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

Decreased hepatic insulin extraction in subjects with mild glucose intolerance

The fact that hyperinsulinemia occurs in simple obesity and mild glucose intolerance has been well established. Altered hepatic insulin extraction may influence the levels of circulating hormone. The simultaneous measurement of insulin and C-peptide concentrations in peripheral blood enables an in vivo estimation of hepatic insulin removal. To evaluate hepatic insulin extraction, insulin and C-peptide responses to oral glucose were studied in 176 obese and nonobese subjects with normal, impaired, or diabetic glucose tolerance. Insulin levels as well as insulin incremental areas in glucose intolerant subjects were significantly higher than in weight-matched controls. The levels of C-peptide as well as C-peptide incremental areas were only slightly enhanced in subjects with impaired glucose tolerance, whereas they were reduced in subjects with diabetic tolerance. The molar ratios of C-peptide to insulin, both in the fasting state and after ingestion of glucose, as well as the relationship between the incremental areas of the two peptides were used as measures of hepatic insulin extraction. They were significantly reduced in glucose intolerant subjects and, to a lesser extent, in nondiabetic obese subjects. These results indicate that peripheral hyperinsulinemia in subjects with simple obesity or impaired glucose tolerance is a result of both pancreatic hypersecretion and diminished hepatic insulin extraction. In subjects with a more severe degree of glucose intolerance, decreased hepatic insulin removal is the primary cause of hyperinsulinemia.

The expanding clinical use of C-peptide radioimmunoassay

C-peptide levels are in many ways a better measure of endogenous insulin secretion than peripheral insulin levels. C-peptide may be measured in either blood or urine. Perhaps the major advantage of measuring C-peptide levels is the ability to readily distinguish endogenous insulin levels in the presence of exogenous administration of insulin. Early C-peptide immunoassays were troubled by lack of sensitivity. This problem has now been overcome, and it is possible to measure C-peptide values down to extremely low levels. The clinical indications for C-peptide measurement include diagnosis of insulinoma and differentiation from factitious hypoglycemia, follow-up of pancreatectomy and evaluation of viability of islet cell transplants. Recently these indications have been dramatically expanded to permit evaluation of insulin dependence in maturity onset diabetes mellitus.

Hyperglucagonemia following cisplatin treatment

These studies were initiated to determine (1) if cisplatin (cis-DDP)-induced hyperglucagonemia is related to decreased hormone degradation, (2) the relationship between impaired kidney function associated with cis-DDP nephrotoxicity and hyperglucagonemia, and (3) the contribution of cis-DDP-induced hyperglucagonemia to disturbances in glucose metabolism in male F-344 rats. Administration of 5 or 7.5, but not 2.5, mg/kg cis-DDP iv increased fasting plasma immunoreactive glucagon (IRG) concentrations. Hyperglucagonemia following cis-DDP treatment was characterized by an increase in the biologically active or true pancreatic form of IRG as well as an increase in an extrapancreatic component. cis-DDP treatment (5 mg/kg) resulted in a prolonged half-life and a reduced rate of plasma disappearance of exogenous glucagon. Reducing cis-DDP nephrotoxicity, via mannitol pretreatment, resulted in a significant reduction in total, true pancreatic, and extrapancreatic plasma IRG. Other nephrotoxicants, such as glycerol or gentamicin, also resulted in hyperglucagonemia, indicating that the effects of cis-DDP on glucagon metabolism are also characteristic of other nephrotoxicants and, therefore, may be secondary to kidney toxicity. Despite marked hyperglucagonemia following cis-DDP treatment, neither severe fasting hyperglycemia nor increased hepatic and renal gluconeogenic enzyme activity was apparent in treated animals. This apparent discrepancy cannot be attributed to glucagon resistance at the target tissue level since cis-DDP-treated animals responded appropriately to exogenous glucagon. These results indicate that hyperglucagonemia following cis-DDP treatment (1) may be related to decreased glucagon degradation associated with impaired renal function and (2) does not markedly disrupt glucose homeostasis.

Oral glucose decreases hepatic extraction of insulin

Peripheral venous (plasma) insulin and C-peptide concentrations were measured in eight normal subjects given oral or intravenous glucose sufficient to produce similar plasma glucose concentrations. The expected increased insulin response to oral as compared with intravenous glucose was not matched by a comparable increase in C-peptide concentration. The ratio of insulin to C-peptide concentrations doubled 30 minutes after oral glucose was given; no comparable rise was seen with intravenous glucose (p = 0.01). This finding is interpreted as evidence for decreased hepatic extraction of insulin after administration of oral glucose. Such a decrease could account for at least half of the well known difference in peripheral insulin concentrations after administration of oral as compared with intravenous glucose.

