Metabolism of proinsulin, insulin, and C-peptide in the rat

A specific and sensitive radioimmunoassay for rat C-peptide

A sensitive and specific radioimmunoassay for rat serum C-peptide (RCP) has been developed and validated using a guinea pig anti-rat C-peptide antibody to synthetic rat C-peptide. Negligible crossreactivity (< 0.01%) to human proinsulin was observed, whereas human insulin, human pancreatic polypeptide (hPP), porcine insulin, porcine C-peptide, bovine insulin, rat insulin, porcine-PP, and glucagon, respectively, did not produce measurable displacement of RCP tracer. Human C-peptide even in a supraphysiological concentration range crossreacted poorly (< 0.1%). The sensitivity limit of the assay calculated at +/- 3 standard deviations was 24.2pM (0.07 ng/mL). RCP standard concentrations ranged from 25-1600pM. The intraassay- and between assay-coefficient of variations (CV) were 3.5-6.1% and 4.1-9.5%, respectively. The mean percentage recovery of RCP added to rat serum samples was 100.8 +/- 2%. Serum volume dilution from 25 to 100 microL did not significantly alter the expected RCP level. Migration of rat serum C-peptide and that of synthetic RCP were identical in a Sephadex G-50 chromatographic analysis. The mean fasting and postprandial plasma RCP levels in normal rats were 102 +/- 15 pM and 485 +/- 75pM, respectively. RCP levels following intravenous glucose tolerance test in diabetic and nondiabetic rats were consistent with expected patterns. In conclusion, we have developed and validated a rat C-peptide assay that is sensitive, simple, and specific for RCP in serum. The assay provides a reliable tool for studies of diabetes using rodent animal models.

Association between stimulated plasma C-peptide and age: the Wadena City Health Study

OBJECTIVE

To assess age-related changes in stimulated plasma C-peptide in a population-based sample of adults.

DESIGN

Cross-sectional study.

SETTING

Wadena, Minnesota, a city of 4,699 residents (1980 census) in west central Minnesota, approximately 150 miles from Minneapolis/St. Paul.

STUDY SUBJECTS

344 non-diabetic subjects (NDDG standards) from a stratified random sample of the total adult population of Wadena, MN. The six-study strata were men and women from three age groups: young, 20-39 years of age; middle-aged, 40-59; and older, greater than 60 years of age.

MEASUREMENTS

During a liquid meal of Ensure-Plus (Ensure-Plus challenge test; EPCT; Ross Laboratories), blood samples were taken for glucose, free fatty acids, creatinine, and C-peptide. Plasma C-peptide taken 90 minutes after the EPCT was used as a surrogate measure for insulin. Clinical tests included one-time samples for hemoglobin, glycosylated hemoglobin, plasma cholesterol, triglycerides, and lipoproteins. Physical measurements included height, weight, and blood pressure. Urine was assayed for C-peptide and creatinine. Assays of urine and plasma C-peptide used antibody M1221 (from Novo; Copenhagen, Denmark).

MAIN RESULTS

No differences were observed for the relationship between age and C-peptide within each of the three age groups for men and the three age groups for women. However, the levels of plasma C-peptide for older men or women were statistically significantly higher than levels for the young age groups of the same sex; fasting plasma glucose also was higher for older groups of both sexes, and postmeal glucose was significantly higher for older women. There were decreases with age in urine C-peptide clearance for women and men; the decline for women was statistically significant. In multiple regression models for men alone and women alone, that controlled for age, post-meal plasma glucose best explained plasma C-peptide levels. For young men, plasma glucose alone provided the best prediction of plasma C-peptide levels; body mass index (BMI) and plasma glucose provided the best prediction for young women. For older men and both middle-aged and older women, a combination of urine C-peptide clearance and plasma glucose best predicted plasma C-peptide levels; for middle-aged men, BMI also contributed to the prediction.

CONCLUSIONS

Secretion of insulin in response to an orally administered mixed meal is undiminished with age in non-diabetic adults.

Exocrine and endocrine functional reserve in the course of chronic pancreatitis as studied by maximal stimulation tests

Thirty patients suffering from chronic alcoholic pancreatitis (18 calcified) were entered into a study of exocrine and endocrine pancreatic function based on two maximal stimulation tests, namely the secretin-cerulein test and the glucagon test with serum assays of C peptide. The glucagon test was also performed in 19 control subjects. In addition, 10 chronic pancreatitis patients and nine controls were subjected to an oral glucose tolerance test (OGTT) with serum insulin determinations. C peptide basal values were decreased only in patients with severe pancreatic exocrine insufficiency (P less than 0.001), while delta C peptide values were also reduced in patients with moderate exocrine insufficiency (P less than 0.001). Lipase output correlated very well with delta C peptide values (P less than 0.001). While serum insulin levels during OGTT and C peptide basal values showed no significant differences between the chronic pancreatitis and control groups, delta C peptide values were significantly reduced in chronic pancreatitis patients (P less than 0.02). Both endocrine and exocrine function are impaired in chronic pancreatitis, as demonstrated by maximal tests, even in early stages of the disease.

Plasma levels of vasoactive regulatory peptides in patients receiving regular hemodialysis treatment

The fasting plasma levels of 10 vasoactive regulatory peptides were measured by radioimmunoassay in 23 stable patients with chronic renal failure receiving regular hemodialysis treatment (RDT) and compared with those of healthy controls. The plasma concentrations of arginine vasopressin, atrial natriuretic peptide, beta-endorphin, methionine-enkephalin, motilin, neuropeptide Y, substance P, and vasoactive intestinal peptide were increased. The plasma level of calcitonin gene-related peptide was not statistically different from that of the controls. The plasma concentration of gamma 2-melanocyte-stimulating hormone was lowered in the RDT-patients. The arterial blood pressure correlated with the plasma levels of motilin and neuropeptide Y. We conclude that patients with chronic renal failure receiving RDT have increased concentrations of 8 out of 10 measured vasoactive regulatory peptides. The elevated levels of vasoactive peptides may contribute to the adaptation of the cardiovascular system to impaired renal function.

Variation of endogenous insulin secretion in association with treatment status: assessment by serum C-peptide and modified urinary C-peptide

The variation of endogenous insulin secretion in association with fasting plasma glucose (FPG) level and the modality of treatment was assessed using serum C-peptide levels before and after breakfast and the corrected value of 24-h urinary C-peptide (24 h-UCP) in inpatients with non-insulin-dependent diabetes mellitus. The corrected value calculated as 24 h-UCP/(urinary C-peptide to creatinine clearance (CCP/CCR) ratio in the fasting state x 10) was correlated with the sum of day-long serum C-peptide levels (r = 0.93) more closely than the measured value of 24 h-UCP (r = 0.79) in 9 patients. In 52 patients treated with diet alone, 38 with sulfonylurea and 28 with insulin, fasting serum C-peptide level did not vary with FPG level, and the increment of serum C-peptide level after breakfast and the corrected value of 24 h-UCP decreased with the rise in FPG level in each treatment. These indexes were the lowest in insulin treatment among the patients with similar FPG levels. In conclusion, 24 h-UCP was demonstrated to be able to reflect day-long endogenous insulin secretion more faithfully after the correction with the CCP/CCR ratio. It was estimated that the insulin response to breakfast and day-long insulin secretion decreased with the rise in FPG level, but basal insulin secretion was maintained over a wide range of FPG levels in each treatment. Endogenous insulin secretion seemed to be somewhat suppressed or rested by exogenous insulin in insulin-treated patients.

