to those obtained by the more laborious column chromatography, could be summarized as follows: (1) proinsulin values in lean, young normal subjects do not vary greatly in response to insulin secretagogues; (2) proinsulin secretion in response to glucose results in a greater percentage of proinsulin in the older age group than in the younger group; (3) in lean adult and juvenile diabetic patients, the percentage of proinsulin is not excessive, whereas obese diabetics and pregnant diabetics appear to secrete relatively greater proinsulin than their diabetic controls; and (4) whereas most hyperinsulinemic states (Cusing's syndrome, adult-onset diabetics, acromegaly, and glucocorticoid therapy) are not associated with an increase in percentage of proinsulin, hyperinsulinemia of insulinoma, selected cases of functional hypoglycemia, and genetic hyperproinsulinemia are associated with a greater percentage of proinsulin. Identification of a possible new proinsulin intermediate(s) in these conditions deserves further investigation...

The relationship between the concentration of adenosine 3':5'-cyclic monophosphate and the anti-lipolytic action of insulin in isolated rat fat-cells

The relationship between cyclic AMP content and lipolysis, as measured by glycerol formation, was studied in isolated rat fat-cells. Inhibition of lipolysis by insulin in the presence of a low concentration of adrenaline was accompanied by little or no lowering of cyclic AMP content, measured after 15min incubation. The time-course of cyclic AMP content after addition of adrenaline showed that the effect of insulin in lowering cyclic AMP content measured after 2-5min was gradually lost over the next hour, mainly because of the fall in cyclic AMP content after an early peak in the presence of adrenaline alone. There was a 44% loss of immunoreactive insulin, from an initial concentration of 0.3nm, during a 1h incubation with fat-cells. Insulin did not affect partitioning of cyclic AMP between cells and incubation medium. When the correlation between cyclic AMP content and rate of lipolysis was investigated for a wide range of adrenaline concentrations, it was found that the lowering of cyclic AMP content by insulin was much less than that required to account for the amount of inhibition of lipolysis. It is concluded that inhibition of adrenaline-stimulated lipolysis by insulin involves factors in addition to a decrease in intracellular cyclic AMP concentration.

Modulation of adenylate cyclase activity in liver and fat cell membranes by insulin

Insulin depresses both the activity of adenylate cyclase stimulated by glucagon, epinephrine, and sodium fluoride in liver cell membranes and the activity of adenylate cyclase stimulated by epinephrine and adrenocorticotropin in particulate preparations from homogenates of isolated fat cells. Significant inhibition is detected with very low concentrations (10(-11) molar) of insulin but not with unphysiologically high (10(-9)molar) concentrations of the hormone. These direct effects of insulin on an enzymatic system in broken-cell -preparations suggest a fundamental role of adenylate cyclase activity and of cyclic adenosine monophosphate in the mechanism of action of insulin.