[Basal C peptide levels in response to intravenous tolbutamide in the detection of incorrect insulin therapy (or prolonged misuse)]

Fifty nine insulin dependant diabetics were hospitalised for a trial withdrawal of insulin: 17 patients rapidly showed signs of lack of insulin, 18 did not develop cetoacidosis but could not be stabilised on diet and oral hypoglycemic agents, 24 were stabilised without insulin. A statistical study (multifactorial analysis of correlations, plotting of ROC graphs) validated the classification of these diabetics into 3 groups. It also showed that in patients with hypoglycemia, the values of C-protein, and after intravenous injection of tolbutamide, were good predictive factors for insulin-dependance: all patients with basal C-protein less than 1,9 ng/ml could not be stabilised without insulin; when the basal C-protein greater than or equal to 1,9 ng/ml and the amplitude of response at the 5th min was greater than or equal to 0,4 ng/ml, the diabetes could be stabilised by diet and oral hypoglycemic agents in 90 p. 100 of cases. This institutes an easy, reliable and economic method of detecting abusive insulin therapy.

Renal degradation of insulin in patients with renal hypertension

Renal processing of insulin was studied over plasma insulin levels of 5-80 mU/l by renal vein catheterization in 14 patients. Kidneys of patients with a normal GFR removed around 30% of the insulin from arterial plasma. In 6 patients having unilateral renal artery stenosis but only moderately impaired renal blood flow and unchanged PAH-extraction, a higher or unchanged fractional insulin extraction was seen in all but one, compared with the contralateral kidney. Due to the high fractional extraction of insulin by the kidneys with renal artery stenosis, a preserved total insulin uptake by these kidneys was seen. In 4 patients with renal hypoplasia (2 of them had renal artery stenosis) low values for insulin extraction and insulin uptake were seen on the affected side. One patient with chronic pyelonephritis and uremia had an extremely low renal insulin extraction and uptake. The results suggest that estimation of renal insulin extraction may be an important renal functional test in renovascular patients.

Feedback inhibition by biosynthetic human insulin of insulin release in healthy human subjects

To determine the impact of biosynthetic human insulin (BHI) on endogenous insulin release, splanchnic output and arterial concentrations of C-peptide were measured in eight healthy men after intravenous administration of 0, 0.5, 1.25, U BHI . m-2 . h-1 for 70 min each. Euglycemia was maintained by a variable glucose infusion. Arterial levels of serum insulin were 48 +/- 6 pmol/liter before and 135 +/- 12, 265 +/- 18, and 593 +/- 47 pmol/liter after BHI infusion. Splanchnic C-peptide output was reduced by BHI infusion from 88 +/- 10 pmol/min before to 50 +/- 9, 28 +/- 10, and 18 +/- 16 pmol/min (P less than 0.0025). Simultaneously, arterial concentrations of C-peptide fell from 539 +/- 54 pmol/liter by 29 and 43% when 1.25 and 2.5 U . m-2 . h-1 of BHI were administered. Hepatic insulin uptake was directly related with BHI infusion rate (r = 0.88) and rose during BHI administration from a basal value of 58 +/- 7 to an uptake of 265 +/- 31 pmol/min when 2.5 U . m-2 . h-1 were infused (P less than 0.0005). Basal hepatic insulin clearance was 4.75 +/- 0.60 ml . kg-1 . min-1 and remained unchanged after BHI infusion as did hepatic fractional extraction of insulin, which was 61 +/- 4% in the basal state. Metabolic clearance rate of immunoreactive insulin (MCRi) was dose-dependently reduced by BHI infusion, whereas the relative share of hepatic insulin clearance in total MCRi rose simultaneously (P less than 0.01). We conclude that feedback inhibition of endogenous insulin release may play an important role in vivo. Furthermore, it appears that nonhepatic insulin degradation is a saturable phenomenon as total MCRi fell in the presence of its unchanged hepatic clearance rate after the infusion of large amounts of BHI.

The renal handling of beta 2-microglobulin in the dog

The renal extraction of beta 2-microglobulin (beta 2M) was investigated under steady-state conditions achieved by constant infusion of human beta 2M. Fifteen animal experiments were conducted. Beta 2 microglobulin was infused at rates ranging from 51 o 269 micrograms/min. The renal arterial and venous blood levels remained constant throughout the study period. The data showed that renal extraction of beta 2M exceeded the rate of filtration at all levels of beta 2M delivered to the kidney. The tubular uptake of filtered beta 2M increased linearly as did the extraglomerular extraction throughout the range investigated. There was no evidence of beta 2M (FE beta 2M) increased linearly with the fractional excretion of filtered water (FE H2O). The results are interpreted to indicate that beta 2M is extracted from renal blood by glomerular filtration and, in addition, by a mechanism independent of glomerular filtration rate (GFR). Under the conditions existing in these experimental animals, the linear relationship between FE beta 2M and FE H2O is evidence to suggest that factors affecting proximal tubular water reabsorption also affect beta 2M reabsorption.