Contribution of hyperglycemia and renal damage to urinary C-peptide clearance in non-insulin-dependent diabetic patients

The variation of urinary C-peptide clearance in relation to hyperglycemia and renal damage was evaluated in 57 patients with non-insulin-dependent diabetes mellitus (NIDDM) with and without overt proteinuria, 14 nondiabetic patients with renal disease (RD) and 18 healthy control subjects. Urinary C-peptide clearance expressed as the ratio of urinary C-peptide to creatinine clearance (CCP/CCR) in the fasting state did not differ in control subjects and RD patients, and was higher equally in NIDDM patients with and without proteinuria. In NIDDM patients without overt proteinuria, urinary levels of C-peptide, beta 2-microglobulin (B2M), N-acetyl-beta-D-glucosaminidase (NAG) and albumin as well as CCP/CCR ratio all decreased significantly with short-term glycemic control (P less than 0.05). Despite a wide range of CCP/CCR ratio (0.07-0.73), a weak but significant correlation (r = 0.56, P less than 0.005) was found between fasting serum and urinary C-peptide levels in NIDDM patients. These results suggest that urinary C-peptide may easily be affected by hyperglycemia per se rather than renal damage, while urinary B2M, NAG and albumin may be affected by both hyperglycemia and renal damage. When the urinary C-peptide level is interpreted, the influence of hyperglycemia on it must be taken into consideration.

The influence of regional adiposity on atherogenic risk factors in men and women with type 2 diabetes

To determine firstly whether body fat distribution could predict the presence of atherogenic risk factors better than overall adiposity in Type 2 diabetes, and secondly whether sex differences in these risk factors could be explained by sex differences in fat distribution, waist-to-hip girth ratio (WHR), serum lipids, lipoproteins, apolipoproteins, plasma lipolytic activity, and blood pressure were assessed in 47 patients with Type 2 diabetes, 21 women matched for age, body mass index (BMI) and blood glucose control with 26 men. The men had higher WHR (0.95 (range 0.83-1.07) vs 0.82 (0.74-0.94), p less than 0.001), lower HDL-cholesterol (1.03 +/- 0.05 vs 1.38 +/- 0.06 mmol l-1, p less than 0.001) and apolipoprotein A1 (1.40 +/- 0.06 vs 1.76 +/- 0.06 gl-1, p less than 0.001) concentrations, and higher hepatic lipase activities (16.2 (6.4-38.0) vs 8.6 (2.3-23.1) mmol h-1 l-1, p less than 0.01). In both men and women, BMI and WHR were positively related to serum triglyceride, insulin and C-peptide concentrations. In women, HDL-cholesterol was negatively related to BMI (r = -0.45, p less than 0.05) but only possibly related to WHR (r = -0.33, NS). In men, by contrast, WHR was related negatively to HDL-cholesterol (r = -0.60, p less than 0.005), HDL2-cholesterol (r = -0.43, p less than 0.05), and apolipoprotein A1 (r = -0.70, p less than 0.001) and positively to hepatic lipase activity (r = 0.65, p less than 0.001), whereas the same relationships with BMI were not significant.(ABSTRACT TRUNCATED AT 250 WORDS)

Extrapancreatic insulin effect of glibenclamide

In eight patients with uncomplicated non insulin dependent diabetes mellitus, serum insulin levels, serum C-peptide levels and blood glucose levels were measured before and after oral administration of glibenclamide 0.1 mg/kg body weight and a test meal, or after a test meal alone. The rise in serum insulin levels persisted longer after glibenclamide. The initial rise in serum insulin was of the same magnitude in both situations, as was the rise in serum C-peptide levels during the entire 5 h study. It is concluded that glibenclamide is able to maintain a more prolonged increase in serum insulin levels by inhibiting the degradation of insulin in the vascular endothelial cells of the liver. The inhibition contributes to the blood glucose lowering effect of glibenclamide.

Clinical pharmacokinetics of epoetin (recombinant human erythropoietin)

Epoetin (recombinant human erythropoietin, EPO) is of proven benefit in the treatment of renal anaemia, and preliminary reports suggest that it may have a role in the management of other anaemic conditions. Pharmacokinetic and therapeutic studies have examined the use of epoetin administered intravenously, intraperitoneally and subcutaneously, and there is accumulating evidence that the last route has several advantages. After intravenous administration, epoetin is distributed in a volume comparable to the plasma volume, and plasma concentrations decay monoexponentially with a half-life of between 4 and 12 hours. Administration of epoetin in peritoneal dialysis fluid results in detectable concentrations in the bloodstream after 1 to 2 hours, and peak concentrations of the order of 2 to 4% of those obtained with the same intravenous dose are found after approximately 12 hours. The bioavailability of epoetin administered intraperitoneally in dialysis fluid is about 3 to 8%, but this may be increased by injecting the drug into a dry peritoneal cavity. Subcutaneous administration results in peak concentrations at about 18 hours which are 5 to 10% of those found after the same intravenous dose. The bioavailability of subcutaneous epoetin is about 20 to 30% and detectable serum concentrations are still present 4 days after administration, in contrast to intravenous administration after which concentrations have returned to baseline within 2 to 3 days. Remarkably little is known about the metabolic fate of either erythropoietin or epoetin. In addition, there is much controversy surrounding the relative roles of the kidney and liver in the catabolism of epoetin. About 3 to 10% of epoetin is excreted unchanged in the urine. In common with other glycoproteins, the carbohydrate residues which constitute 40% of its molecular size are essential for maintaining the stability of epoetin in circulation. Desialated epoetin, although biologically active in vitro, is cleared very rapidly from plasma with resultant loss of activity. Further work is required, however, in identifying the pathways of metabolism and elimination of this glycoprotein hormone.

Proteinuria: changes and mechanisms in toxic nephropathies

During the last few decades, considerable progress has been made in the understanding of the pathophysiological mechanisms of proteinuria. A great variety of hemodynamic or biochemical mechanisms acting at different sites of the nephron have been shown to alter the renal handling and the urinary excretion of proteins. The finding which perhaps has had most practical implications is that the pattern of protein excretion quantitatively and qualitatively varies with the site and severity of renal damage. This has led to the development of a large array of methods for the identification and quantitation of specific urinary proteins. These methods have been most extensively used by toxicologists in experimental, epidemiological, or clinical studies on potentially nephrotoxic chemicals (e.g., drugs, heavy metals, solvents, etc.). The present review summarizes the current state of knowledge on the mechanisms of proteinuria and the use of urinary proteins as indicators of nephrotoxicity.

Glucose intolerance and hyperinsulinemia of cirrhosis are not results of spontaneous or surgical portosystemic shunting

To assess if spontaneous portosystemic shunting from collaterals contributes to the hyperinsulinemia of cirrhosis, 12 patients with alcoholic cirrhosis underwent a 5-hour oral glucose tolerance test 1 day before and 10 days after an elective side-to-side portacaval shunt. The glucose, insulin, and C peptide responses to oral glucose post-shunt were exaggerated but comparable to preoperative values. Compared with preoperative values, the fasting molar ratio of C peptide to insulin postoperatively had increased 40% (6.0 +/- 1.2 versus 8.4 +/- 0.7), indicating improved hepatic function. These results suggest that extrahepatic portosystemic shunting secondary to spontaneous splanchnic collaterals plays little or no role in the hyperinsulinemia of cirrhosis. It appears that decreased hepatic degradation of insulin in these patients is secondary to hepatocellular dysfunction rather than a result of shunting of portal blood around the liver.

The reduction of peripheral insulin concentrations in obese subjects following a hypocaloric diet: reduced pancreatic secretion or increased hepatic extraction?

The fasting hyperinsulinaemia characteristic of obesity is due to pancreatic hypersecretion and is accompanied by a corresponding increase in plasma C-peptide concentrations. The exaggerated plasma insulin and C-peptide responses to oral glucose in obese subjects which is greater for insulin than C-peptide suggests that the stimulated hyperinsulinaemia is due to both pancreatic hypersecretion and decreased fractional hepatic extraction of insulin. In obese subjects fasting and stimulated insulin concentrations, but not those of C-peptide, are lower after 3 weeks of energy restriction than before it. This reduction in fasting and stimulated insulin levels is, therefore, due to an increase in the fractional extraction of insulin by the liver rather than to a reduction in pancreatic secretion as had previously been supposed.

Clinical significance of urinary C-peptide excretion in children with insulin-dependent diabetes mellitus

In order to evaluate the accuracy of urinary C-peptide determination and the clinical significance of C-peptiduria for the early course of insulin-dependent diabetes (IDDM), the rate of urinary excretion of C-peptide was determined in 32 children and adolescents with IDDM and correlated with serum C-peptide concentration, urinary excretion of albumin and beta 2-microgloublin and with the glomerular filtration rate (GFR) measured in terms of the clearance of 99mTc-DTPA. The age of the subjects ranged from 9.1 to 17.1 years (mean 13.1) and the duration of diabetes from 0.3 to 11.9 years (mean 4.6). There was a good correlation between postprandial serum C-peptide concentration and the 24-hour urinary C-peptide excretion rate (r = 0.81; p less than 0.001). GFR and urinary albumin excretion were slightly elevated in the diabetic patients as compared with non-diabetic subjects (p less than 0.05 and p less than 0.001, respectively), but C-peptide excretion was unrelated to the degree of hyperfiltration or albuminuria, neither was there any correlation between the excretion rate of beta 2-microglobulin and C-peptide. Glycaemic control was poorer in the diabetic children who had only trace amounts of C-peptide in their urine (less than 0.05 nmol/m2/24 h) than in those with minimal (0.05-1.0 nmol/m2) or moderate 24-hour urinary C-peptide excretion (greater than 1.0 nmol/m2). It is concluded that urinary C-peptide excretion serves very well to reflect residual beta-cell function and is unrelated to the slight renal hyperfunction and albuminuria often seen in diabetic subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in endogenous insulin secretion during childhood as expressed by plasma and urinary C-peptide

Basal fasting values of plasma C-peptide (CP), plasma insulin and 24 h urine CP were determined in 224 normal non-obese subjects of both sexes ranging in age from 1 to 20 years. Analysis of the results by age, pubertal rating, sex and bone age (BA) during childhood showed that mean +/- SD plasma CP levels in both sexes rose from 0.07 +/- 0.08 pmol/ml at the age of 1-2 years to 0.21 +/- 0.11 pmol/ml at 8-10 years. Mean +/- SD plasma insulin levels in both sexes rose from 3.2 +/- 4.3 microU/ml at the age of 1-2 years to 5.9 +/- 4.5 microU/ml at 8-10 years. Mean +/- SD urine CP levels rose from 6.5 +/- 2.8 pmol/mg creatinine per 24 h at the age of 2-8 years to 7.7 +/- 3.5 pmol/mg creatinine per 24 h at 8-11 years in both sexes. During puberty, plasma and urine CP and plasma insulin levels rose further to peak at pubertal stage P3, the values in females being higher (CP = 0.32 +/- 0.06 pmol/ml) than those in males (CP = 0.22 +/- 0.06 pmol/ml) (P less than 0.005). Plasma insulin levels in females were 13.2 +/- 6.9 microU/ml and 6.4 +/- 3.1 microU/ml in males (P less than 0.05). Urine CP levels were 14.5 +/- 5.7 pmol/mg creatinine per 24 h and 10.8 +/- 5.4 pmol/mg creatinine per 24 h in females and males respectively (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Scintigraphic distribution of 123 I labelled proinsulin, split conversion intermediates and insulin in rats

Insulin, biosynthetic human proinsulin and 2 human proinsulin conversion intermediates, des (64, 65) human proinsulin and des (31, 32) human proinsulin, were labelled with 123 I and the derivatives monosubstituted on Tyr A14 were purified by reverse phase high performance liquid chromatography. The four tracers were injected into anaesthetized rats via a jugular or a portal vein and time activity curves were generated for the liver and kidneys using a gamma camera and an online computer. Liver extraction coefficients varied in the order insulin (38%), des (64, 65) human proinsulin (11.7%), des (31, 32) human proinsulin (3.2%), human proinsulin (1.6%); whereas half-life of hepatic activity varied in the reverse order, from 6 min for insulin, to 45 min for human proinsulin. As expected for a non-receptor mediated process, kidney extraction varied conversely to liver extraction, being highest for human proinsulin and lowest for insulin. It is concluded that the kinetics of human proinsulin conversion intermediates depends upon the site of cleavage and deletion and is intermediate between those of insulin and intact human proinsulin.

Binding and degradation of 125I-insulin by isolated rat renal brush border membranes: evidence for low affinity, high capacity insulin recognition sites

The kidney plays a major role in the handling of circulating insulin in the blood, primarily via reuptake of filtered insulin at the luminal brush border membrane. 125I-insulin associated with rat renal brush border membrane vesicles (BBV) in a time- and temperature-dependent manner accompanied by degradation of the hormone to trichloroacetic acid (TCA)-soluble fragments. Both association and degradation of 125I-insulin were linearly proportional to membrane protein concentration with virtually all of the degradative activity being membrane associated. Insulin, proinsulin and desoctapeptide insulin all inhibited the association and degradation of 125I-insulin by BBV, but these processes were not appreciably affected by the insulin-like growth factors IGF-I and IGF-II or by cytochrome c and lysozyme, low molecular weight, filterable, proteins, which are known to be reabsorbed in the renal tubules by luminal endocytosis. When the interaction of 125I-insulin with BBV was studied at various medium osmolarities (300-1100 mosM) to alter intravesicular space, association of the ligand with the vesicles was unaffected, but degradation of the ligand by the vesicles decreased progressively with increasing medium osmolarity. Therefore, association of 125I-insulin to BBV represented binding of the ligand to the membrane surface and not uptake of the hormone or its degradation products into the vesicles. Attempts to crosslink 125I-insulin to a high-affinity insulin receptor using the bifunctional reagent disuccinimidyl suberate revealed only trace amounts of an 125I-insulin-receptor complex in brush border membrane vesicles in contrast to intact renal tubules where this complex was readily observed. Both binding and degradation of 125I-insulin by brush border membranes did not reach saturation even at concentrations of insulin approaching 10(-5) M. These results indicate the presence of low-affinity, high-capacity binding sites for 125I-insulin on renal brush border membranes which can clearly distinguish insulin from the insulin-like growth factors and other low molecular weight proteins and polypeptides, but which do not differentiate insulin from its analogues as do the biological receptors for the hormone. The properties and location of these binding sites make them attractive candidates for the sites at which insulin is reabsorbed in the renal tubule.