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

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

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

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

Glucose ingestion in dogs alters the hepatic extraction of insulin. In vivo evidence for a relationship between biologic action and extraction of insulin

Oral glucose (25 g) fed to seven healthy, conscious dogs resulted in an increase in peripheral plasma glucose from 109 +/- 3 to 178 +/- 10 mg/dl. Concurrently serum insulin increased in the portal vein to levels approximately threefold greater than those in the periphery. Hepatic insulin delivery rose from 10.8 +/- 0.7 to 59.0 +/- 19.9 m U/min at 60 min. coincident with an increased hepatic insulin extraction from 3.3 to 41.4 mU/min (corresponding to an increase in hepatic extraction from 31 +/- 4 to 59 +/- 7%), both returning to basal at 3 h. In each animal there was a positive correlation between hepatic insulin delivery and extraction (r = 0.80, P less than 0.001 for the seven experiments combined). These changes in heptic insulin delivery and extraction after glucose metabolism associated with insulin action. As hepatic insulin extraction increased, hepatic glucose output declined, both parameters returning to basal levels by 3 h, indicating a negative correlation between hepatic insulin extraction and hepatic glucose output (r = 0.63, P less than 0.001; n = 7). The factors that mediate this marked and rapidly occurring increase in hepatic insulin extraction after oral glucose are unknown, and may include hepatic insulin delivery, glucose levels in the blood flow, and gut factors released by oral glucose intake. The association of changes in hepatic insulin extraction in vivo with an insulin effect on the liver as measured hepatic glucose output is consistent with in vitro observations relating insulin degradation to receptor binding.

Metabolic events in infants of diabetic mothers during first 24 hours after birth. I. Changes in plasma glucose, insulin and glucagon

Changes in plasma glucose, nonantibody-bound insulin and glucagon concentrations were studied in 32 newborn infants of diabetic mothers (IDM) during the first 24 hours after birth. Ten infants were born to White class A mothers and 22 to class B-F mothers. The infants were kept fasting during the investigative period and blood was sampled from an umbilical artery catheter. At birth, plasma glucose and glucagon levels were similar in the class A and B-F infants, whereas nonantibody-bound insulin levels were approximately 15-fold higher in the class B-F infants than in the class A infants (p less than 0.001). After birth, plasma glucose fell in all infants, the nadir being reached at two hours (p less than 0.01). Plasma glucose fell by approximately 35% in the class A infants and 63% in the class B-F infants (p less than 0.01). Eight IDM had asymptomatic hypoglycemia (plasma glucose less than 1.9 mmol/l) and four of these infants had glucose levels below 1.7 mmol/l and were withdrawn from further study. In the remaining four hypoglycemic IDM, plasma glucose was about 1.6-fold higher (p less than 0.01) and insulin about 11-fold higher (p less than 0.001) at birth compared to the 24 normoglycemic IDM. The hypoglycemia was attended by unchanged insulin levels in the class A infants, whereas insulin fell in the class B-F infants (p less than 0.01). However, during the whole investigative period, plasma insulin of the class B-F infants was higher than that of the class A infants (p less than 0.01). After birth, plasma glucagon increased slowly in all IDM and peak values were reached after 12 hours in the class A infants (p less than 0.05) and 24 hours in the class B-F infants (p less than 0.01). Only those infants who became hypoglycemic after birth exhibited a significant increment in plasma glucagon from 0.2 hours (p less than 0.05). These results suggest that neonatal hypoglycemia of IDM results from high plasma levels of nonantibody-bound insulin together with a very retarded increment in plasma glucagon levels. The degree of neonatal hypoglycemia and hyperinsulinemia of an individual IDM seems to be positively correlated to the severity of the diabetes of the mother.

Insulin, C-peptide and glucagon levels during OGTT in hepatic cirrhosis and in patients with prehepatic block

In order to investigate pancreatic B-cell function in hepatic cirrhosis and to elucidate the role of porto-caval shunt-circulation in the development of hyperinsulinism and hyperglucagonemia in cirrhotic patients, blood glucose, plasma insulin and glucagon, and serum C-peptide concentrations were measured during OGTT in 11 control and 16 cirrhotic subjects as well as in 7 patients with prehepatic block secondary to thrombosis of the portal vein. Insulin and glucagon levels were significantly higher in the cirrhotic than in the control group (for insulin: p less than 0.01, less than 0.001, less than 0.01 and less than 0.05 at 0, 60, 90 and 120 min, respectively; for glucagon: p less than 0.01, less than 0.01, and less than 0.05 at 0, 30 and 60 min, respectively). Serum C-peptide levels were, however, similar in the two groups with the exception of the 30-min value, which was significantly lower in the cirrhotic group (p less than 0.05). Plasma insulin and glucagon concentrations in patients with prehepatic block were similar to those of the controls but significantly lower than the values found in cirrhotic patients (for insulin: p less than 0.05 at 0, 30, 60 min, respectively). Serum C-peptide levels of these patients were not significantly different either from the control values or from those obtained in the cirrhotic group. Accordingly, pancreatic B-cell secretion is not increased in hepatic cirrhosis. Hence, the hyperinsulinism is due to decreased heptic degradation of the hormone. Decreased degradation of both insulin and glucagon should be attributed mainly to parenchymal liver damage, rather than porto-systemic shunting.