Insulin and C-peptide plasma levels in patients with severe chronic pancreatitis and fasting normoglycemia

The aim of the present study was to evaluate insulin secretion by the pancreatic B cell in a group of patients with severe chronic pancreatitis and without overt diabetes. For this purpose we have measured plasma insulin and C-peptide peripheral levels in the fasting state and after a 100-g oral glucose load in 10 patients with severe chronic pancreatitis and fasting normoglycemia, and in 10 sex-, age-, and weight-matched healthy controls. As compared to normal subjects, patients with chronic pancreatitis showed: (1) significantly higher plasma glucose levels after oral glucose load (area under the plasma glucose curve 1708 +/- 142 vs 1208 +/- 47 mmol/liter X 240 min, P less than 0.005); (2) plasma insulin levels significantly higher at fasting (0.11 +/- 0.008 vs 0.08 +/- 0.005 nmol/liter, P less than 0.01) but not after oral glucose administration (area under the plasma insulin curve 79 +/- 12 vs 88 +/- 16 nmol/liter X 240 min); (3) significantly lower plasma C-peptide concentrations both in the fasting state (0.15 +/- 0.01 vs 0.54 +/- 0.05 nmol/liter, P less than 0.001) and after oral glucose load (area under the plasma C-peptide curve 211 +/- 30 vs 325 +/- 37 nmol/liter X 240 min, P less than 0.05). The finding of diminished plasma C-peptide levels suggests that chronic pancreatitis is associated with an impaired B-cell function even in the absence of overt diabetes. The increased or unchanged plasma insulin levels in spite of decreased plasma C-peptide concentrations indicate that in chronic pancreatitis insulin metabolism is reduced, most likely within the liver.

C-peptide in non-alcoholic cirrhosis and hepatocellular carcinoma

Fourteen normal controls, eleven patients with non-alcoholic cirrhosis, twenty-nine with hepatocellular carcinoma (HCC) and six with HCC and hypoglycemia were studied. The tests performed include iv glucose tolerance test (25 g) and glucagon challenge test (2 mg). In cirrhosis, glucose intolerance and insulin resistance were demonstrated. The fasting hyperinsulinemia in cirrhosis is the result of decreased degradation as shown by the normal fasting C-peptide. The increased insulin responses to glucose, despite a normal C-peptide response, further supports the importance of impaired degradation in the pathogenesis of hyperinsulinemia after challenge. Despite a strong etiological association between cirrhosis and HCC, patients with HCC do not have significant hyperinsulinemia or glucose intolerance. This provides metabolic evidence to support the clinico-pathological observation that HCC occurred when cirrhosis was not advanced or in a precirrhotic stage. In HCC patients with clinically overt hypoglycemia, the fasting glucose, insulin and C-peptide were very low. The C-peptide responses to glucose and glucagon challenges were suppressed despite pharmacologic stimulation. This can be explained by the suppression of insulin secretion by a circulating substance secreted by hepatoma. The results support the pathogenetic importance of insulin-like activities recently detected in HCC patients with hypoglycemia.

Differences in renal metabolism of insulin and cytochrome c

Kidneys degrade small proteins such as cytochrome c (CYT c) by the classic lysosomal pathway. However, because alternate routes for the transport and degradation of protein hormones have been identified in other tissues, we set out to determine whether extralysosomal sites might participate in the renal degradation of insulin. First, we compared the effect of the lysosomal inhibitor NH4Cl on insulin and CYT c degradation by isolated perfused rat kidneys. After kidneys were loaded with radiolabeled proteins to allow for absorption and transport to lysosomes, degradation was measured in the presence or absence of inhibitors. Control kidneys degraded 45 +/- 1.5% of the trapped CYT c per hour, and this was inhibited 62 +/- 1.3% by NH4Cl. In contrast, 86 +/- 2.4% of the trapped insulin was degraded per hour, and this was inhibited 26 +/- 4% by NH4Cl. Next we followed the subcellular distribution of 125I-labeled insulin in kidneys exposed to 125I-labeled insulin in vivo or when isolated and perfused. Under both circumstances the distribution of insulin on a linear sucrose gradient differed from that of the lysosomal enzyme N-acetyl-beta-glucosaminidase. In contrast, [14CH3]CYT c, injected in vivo, distributed over a density similar to the lysosomal marker. Thus important differences exist between the renal metabolism of CYT c, which proceeds in lysosomes, and the renal metabolism of insulin. These include rate of degradation, sensitivity to NH4Cl, and subcellular sites of localization. Accordingly, we suggest that insulin degradation may occur, at least in part, in a different compartment from the classic lysosomal site of protein degradation.

Insulin-induced drinking: an analysis of the involvement of renal angiotensin II and insulin-induced changes in plasma volume

Male Long-Evans rats were used to investigate the potential hydrational mechanisms underlying insulin-induced drinking (IID). Plasma volume effects of insulin were assessed using both hematocrit and dye dilution procedures. Evidence is presented indicating that insulin produces a long-lasting and dose-dependent reduction in plasma volume. However, it does not appear that the drinking response is tightly tied to the reduced plasma volume because a 50% blockade of this effect does not reduce water intake. In addition, it does not appear that insulin-induced drinking is related to a release of renal angiotensin because IID is not blocked by nephrectomy. The mechanism underlying IID may be related to the activation of the recently described brain insulin receptor system.

Accuracy of C-peptide:insulin molar ratio as a measure of hepatic removal of insulin

We measured transhepatic C-peptide and insulin concentrations in plasma, and hepatic removal of insulin, to examine whether the practice of reporting the C-peptide:insulin molar ratio as a measure of the hepatic removal of insulin is valid. In anesthetized dogs (n = 6), during electromagnetic hepatic blood flow monitoring, endogenous insulin was suppressed with somatostatin, while equimolar proportions of porcine insulin and simian C-peptide (2.4 and 6.0 pmol/kg.min) were infused during two consecutive 45-min periods. Insulin reached steady state within 20 min (t1/2 = 4.5 min); however, C-peptide concentrations continued to rise (t1/2 V 12.5 min). The ratio decreased when the peptide infusion was changed to the higher rate and increased when it was stopped, reflecting the more rapid removal of insulin than of C-peptide. Hepatic removal of insulin remained constant during the two infusion periods (average 60% extraction) and never correlated with the changing molar ratios. Hepatic net flux of insulin correlated with the ratio (P less than 0.05) only while plasma insulin concentrations were rising during constant-rate infusion. We therefore conclude that the molar ratio is not a reliable measure of the hepatic removal of insulin during non-steady states of insulin or C-peptide.

Metabolism and placental transfer of 125I-proinsulin and 125I-tyrosylated C-peptide in the pregnant rhesus monkey

125I-Proinsulin or 125I-tyrosylated-C-peptide (125I-tyr-CP) was administered to pregnant Rhesus monkeys by bolus followed by constant infusion to examine placental transfer of these peptides. At the end of each infusion, fetuses were exsanguinated in situ via the umbilical vein. The bolus-constant infusion technique produced a steady state in maternal plasma of immunoprecipitable label, measured using excess insulin or C-peptide antiserum. In animals infused with 125I-proinsulin, analysis of umbilical venous plasma revealed no apparent transfer to the fetus of immunoprecipitable label. In animals infused with 125I-tyr-CP, 3-13% of the umbilical venous plasma radioactivity was immunoprecipitable, representing 1.4-5.8% of the immunoprecipitable radioactivity in maternal plasma at delivery. Gel filtration chromatography of umbilical venous plasma revealed that the immunoprecipitated moiety was a fragment of 125I-tyr-CP. Analysis of maternal plasma showed that the predominant peak of radioactivity represented intact C-peptide. A peak corresponding to the fetal immunoprecipitable peak was also present. Analysis of simultaneous maternal arterial and uterine vein plasma samples showed that degradation of 125I-tyr-CP occurred across the uterus. Studies in one nonpregnant and three postpartum animals indicated that pregnancy increased the rate of metabolism of 125I-tyr-CP. When 125I-tyr-CP was incubated with trophoblastic cells in culture, degradation to a species corresponding on gel filtration to the immunoprecipitable fetal metabolite was found. We conclude that proinsulin, like insulin, does not traverse the placenta. Immunoreactive fragments of C-peptide do cross, however, and pregnancy alters the metabolism of 125I-tyr-CP, probably owing to placental degradation.