Alterations in glomerular RNA in diabetic rats: roles of glucagon and insulin

Incorporation in vivo of labeled orotate into RNA and total nucleotides was measured in isolated glomeruli and whole renal cortex. In 2-day diabetic animals, glomerular RNA was increased, and there was greater incorporation of orotate into total nucleotides and RNA as compared with controls. Insulin reversed the exaggerated incorporation at infusion rates that corrected hyperglucagonemia without reducing plasma glucose and with only minimal changes in insulin concentrations. The addition of glucagon to insulin infusions reproduced the increased incorporation observed in untreated diabetics. Similar changes occurred in renal cortex, where differences in orotate incorporation into nucleotide precursors seemed to be the main cause for alterations in RNA labeling. Isotope incorporation in glomeruli correlated positively with plasma glucagon, but not with insulin or glucose concentrations. Although in 7-month diabetic animals orotate incorporation into RNA was less than in controls, probably as a consequence of renal disease, 24-hour insulin infusion decreased it further. Our results confirm that in the diabetic kidney, abnormal uracil nucleotide metabolism and increased cellular content of RNA are demonstrable in glomeruli as in the renal cortex. These changes appear to be related directly to hyperglucagonemia.

Circulating C peptide: measurement and clinical application

Pancreatic beta-cell function is usually assessed by the measurement of plasma insulin concentration in various clinical situations. However, the advent of an assay for the measurement of connecting-peptide (C-peptide) concentration in plasma has provided a further method for the assessment of the secretory capacity of the pancreatic beta cell in clinical disorders, particularly in the investigation of hypoglycaemia. The metabolism and immunoassay methodology of C-peptide are reviewed, and its application in clinical practice is outlined.

C-peptide blood levels in keto-acidosis and in hyperosmolar non-ketotic diabetic coma

For further evaluation of B-cell secretion in diabetic keto-acidosis (KA) and in non-ketotic hyperosmolar coma (NKHC), basal and post-i.v. tolbutamide blood CPR and IRI values were measured in 34 patients (22 KA and 12 NKHC). FFA, cortisol and HGH measurements were also performed. IRI was low in both KA and NKHC (0.07 +/- 0.01 and 0.082 +/- 0.01 nmol/l) as opposed to CPR which was significantly higher in NKHC (1.14 +/- 0.1 nmol/l) than in KA (0.21 +/- 0.03 nmol/l). After tolbutamide injection, CPR and IRI levels did not change in any of the KA cases, whereas they significantly increased in half of the NKHC cases. Cortisol and FFA values were similarly increased in both situations, as opposed to HGH which was significantly higher (6.1 +/- 1.2 ng/ml) in KA than in NKHC (1.9 +/- 0.2 ng/ml). These results suggest that B-cell function is less deficient in NKHC than in KA. Residual insulin amounts reaching the liver via the portal vein could partly account for the absence of ketosis in NKHC.

Gastrin and gastric acid secretion in renal failure

In 10 anephric patients awaiting transplantation, 15 patients with chronic renal failure and 30 patients with acute renal failure, daily basal plasma gastrin levels and basal and stimulated gastric acid secretion were measured. Significant elevated plasma gastrin levels were found in all of the anephric patients and in 50 percent of the patients with acute and 55 percent of those with chronic renal failure. Elevated plasma gastrin levels decreased to normal after kidney transplantation or when kidney function returned to normal in the patients with acute renal failure. Gastric acid secretion studies showed a consistent pattern in all three groups of patients with a low basal acid output, a high basal intragastric pH and a very significant peak acid output, perhaps secondary to elevated plasma gastrin levels due to inadequate renal inactivation of gastrin. This may partly explain the increased incidence of gastrointestinal bleeding and gastritis seen in patients with different degrees of renal failure.

Peripheral metabolism of intact parathyroid hormone. Role of liver and kidney and the effect of chronic renal failure