The effects of glucose-dependent insulinotropic polypeptide infused at physiological concentrations in normal subjects and type 2 (non-insulin-dependent) diabetic patients on glucose tolerance and B-cell secretion

The effects of porcine glucose-dependent insulinotropic polypeptide given by continuous intravenous infusion in normal subjects (n = 6) and Type 2 (non-insulin-dependent) diabetic patients (n = 6) have been investigated. The subjects were studied on 2 separate days after overnight fasts. On each day 25 g of glucose was infused from 0-30 min plus an infusion of either porcine glucose-dependent insulinotropic polypeptide (0.75 pmol . kg-1 . min-1) or control solution. During the glucose-dependent insulinotropic polypeptide infusion plasma glucose values were reduced in normal subjects from 30-60 min (p less than 0.01) and in Type 2 diabetic patients at 45 and 60 min (p less than 0.05). In the normal subjects insulin concentrations were greater from 10-35 min (p less than 0.01) following glucose-dependent insulinotropic polypeptide infusion and peak values were increased by 123%. In the Type 2 diabetic patients following glucose-dependent insulinotropic polypeptide infusion insulin levels were increased from 4-40 min (p less than 0.01) but peak values were only increased by 27%. In the normal subjects C-peptide values were greater from 25-45 min (p less than 0.01) following glucose-dependent insulinotropic polypeptide infusion and peak C-peptide levels were increased by 82%. In the Type 2 diabetic patients following the glucose-dependent insulinotropic polypeptide infusion C-peptide levels were increased from 6-55 min (p less than 0.01) and peak values were increased by 20%. Plasma glucose-dependent insulinotropic polypeptide levels were within the physiological post prandial range during the glucose-dependent insulinotropic polypeptide infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered C-peptide/insulin molar ratios and glucose turnover rates after stimulation in nondiabetic offsprings of type II diabetic patients

We evaluated the serum glucose/insulin/C-peptide dynamics and C-peptide/insulin molar ratios during sequential standard meal and intravenous (IV) glucagon testing for 240 minutes in eight genetically predisposed but nondiabetic female offsprings of type II diabetic patients and seven weight-matched, normal female controls. Glucose turnover rates and metabolic clearance rates of glucose (MCRG) were also determined isotopically by the D-[3-3H]glucose infusion technique. All the subjects had normal fasting serum glucose and glycosylated hemoglobin (HbA1) values. After meal ingestion, mean serum glucose concentrations were not different except for 120 to 180 minute values, which were significantly higher in the offsprings v controls. After intravenous glucagon infusion, percent maximum increments of glucose were no different. Mean basal immunoreactive insulin (IRI) levels were significantly (P less than 0.02) higher in the nondiabetic offsprings v controls. Following meal ingestion, mean IRI rose to a peak at 40 minutes in both groups, but values were significantly (P less than 0.001) higher in the offsprings v controls. After glucagon administration, the percent maximum increment was significantly (P less than 0.05) lower in the offsprings v controls. Despite exaggerated IRI levels in the offsprings, the mean fasting and stimulated C-peptide levels were identical in both groups throughout the study period. Basal and stimulated C-peptide/IRI molar ratios were quantitatively lower but qualitatively no different in the nondiabetic offsprings v controls throughout the study period. Mean basal hepatic glucose output (HGO) was higher but not statistically different in the offsprings compared with the controls (2.10 +/- 0.28 v 1.65 +/- 0.15 mg/kg X min).(ABSTRACT TRUNCATED AT 250 WORDS)

Fractional hepatic extraction of insulin in man: is it constant?

The present study was designed to compare insulin extraction by the liver following oral glucose administrations of different size, in order to evaluate insulin removal by the liver in relation to the insulin exposure, and to the amount of ingested glucose. Insulin secretion by the pancreas was estimated by the measurement of peripheral C-peptide levels, and insulin extraction by the liver by the analysis of peripheral C-peptide to insulin ratios and relations. Ten healthy subjects (5 males and 5 females), aged 16 to 66 yr, with normal bw, and without family history of diabetes mellitus were investigated by means of the administration, on alternate days, of 50 and 150 g oral glucose loads. After the 150 g oral glucose load plasma glucose levels were significantly higher than after the 50 g oral glucose administration: glucose incremental areas of 1.45 +/- 0.12 vs. 0.55 +/- 0.04 mmol/l X min, respectively (p less than 0.001). Similarly, insulin concentrations were significantly higher following 150 g than after 50 g glucose ingestion: insulin incremental areas of 0.52 +/- 0.09 vs. 0.20 +/- 0.04 nmol/l X min (p less than 0.001). Also C-peptide levels were higher after 150 vs. 50 g oral glucose load: C-peptide incremental areas of 1.85 +/- 0.41 vs. 0.64 +/- 0.13 nmol/l X min (p less than 0.01). C-peptide to insulin molar ratios were similar during the two glucose challenge, and averaged 5.25 +/- 0.42 vs. 5.08 +/- 0.50 after 50 and 150 g oral glucose loads, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Basal and 24-h C-peptide and insulin secretion rate in normal man

An understanding of the metabolic abnormalities rising from inappropriate insulin delivery in diabetic patients demands a knowledge of 24-h and basal insulin secretion rates in normal man. We have used biosynthetic human C-peptide to determine its kinetic parameters in 10 normal subjects and applied these to measurements of plasma concentrations in the same subjects to determine pancreatic secretion rate. Metabolic clearance rate measured by stepped primed infusion of biosynthetic human C-peptide at rates of 10, 19 and 26 nmol/h was 4.7 +/- 0.7 (+/- SD) ml X kg-1 X min-1, and was independent of infusion rate. Fractional clearance (T1/2, 26 +/- 3 min) and distribution volume (0.178 +/- 0.039 l/kg) were calculated from the decline in concentration after cessation of the highest rate infusion. Basal insulin secretion calculated from the C-peptide metabolic clearance rate and plasma concentrations for the period 02.00 to 07.00 hours was 1.3 +/- 0.4 U/h. Over 24 h total insulin secretion on a standard high carbohydrate diet was 63 +/- 15 U, calculated from the area under the C-peptide concentration curve. Basal insulin secretion, therefore, accounted for 50 +/- 8% of total insulin secretion. Although only 5.6 +/- 1.1% of C-peptide was detected in 24-h urine collections, urinary C-peptide excretion was significantly related to 24-h C-peptide secretion (r = 0.74, p less than 0.02).

Insulin but not GH directly stimulates growth of transplanted fetal rat paws

We have investigated whether insulin or growth hormone (GH) can affect growth of transplanted fetal rat paws by direct and/or indirect actions. Paws from 15-day rat fetuses were transplanted under the kidney capsule of adult female hosts, and grown for 7 days. In hypophysectomized (HX) or diabetic hosts their growth was reduced by 65% and 35%, respectively. The direct effects of insulin and GH were studied by inserting a catheter connected to an osmotic minipump into the left renal artery. Thus transplants on the left (infused) kidney were exposed directly to much higher concentrations of hormone than were those on the right (uninfused) side. In HX hosts, infusion of GH (0.1 microgram X g body wt-1 X day-1) had no effect on growth of paws on either kidney. At 0.3 or 1.0 microgram X g body wt-1 X day-1 GH caused a dose-related restoration of transplant growth that was equivalent on both sides. In diabetic hosts infusion of insulin (2 U X kg body wt-1 X day-1) partially restored growth of paws on the infused kidney without affecting those on the right. Insulin at 6 U X kg body wt-1 X day-1 caused full restoration of growth of the paws on the left and a partial increase in those on the right. At 9 U X kg body wt-1 X day-1 insulin caused full restoration of transplant growth on both kidneys. These results indicate that insulin promotes normal growth of transplanted fetal paws by direct and possibly indirect mechanisms, but the effects of GH are apparently only indirect.