The plasma disappearance rate (metabolic clearance rate) of administered intact parathyroid hormone (intact PTH) was analyzed in awake dogs with indwelling hepatic and renal vein catheters. The metabolic clearance rate (MCR) of intact PTH was found to be very rapid, 21.6 +/- 3.1 ml/min per kg in 11 normal dogs. The liver accounted for the greatest fraction of the MCR of intact PTH (61 +/- 4%) by virtue of an arterial minus venous (a - v) difference across the liver of 45 +/- 3%. The renal uptake of intact PTH accounted for 31 +/- 3% of the MCR of intact PTH. The renal a - v difference for intact PTH of 29 +/- 2% was significantly greater than the filtration fraction indicating renal uptake of intact PTH at sites independent of glomerular filtration. Together, the hepatic and renal clearances of intact PTH accounted for all but a small fraction of the MCR of intact PTH. The MCR of intact PTH, rendered biologically inactive by oxidation, was markedly decreased to 8.8 +/- 1 ml/min per kg. The a - v difference of oxidized intact PTH was reduced both in the liver and kidney. These data suggested that the high uptake rates of intact PTH are dependent, at least in part, upon sites recognizing only biologically active PTH. Chronic renal failure (CRF) decreased the MCR of intact PTH to 11.3 +/- 1.3 ml/min per kg (n = 10). Both the hepatic and renal a - v differences of intact PTH were reduced in dogs with CRF. This resulted in reductions in the hepatic and renal clearances of intact PTH. These studies identify the liver as a major extrarenal site of PTH metabolism affected by CRF. They suggest that CRF impairs the function of the major uptake sites involved in intact PTH metabolism.

Renal filtration, absorption and catabolism of human alpha interferon

The renal handling of human interferon-alpha has been evaluated by using an isolated and perfused rabbit kidney. IFN-alpha disappears from plasma with a t1/2 of 81 min and the fractional turnover rate is 0.84%/min. About 47 molecules of IFN-alpha are filtrated with 100 molecules of creatinine but most of the IFN is absorbed by tubular cells. This is the first report showing that human IFN-alpha is filtrated by the kidney, largely absorbed, most probably catabolized within tubular epithelium and excreted in negligible amounts with the urine.

Amniotic fluid C-peptide in normal and insulin-dependent diabetic pregnancies

Glucose, insulin and C-peptide were determined in amniotic fluid from 28 normal and 46 insulin-treated diabetic pregnant women. Glucose, insulin and C-peptide concentrations in amniotic fluid were higher in the diabetics than in the normal subjects. In diabetic women insulin levels did not correlate with birth weight or birth weight adjusted for gestational age, but C-peptide did. C-peptide correlated poorly with insulin (p < 0.05) in diabetics but closely (p < 0.002) in normal subjects. These results suggest that amniotic fluid investigations in insulin-treated diabetic women should use C-peptide assays as these seem to reflect more closely the insulin production of the fetus than do insulin assays. There were no differences in amniotic fluid glucose, insulin and C-peptide concentrations where the amniotic fluid lecithin-sphingomyelin ratio indicated fetal pulmonary maturity or immaturity.

Insulin production rate following glucose ingestion estimated by splanchnic C-peptide output in normal man

Insulin production rate has been estimated in healthy male volunteers (n = 16), and evaluated with respect to splanchnic glucose exchange. Insulin production rate was calculated from splanchnic immunoreactive C-peptide output. C-peptide secretion was estimated by the hepatic venous catheter technique both in the basal state and for 2 h following ingestion of various glucose loads (0, 12.5, 25, 50, 75, and 100 g). The results demonstrate a basal insulin production rate of 0.017 +/- 0.002 U/min (mean +/- SEM) or 2.04 U/2 h. Values rose in a dose dependent manner from 2.6 +/- 1.1 U/2 h after ingestion of 12.5 g of glucose to 10.8 +/- 1.1 U/2 h following a glucose load of 100 g. Insulin retention by the liver was estimated at 0.012 +/- 0.001 U/min in the basal state, and ranged from 47-85% (70 +/- 2%) of production following an oral glucose load. It was also demonstrated 1) that the relative splanchnic glucose output was inversely related to the amount of ingested glucose, and reached a minimum when glucose in excess of 50 g was ingested; and 2) that hepatic glucose retention was directly proportional to insulin production rate (r = 0.83; p less than 0.001; n = 15). It is suggested that the adaptive capacity of the splanchnic bed to retain glucose depending on the amount of ingested glucose guarantees that splanchnic glucose output fluctuates in healthy man only within a narrow range.

Renal tubular transport and catabolism of proteins and peptides

The kidney plays an important role in the metabolism of proteins and peptides. Current evidence indicates that only the proximal tubule possesses the mechanism for degradation or transport of these substances and reabsorption of metabolic products. Proteins and large polypeptides filtered at the glomerulus are absorbed from proximal tubular fluid by luminal endocytosis into apical vacuoles. These fuse with primary lysosomes, where hydrolysis occurs followed by diffusion of metabolites out of the cells and into the blood. Recent evidence indicates that small linear peptides are handled by a different mechanism. It is likely that small peptides are degraded at the luminal surface of the brush border of proximal tubules, which contains many hydrolytic enzymes, by the process of membrane or contact digestion with reabsorption of the breakdown products. The probable biological significance of proximal tubular mechanisms for handling of proteins and peptides are conservation of amino acids, inactivation of toxic substances, and participation in the regulation of the circulating level of protein and peptide hormones.