Inhibition by proinsulin of endogenous C-peptide release in healthy man

To determine the role of proinsulin on endogenous insulin release, splanchnic output and arterial concentrations of C-peptide were measured in healthy subjects before and during infusion of human (HPI) and porcine (PPI) proinsulin at increasing rates for 70 min each (HPI, 328 and 656 micrograms X m-2 X h-1; PPI, 54, 134, and 268 micrograms/m-2 X h-1), while euglycemia was maintained by variable glucose infusion. By using this approach splanchnic C-peptide output was reduced by human proinsulin infusion from 143 +/- 16 (mean +/- SE) pmol/min to 111 +/- 18 and 75 +/- 11 pmol/min (P = 0.01). Simultaneously, arterial concentrations of C-peptide decreased from 716 +/- 40 pmol/l by 23 and 32%. Similar inhibition was induced by porcine PPI of splanchnic C-peptide output at an infusion rate of 268 micrograms X m-2 X h-1. Mean metabolic clearance rate was 2.7 and 3.7 ml X kg-1 X min-1 for HPI and PPI, respectively. Splanchnic glucose output was almost completely suppressed by human and porcine proinsulin at maximal infusion rates. This effect preceded both inhibition by proinsulin of splanchnic C-peptide output and stimulation of peripheral glucose utilization. We conclude that human and porcine proinsulin suppress endogenous insulin secretion at pharmacological concentrations. The observed constancy of the metabolic clearance rate of HPI demonstrates that its clearance remains a nonsaturable process up to supraphysiological HPI concentrations, while clearance of PPI appears to be subject to saturation. Furthermore, it appears that splanchnic glucose output responds earlier to proinsulin exposure than suppression of C-peptide release or stimulation of peripheral glucose utilization.

Luminal uptake and intracellular transport of insulin in renal proximal tubules

It is generally accepted that proteins taken up from the renal tubular fluid are transported into lysosomes in proximal tubule cells. Recently, however, it has been postulated that insulin in isolated perfused rat kidneys did not accumulate in lysosomes but to a certain degree in the Golgi region. The present study was undertaken to investigate the intracellular handling of biologically unaltered insulin in rat renal proximal tubule cells. Rats were prepared for in vivo micropuncture and either a colloidal gold insulin complex or insulin monoiodinated in the A-14 position (125I-insulin) was microinfused into proximal tubules. After 5, 10, 25 or 60 min the tubules were fixed by microinfusion of glutaraldehyde and processed for electron microscopy or electron microscope autoradiography. A qualitative analysis of tubules infused with colloidal gold insulin or 125I-insulin showed that insulin was taken up by endocytosis and transported to lysosomes, and a quantitative autoradiographic analysis of the 125I-insulin microinfused tubules showed that the grain density after five min was significantly increased for endocytic vacuoles and for lysosomes. After 60 min the grain density was still significant over lysosomes. The accumulation of grains was non-significant over all other areas analyzed at any time. This study shows that insulin is taken up from the luminal side of the proximal tubule by endocytosis and transported to the lysosomes. There was no significant transport to the Golgi region.

The role of ranitidine infusion on glucose, insulin and C-peptide serum levels induced by oral glucose tolerance test in healthy subjects

In 9 healthy subjects we evaluated the effect of a constant ranitidine infusion (100 mg) on glucose (mg/dl), insulin (microU/ml) and C-peptide (ng/ml) serum levels promoted by oral glucose tolerance test (75 g). Ranitidine significantly increased the area under concentration/time curves for glucose and insulin but not that of C-peptide. Our data indicate that ranitidine does not affect pancreatic insulin release nor peripheral glucose utilization and are consistent with the hypothesis that ranitidine influences the hepatic clearance of glucose and insulin both of which undergo high first-pass liver extraction.

The effects of physical training on insulin secretion and effectiveness and on glucose metabolism in obesity and type 2 (non-insulin-dependent) diabetes mellitus

Obese subjects with normal glucose tolerance (n = 55), and, in another study, a group of patients with Type 2 (non-insulin-dependent) diabetes (n = 33), and controls (n = 13) matched for body weight and age but with normal glucose tolerance, participated in an individualized physical training program for 3 months. Under controlled dietary conditions, metabolic studies were performed before and in steady state after the last exercise session after training in the subjects showing signs of physical training in VO2 max and heart rate measurements. No changes occurred in body weight, body cell mass, body fat or adipose tissue cellularity. Oral glucose tolerance was improved in the patients with diabetes mellitus only. In both diabetic and control subjects initially elevated C-peptide concentrations decreased, while low C-peptide values increased and which was particularly pronounced in diabetic subjects with subnormal values. Peripheral insulin values did not change. Glucose disposal rate measured with the glucose clamp technique was similar in diabetic patients and control subjects. An improvement was seen at both submaximal and maximal insulin levels in both groups, correlating with improvement in glucose tolerance in the diabetic subjects. No changes were found in adipocytes in insulin binding or the antilipolytic effect of insulin at submaximal insulin levels, but there was a normalization of a decreased glucose incorporation into triglycerides. These results indicate that both insulin secretion and effectiveness are altered by physical training in different ways in different clinical entities. They suggest that in insulin resistant conditions with high insulin secretion (as indicated by high C-peptide concentrations) the increased peripheral insulin sensitivity is followed by a decreased insulin secretion. This is not associated with an improvement of glucose tolerance. In Type 2 diabetes with low insulin secretion, an increased insulin secretion results from physical training, perhaps due to accompanying sensitization of the autonomic nervous system. Peripheral insulin concentrations are not altered, suggesting that the extra insulin produced is captured by the liver. This mechanism, as well as the improved peripheral insulin responsiveness seen in the whole body and also seen at the cellular level, probably both contribute to an improvement in glucose tolerance.

Biochemical and clinical implications of proinsulin conversion intermediates

Since a complete map of insulin-related peptides in humans requires consideration of proinsulin, Arg32/Glu33-split proinsulin, Arg65/Gly66-split proinsulin, des-Arg31,Arg32-proinsulin, des-Lys64, Arg65-proinsulin, and insulin, we applied high performance liquid chromatography coupled with radioimmunoassay to investigate the formation of proinsulin conversion intermediates in vitro and in vivo. Kinetic analysis of proinsulin processing by a mixture of trypsin and carboxypeptidase B (to stimulate in vivo processes) revealed (a) a rapid decline in proinsulin concommitant with formation of conversion intermediates, (b) formation of des-Arg31, Arg32-proinsulin and des-Lys64,Arg65-proinsulin in the ratio 3.3:1 at steady state, and (c) complete conversion of the precursor to insulin during extended incubation. Studies on normal human pancreas identified a similar ratio of des-Arg31,Arg32-proinsulin to des-Lys64,Arg65-proinsulin (approximately 3:1), whereas two insulinomas contained sizable amounts of des-Arg31,Arg32-proinsulin, but barely detectable amounts of des-Lys64,Arg65-proinsulin. None of the tissues contained measurable quantities of Arg32/Glu33- or Arg65/Gly66-split proinsulin. Analysis of plasma from three diabetic subjects managed by the intravenous infusion of human proinsulin revealed less than 1% processing of the circulating precursor to conversion intermediates and no processing of the precursor to human insulin. Nevertheless, analysis of plasma from the same subjects managed by the subcutaneous infusion of proinsulin revealed 4-11% processing of the precursor to intermediates that had the properties of des-Arg31,Arg32-proinsulin and Arg65/Gly66-split proinsulin. We conclude that (a) processing of proinsulin to insulin in vivo as in vitro likely occurs by preferential cleavage at the Arg32-Glu33 peptide bond in proinsulin, (b) proinsulin is inefficiently processed in the vascular compartment, and (c) subcutaneous administration of the precursor can result in the formation of conversion intermediates with the potential for contributing to biological activity.