Evaluation of insulin secretory capacity in diabetes mellitus

The capacity of the pancreatic beta cells to secrete insulin can be evaluated in vivo by measurement either of circulating immunoreactive insulin or of C peptide, a by-product of insulin synthesis. Evaluation of serum levels is complicated, however, by the variable degradation rates and distribution spaces of these peptides. Also, interpretation of peripheral vein concentrations of insulin, and perhaps C peptide, is more difficult because of hepatic catabolism. Quantitation of these peptides in the urine may provide an integrated measure of insulin secretion. In insulin treated diabetic patients who develop circulating insulin antibodies, beta cell secretory capacity may be assessed by C peptide measurement, or by techniques that allow separate determinations of "free" and "total" insulin. A variety of stimulatory tests may be used to investigate insulin secretory capacity.

Modeling of plasma disappearance of unlabeled insulin in man

Fifty-two portal and 68 peripheral, brief infusions of unlabeled insulin were given to ambulant, nondiabetic patients. After intraportal insulin infusion (5--50 mU/kg), plasma clearance rate (PCR, dose/area of the incremental plasma insulin concentrations) decreased with increasing dose, varying from 32 to 14 ml-min-1-kg-1 at normoglycemia. After peripheral insulin infusion (5--30 mU/kg), PCR (mean value 15 ml-min-1-kg-1) showed no certain dose-dependence, but transfer rate constants and distribution volumes did. Despite a detectable reentry of insulin from one or more extravascular pools to the plasma pool, transfer rate constants or distribution volumes could not be accurately determined. The shortcomings of conventional noncompartmental and compartmental models did not appear to be due to the dose-dependence demonstrated. Instead, the limitations of these models were caused mainly by the difficulty of defining a proper base-line concentration and, in particular, by the imprecision of the experimental data, indicating that it will be difficult to find more appropriate models from data obtained with unlabeled insulin.

Plasma C-peptide in uraemic patients

The kidney has been suggested as the main organ for the degradation of C-peptide. This hypothesis was tested in subjects with normal fasting blood glucose concentration and varying degrees of renal failure. Forty-nine subjects with endogenous creatinine clearance ranging from 0--25 ml/min were studied. The basal steady state concentrations of C-peptide (CP) and the immunoreactivity of insulin (IRI) were determined in plasma from fasting patients. The average IRI was similar to that found in normal subjects while a higher CP was found in all patients but two. The average CP in the nephrectomized patients was six times higher than the mean CP in normal subjects (0.35 pmol/ml). There was a significant inverse correlation between clearance and CP (r = 0.51, P less than 0.001) with the highest CP in nephrectomized patients. It is concluded that the increased CP in renal failure, and especially the markedly increased CP in the nephrectomized group supports the hypothesis of the kidney being the organ mainly responsible for the degradation of C-peptide also in man.

Endocrine abnormalities associated with chronic renal failure

It is evident that chronic renal failure has far-reaching metabolic consequences because endocrine aberrations are common. Uremia may alter endocrine function through its effect on the hypothalamopituitary axis, the individual end organs, and the peripheral metabolism of various hormones. Deficiency of some hormones and excess of others coexist in patients with renal failure. Since the physiologic effects of many of these abnormalities are still not well defined, no treatment is necessary with the exception of true deficiency states such as testosterone deficiency. In the latter instance, exogenous hormonal supplementation is recommended.

[Concentration of C-peptide and insulin in serum of patients with acute virus hepatitis (author's transl)]

Intravenous glucose tolerance tests were performed in 10 patients with acute virus hepatitis. The assimilation coefficient of glucose and the level of insulin and C-peptide in serum were determined before and in the course of the glucose tolerance tests. In comparison to healthy normal weight persons C-peptide concentration in patients with acute hepatitis increased twice as high whereas the pattern of insulin secretion did not differ significantly. The higher levels of C-peptide indicate an increase of the beta-cell secretion in acute hepatitis. One could suppose an increased hepatic destruction of insulin in acute hepatitis, because there is no significant difference among the insulin levels. More likely, there is a reactive increase of secretion of the beta-cell due to a reduction of insulin sensitivity and this is indicated much better by C-peptide- than insulin levels because of the longer half live of the the C-peptide molecule.

Glucagon metabolism in the rat

The renal handling of the biologically active glucagon component (the 3,500-mol wt fraction of immunoreactive glucagon [IRG]) and the contribution of the kidney to its overall peripheral metabolism were studied in normal and uremic rats. The metabolic clearance rate of glucagon was 31.8 +/- 1.2 ml/min per kg in normal animals and was diminished by approximately one-third in each of three groups of rats with compromized renal function: 22.3+/-1.6 ml/min per kg in partially (70%) nephrectomized; 22.9+/-3.3 ml/min per kg in bilaterally ureteral ligated; and 23.2+/-1.2 ml/min per kg in bilaterally nephrectomized animals. In normal rats the kidney contributed 30% to the overall metabolic clearance of the hormone and the renal extraction of endogenous and exogenous glucagon was similar, averaging 22.9+/-1.6% and was independent of plasma IRG levels over a wide range of arterial concentrations. The remnant kidney of partially (70%) nephrectomized animals continued to extract substantial amounts (16.6+/-4.2%) of the hormone, but accounted for only 8% of the total peripheral catabolism of IRG. In the two groups of animals with filtering kidneys, renal glucagon uptake was linearly related to its filtered load and could be accounted for by glomerular filtration and tubular reabsorption. However, the kidneys of animals with both ureters ligated (renal extraction of inulin = 3.2+/-1.8%) and hence virtual absence of glomerular filtration, continued to extract 11.5+/-1.9% of the renal arterial glucagon, contributing by 9% to its overall metabolic clearance, indicating that IRG uptake occurs also from the post glomerular capillaries.

Factors influencing the handling of insulin by the isolated rat kidney

The renal handling of immunoreactive insulin was studied in the isolated perfused normothermic rat kidney to determine (a) the relative contributions of glomerular clearance and peritubular clearance to the renal clearance of insulin under different conditions, (b) what metabolic factors influence the ability of tubular cells to remove insulin from the glomerular filtrate and the peritubular circulation, and (c) whether the same factors influence the luminal and contraluminal uptake of insulin.In control kidneys the organ clearance of insulin (OCi) was 974+/-63 mul/min (SEM), of which a maximum of 46% could theoretically be accounted for by filtration. OCi was not altered by fasting, lack of exogenous fuel (glucose), or the addition of cyanide. The glomerular filtration rate did not correlate with the OCi, but there was a significant (P < 0.001) negative correlation (r = -0.828) between the peritubular clearance and glomerular filtration rate. Both N-ethylmaleimide and cold (10 degrees C) reduced the rate of insulin removal. Fractional excretion of filtered insulin (9.7+/-1.7% in controls) was not significantly altered by fasting or perfusing without glucose. In contrast, KCN increased fractional excretion of insulin to 41.9+/-3.7% whereas cold increased fractional excretion to 69.0+/-3.3%. This study indicates that renal tubular cells remove insulin from the tubular lumen and the peritubular compartment. Furthermore, the data suggest that insulin removal by tubular cells is a temperature-sensitive process consisting of two different systems. The system associated with the luminal aspect of the cell appears to be dependent on oxidative metabolism, whereas the system associated with the contraluminal aspects of the cell appears to be independent thereof. Under several circumstances when the glomerular clearance of insulin falls thereby reducing the amount of insulin absorbed by the luminal aspect of the cell, contraluminal uptake increases, and a constant rate of insulin removal is maintained by the kidney.

Kinetics of human connecting peptide in normal and diabetic subjects

The metabolic clearance rate (MCR) of synthetic human connecting peptide (C-peptide) was measured with a single-dose injection technique in six normal and seven diabetic subjects and with a constant infusion technique in one normal subject. The MCR of C-peptide did not differ in normal subjects (4.4 ml/min per kg; range, 3.7-4.9) and in diabetic subjects (4.7 ml/min per kg; range, 3.7-5.8). Employment of both techniques in one subject gave similar MCR. The average half-life of C-peptide in plasma calculated from the last 1-h period of the single-dose injection studies was longer in the insulin-dependent diabetics (42.5 min; range, 39.4-48.5) than in the normal subjects (33.5 min; range, 24.9-45.3). These results indicate that the beta-cell secretory capacity of normal and insulin-dependent diabetic subjects can be compared by measuring the C-peptide concentration in peripheral venous plasma. The difference in the half-life of C-peptide in plasma between diabetics and normals suggests an altered kinetics of the disappearance of the peptide, while the overall metabolism, as expressed by the MCR, is similar.

Demonstration of the insulin receptor in vivo in rabbits and its possible role as a reservoir for the plasma hormone

Based on studies of the interaction of insulin with its receptors in vitro, we calculated that a receptor compartment should be measurable directly in vivo. For this purpose, rabbits were injected intravenously with a labeled insulin that has low affinity for receptors in combination with a radioiodinated insulin that has high affinity for receptors. Plasma concentrations of labeled insulins were measured at selected intervals after injection. Apparent volumes of distribution were calculated by extrapolation of plasma distribution were calculated by extrapolation of plasma disappearance curves; high affinity insulins consistently distributed into spaces that were two-three times greater than those of the low affinity insulins. Injections of unlabeled pork insulin before tracer insulins decreased the distribution space of the high affinity insulin in a dose-dependent manner while having little or no effect on the distribution space of the low affinity labeled insulin. When unlabeled insulin was injected after the tracer insulins, there was an immediate rise in the plasma concentration of the high affinity insulin with only a slight change in the plasma concentration of the low affinity insulin. These results demonstrate that high affinity insulins distribute into a body compartment which has many properties of the insulin receptor previously studied in vitro. This receptor compartment: (a) recognizes insulins based on their biological potencies; (b) is saturated by elevated concentrations of insulin; and (c) insulin bound to receptors is in equilibrium with free hormone in plasma. Further, the bound to free ratios for hormone, calculated from these data, suggest that in vivo greater than 50% of the extrapancreatic insulin is bound to receptors during normal physiological states.