Serum free C-peptide response to oral glucose loading as a parameter for the monitoring of pancreatic B-cell function in diabetic patients

As a parameter for evaluating pancreatic B-cell function, the accuracy of measuring serum free C-peptide immunoreactivity (CPR) was compared with that of measuring plasma immunoreactive insulin (IRI) and urine CPR in diabetic patients during a 100 g oral glucose tolerance test. In 25 non-obese patients receiving oral hypoglycemic agent or diet treatment alone, a positive correlation between the sum of serum free CPR (sigma serum free CPR) and the sum of plasma IRI (sigma plasma IRI) was noted (r = 0.68, P less than 0.001). However, the sum of blood glucose values was found to be negatively correlated to sigma free CPR (r = -0.56, P less than 0.0025), but not to sigma plasma IRI (r = -0.25, NS). In 23 patients receiving diet, oral hypoglycemic agent or insulin treatment, a positive correlation between sigma serum free CPR and urine CPR was noted (r = 0.75, P less than 0.001). However, no significant correlation was found when only insulin-treated patients were investigated (r = 0.37, NS, n = 17). In addition, patients with insulin-dependent diabetes mellitus and non-insulin-dependent diabetes mellitus were better differentiated by measuring sigma serum free CPR than urine CPR. Thus, we concluded that the measurement of serum free CPR during OGTT provides an extremely valuable method for monitoring pancreatic B-cell function in diabetic patients, whether they are receiving insulin treatment or not.

Metabolism of glycine- and hydroxyproline-containing peptides by the isolated perfused rat kidney

Isolated perfused rat kidneys removed considerable quantities of glycyltyrosine, glycylhydroxyproline, tetraglycine and prolylhydroxyproline from the perfusate. The component amino acids are released into the perfusate and, in the case of the glycine-containing peptides, there is increased synthesis of serine. Removal of peptides was more than could be accounted for on the basis of filtration, so antiluminal metabolism is indicated. Metabolism of such peptides by the kidney may contribute to renal serine synthesis in vivo.

Differing effects of antiinsulin serum and antiinsulin receptor serum on 123I-insulin metabolism in rats

Anesthetized rats were treated with saline, antiinsulin receptor serum, or antiinsulin serum, and the biodistribution of high pressure liquid chromatography-purified 123I-Tyr A14-insulin was studied by scintillation scanning. Time activity curves over organs of interest were calibrated by sacrificing the rats at the end of the experiment and directly determining the radioactivity in the blood, liver, and kidneys. Saline-treated rats exhibited normal insulin biodistribution. The highest concentration of 123I-insulin was found in the liver, and reached 30% of total injected dose between 3 and 5 min after injection. After this peak, activity rapidly decreased with a t1/2 of 6 min. Activity of 123I-insulin in kidney showed a more gradual rise and fall and was approximately 15% of injected dose at its maximum. In rats treated with antiinsulin antiserum, insulin biodistribution was markedly altered. Peak liver activity increased with increasing antibody concentration with up to 90% of injected dose appearing in the liver. In addition, there was no clearance of the liver 123I-insulin over 30 min. Autoradiographic studies demonstrated that in contrast to the normal rats in which radioactivity was associated with hepatocytes, in rats passively immunized with anti-insulin serum, 125I-insulin was associated primarily with the Kuppfer cells. In contrast, antibodies to the insulin receptor markedly inhibited 123I-insulin uptake by the liver. Kidney activity increased, reflecting the amount of free 123I-insulin that reached this organ. This is similar to the pattern observed when insulin receptors are saturated with a high concentration of unlabeled insulin. Thus, both insulin antibodies and anti-receptor antibodies alter the distribution of insulin, but with very different patterns. The use of 123I-insulin and scintillation scanning allows one to study specific alterations in insulin distribution in animal models of insulin-resistant states, and should also be useful in human disease states.

Mechanism of insulin resistance in human liver cirrhosis. Evidence of a combined receptor and postreceptor defect

Insulin resistance in liver cirrhosis may depend on either reduced sensitivity (receptor defect) and/or reduced response to insulin (postreceptor defect). To clarify the mechanism of such resistance, a [3H]glucose infusion (0.2 microCi/min) was performed for 120 min before and during a euglycemic clamp at approximately 100, 1,000, and 10,000 microU/ml steady state plasma insulin concentration in 18 compensated cirrhotics with portal hypertension and impaired glucose tolerance, and 18 healthy volunteers with no family history of diabetes, matched for sex, age, and weight. Mean fasting plasma insulin (29.2 +/- 3.4 SEM vs. 14.8 +/- 1.1 microU/ml) was significantly higher (P less than 0.001) in cirrhotics, while fasting plasma glucose was much the same in the two groups. Glucose use (milligrams per kilogram per minute) was significantly lower in cirrhotics at all three steady state plasma insulin levels: 3.04 +/- 0.34 vs. 7.72 +/- 0.61 (P less than 0.001) at approximately 100; 6.05 +/- 1.07 vs. 11.45 +/- 1.24 (P less than 0.001) at approximately 1,000; and 11.69 +/- 0.69 vs. 14.13 +/- 0.74 (P less than 0.05) at approximately 10,000 microU/ml. Mean plasma C-peptide was significantly higher in cirrhotics both basally and during the steady states (P less than 0.001); it was completely suppressed at approximately 10,000 microU/ml in controls and only 57.5% of the baseline in cirrhotics. Endogenous glucose production (milligrams per kilogram per minute) was much the same in the two groups in the fasting state and almost entirely suppressed in the controls (0.10 +/- 0.05 vs. 0.48 +/- 0.11, P less than 0.001) at approximately 100 microU/ml; at approximately 1,000 microU/ml a residual glucose production, 0.07 +/- 0.05, was observed in the cirrhotics only. In addition, insulin binding and 3-ortho-methyl-glucose transport were studied in vitro in six cirrhotics and six controls. Insulin binding to circulating monocytes and isolated adipocytes was significantly lower (P less than 0.025) in cirrhotics in all insulin concentration studies. Glucose transport values on isolated adipocytes were significantly lower in cirrhotics both basally (P less than 0.001) and at maximal insulin concentration (P less than 0.05). These results suggest that insulin resistance in human cirrhosis is more dependent on depressed peripheral glucose use than on increased endogenous glucose production, and that a combined receptor and postreceptor defect in insulin action on target cells seems to be present.

New application of high-performance liquid chromatography for analysis of urinary C-peptide

A successful application of high-performance liquid chromatography for analysis of urinary C-peptide is described. Samples (1.0 ml of human urine) were first subjected to gel chromatography to remove interfering substances, and then applied to a reversed-phase column (LiChrosorb RP-18, 7 micron). The detection of C-peptide was performed using a highly specific radioimmunoassay. With the newly developed techniques, at least four forms of immunoreactive C-peptide were detected in human urine. One of these peptides was indistinguishable from authentic C-peptide. The present study has clearly demonstrated the heterogeneity of urinary C-peptide.

Starvation enhances the ability of insulin to inhibit its own secretion

To examine whether decreased insulin secretion during starvation is related to a change in the ability of insulin to inhibit its own secretion, plasma C-peptide was measured after plasma insulin levels were acutely raised by intravenous (IV) insulin infusion in a dose of 40 and 80 mU/M2/min in obese subjects before and after a 72 hour fast. Plasma glucose concentration was maintained +/- 4% of basal levels by a variable glucose infusion. During the 80 mU infusion, at plasma insulin levels of 200 microU/mL, plasma C-peptide fell by 0.17 pmol/mL in the fed state. In the fasted state, despite basal levels that were 36% lower, C-peptide decreased by 0.21 pmol/mL. Highly significant increases in percent suppression after fasting were noted during both 40 mU and 80 mU studies. The plasma C-peptide response was related to the insulin infusion dose in both the fed and fasted state. In contrast, alpha cell suppression by insulin, as determined by plasma glucagon levels, was not altered by fasting. It is concluded that enhanced inhibitory influences of insulin on the beta cell during starvation may be a physiologically important mechanism for diminished insulin secretion during the transition from the fed to the fasting state.

Hyperinsulinaemia and insulin resistance of cirrhosis: the importance of insulin hypersecretion

The mechanism for the hyperinsulinaemia in cirrhosis was investigated using two different approaches. In the first study, the metabolic clearance rate of insulin was measured at steady state in 13 cirrhotic and 13 weight-matched control subjects. With comparable insulin infusion rates (1.00 +/- 0.19 versus 1.07 +/- 0.15 mU/kg/min), steady-state plasma insulin concentrations (104 +/- 25 versus 87 +/- 12 microU/ml; P greater than 0.5) and MCRIRI (13.6 +/- 1.6 versus 15.4 +/- 2.0 ml/kg/min; P greater than 0.5) were similar. In the second study, fasting and oral glucose stimulated C-peptide/insulin ratios were compared in 16 cirrhotic and 18 weight matched control subjects. Although fasting glucose levels were significantly higher in the cirrhotic groups, all values were in the normal range (5.5 +/- 0.3 versus 4.8 +/- 0.1 mmol/l, P less than 0.02). However, fasting insulin (0.171 +/- 0.02 versus 0.068 +/- 0.004 nmol/l) and C-peptide (1.02 +/- 0.13 versus 0.42 +/- 0.02 nmol/l) were strikingly higher (P less than 0.001) in cirrhotic subjects. On the other hand, fasting C-peptide/insulin ratios were not statistically different in the two groups (6.18 +/- 0.52 versus 6.77 +/- 0.46; P greater than 0.3). This suggests that beta cell hypersecretion was the principal cause of the fasting hyperinsulinaemia, rather than decreased insulin hepatic extraction. Following the glucose load in 13 of the control and seven of the cirrhotic group, the C-peptide/insulin ratio fell in both groups but was significantly lower in the cirrhotic compared to control subjects at 30, 60 and 120 min, consistent with possible impairment of hepatic insulin extraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the kidney in metabolism of gonadotropins in rats

The role of the kidney in the metabolic disposal of homologous gonadotropins [renal luteinizing hormone (rLH) and renal follicle-stimulating hormone (rFSH)] was studied in rats. In analogy with other protein hormones, renal mechanisms contributed importantly to their metabolic clearance rates (MCR), which were profoundly and comparably decreased following nephrectomy (by 94 and 78% for rLH and rFSH, respectively). Absolute MCR and renal organ clearance rates of gonadotropins were, however, markedly lower and urinary clearance rates proportionally higher than those of nonglycosylated protein hormones reported previously. Nonetheless, handling of both LH and FSH by the kidney probably involves, in addition to their excretion in the urine, also intrarenal degradation because their urinary clearance rates accounted for at most a third of their respective MCR, considerably less than the striking reduction of MCR seen after acute renal ablation. Moreover, losses of LH immunoreactivity across the renal circulation were over and above those accountable for by urinary excretion alone. Thus, handling of gonadotropins by the kidney differs from that of nonglycoprotein hormones both in magnitude and in that it involves, in addition to intrarenal degradation, also substantial urinary excretion, a pattern that appears to be representative of the way the kidney disposes of glycoprotein hormones in general.

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.

Regulation of peripheral insulin/glucagon levels by rat liver

The concentrations of insulin and glucagon were measured in the portal and hepatic vein, the abdominal aorta and caval vein in the rat during a normal 24-h feeding cycle. Portal insulin levels showed little diurnal variation while hepatovenous and peripheral values were clearly increased during the eating phase. Conversely, portal glucagon levels were maximal during the fasting period while hepatovenous and peripheral concentrations showed little diurnal variation. The removal of insulin and glucagon by the liver was not constant, but independently regulated. During meals the liver increased the high portal insulin/glucagon ratio further to an even higher peripheral ratio favouring glucose utilization, e.g. by muscle and adipose tissue. During a short fast the liver decreased the low portal insulin/glucagon ratio further to an even lower peripheral ratio leading to glucose saving, e.g. by muscle and adipose tissue in favour of the brain and erythrocytes. The results indicate that the liver has an important role in the regulation of peripheral insulin/glucagon levels.

Hepatic extraction of exogenous insulin in depancreatized conscious dogs

Hepatic and mesenteric extraction of exogenous insulin and glucose appearance and clearance were compared in conscious depancreatized and normal dogs after intraportal or peripheral intravenous insulin infusion. Portal vein insulin levels were higher, whereas arterial insulin levels were lower after intraportal compared with intravenous peripheral infusion of insulin. During the intraportal infusion of 1 and 2 mU X kg-1 X min-1 insulin, 40 +/- 3% of the insulin presented to the liver was extracted by that organ in the diabetic dogs, similar to the value obtained in normal dogs (39 +/- 5%). Hepatic extraction of insulin after intravenous peripheral infusion of that hormone was similar in normal and diabetic dogs and was not significantly different from intraportal infusion. Mesenteric extraction of insulin in the diabetic dogs (13 +/- 2%) was similar to the 19 +/- 3% in the normal animals. The blood sugar changes were similar after both routes of insulin infusion. Suppression of glucose appearance in diabetic dogs was also similar during both routes of infusion. Glucose clearance during the peripheral intravenous infusion of insulin in diabetic dogs was greater than during intraportal insulin. These findings indicate that hepatic extraction of exogenous insulin was similar in normal and depancreatized dogs and was not influenced by the different infusion routes. Suppression of glucose appearance in diabetic dogs was similar after both routes despite different portal vein insulin levels. The peripheral action of intravenous peripheral infused insulin in diabetic dogs was greater than that of intraportal insulin reflecting the higher arterial insulin levels.

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.

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.

In vivo insulin sensitivity in the rat determined by euglycemic clamp

Our aim was to develop the glucose clamp (GC) technique in the conscious rat for assessment of in vivo insulin sensitivity. A 2-h euglycemic GC could be performed in chronically cannulated rats using 625 microliter blood. Overnight-fasted rats were infused with porcine insulin (1.67 mU . kg-1 . h-1). Insulin levels of 41 +/- 2 (SE) mU/liter were produced in rats aged 91 +/- 4 days with a 60- to 120-min glucose infusion rate (GIR60-120) of 10.6 +/- 0.6 mg . kg-1 . min-1 (n = 9) during euglycemia. GIR60-120 was significantly (P less than 0.025) reduced in rats aged greater than 130 days (mean, 169 +/- 16 days) to 7.7 +/- 1.2 mg . kg-1 . min-1 (n = 7). Metabolic clearance rate of porcine insulin (46 +/- 3 ml . kg-1 . min-1) and GIR60-120 compared with plateau plasma insulin levels are higher than values reported in humans. The latter may be due to suppression of a higher basal hepatic glucose production or increased potency of porcine compared with native insulin. We conclude that the GC can be accomplished in the rat. When combined with tracer administration and subsequent killing, it should provide a quantitative in vivo measurement of insulin sensitivity in individual tissues.