The renal handling of parathyroid hormone. Role of peritubular uptake and glomerular filtration

The mechanisms of uptake of parathyroid hormone (PTH) by the kidney was studied in anesthetized dogs before and after ureteral ligation. During constant infusion of bovine PTH (b-PTH 1-84), the renal arteriovenous (A-V) difference for immunoreactive PTH (i-PTH) was 22+/-2%. After ureteral ligation and no change in renal plasma flow, A-V i-PTH fell to 15+/-1% (P < 0.01), indicating continued and significant uptake of i-PTH at peritubular sites and a lesser role of glomerular filtration (GF) in the renal uptake of i-PTH. Since, under normal conditions, minimal i-PTH appears in the final urine, the contribution of GF and subsequent tubular reabsorption was further examined in isolated perfused dog kidneys before and after inhibition of tubular reabsorption by potassium cyanide. Urinary i-PTH per 100 ml GF rose from 8+/-4 ng/min (control) to 170+/-45 ng/min after potassium cyanide. Thus, i-PTH is normally filtered and reabsorbed by the tubular cells. The physiological role of these two mechanisms of renal PTH uptake was examined by giving single injections of b-PTH 1-84 or synthetic b-PTH 1-34 in the presence of established ureteral ligation. After injection of b-PTH 1-84, renal A-V i-PTH was 20% only while biologically active intact PTH was present (15-20 min). No peritubular uptake of carboxyl terminal PTH fragments was demonstrable. In contrast, after injection of synthetic b-PTH 1-34, renal extraction of N-terminal i-PTH after ureteral ligation (which was 13.4+/-0.6% vs. 19.6+/-0.9% in controls) continued for as long as i-PTH persisted in the circulation. These studies indicate that both GF and peritubular uptake are important mechanisms for renal PTH uptake. Renal uptake of carboxyl terminal fragments of PTH is dependent exclusively upon GF and tubular reabsorption, whereas peritubular uptake can only be demonstrated for biologically active b-PTH 1-84 and synthetic b-PTH 1-34.

The role of the liver in glucagon metabolism

Total plasma immunoreactive pancreatic glucagon (IRG) was measured in samples taken simultaneously from the proximal portal vein and superior vena cava of 26 healthy rats. The portal-peripheral ratio of IRG was 2.80+/-0.25, the portal-peripheral difference (Delta) 124+/-15 pg/ml, and percentage extraction 58+/-3. Gel filtration of paired portal and peripheral vein samples showed that reduction in the 3,500-dalton IRG component (glucagon) in peripheral samples accounted for almost all the differences, there being minimal and inconsistent changes in the high molecular weight (>40,000) fraction. The portal-peripheral ratio of the 3,500-dalton glucagon was 5.24+/-1.10, the portal-peripheral difference 130+/-33 pg/ml, and the percentage extraction 81+/-5. To study the transhepatic differences in the 9,000-dalton "proglucagon-like" material, the experiment was repeated in nine rats 24 h after bilateral nephrectomy, a procedure which increases plasma levels of this fraction. The portal-peripheral ratio for plasma IRG in these rats was 1.48+/-0.12, the portal-peripheral difference 140+/-29 pg/ml, and percentage extraction 28+/-5. Gel filtration revealed no consistent differences between portal and peripheral concentrations of the 9,000- and >40,000-dalton components, which comprised 40 and 13%, respectively, of the mean IRG level of 492+/-35 pg/ml. In contrast, there were marked differences between portal and peripheral levels of the 3,500-dalton component the ratio being 3.42+/-0.63, the portal-peripheral difference 182+/-32 pg/ml, and percentage extraction 64+/-5. Similar studies in a healthy dog, in which species there are significant circulating levels of the 9,000-dalton IRG component, confirmed the selective hepatic extraction of the 3,500-dalton fraction. We conclude that the various IRG fractions are metabolized differently by the liver, and that portal-peripheral ratios based on direct assay of plasma IRG will vary depending on the percentage glucagon immunoreactivity in each fraction; the greater the combined contribution of fractions other than the 3,500-dalton component to total plasma IRG, the lower will be the ratio. Because of the heterogeneity of circulating IRG and significant differences in the metabolism of its various components, gel filtration of plasma samples is necessary for precise quantitation of the hepatic uptake of each particular fraction.

[Concentration of C-peptide in correlation to kidney function (author's transl)]

Recently, the radioimmunological determination of C-peptide came into interest because of the jugdement of the remaining function of the islet apparatus in insulin-dependent diabetics. As the degradation of C-peptide preferably takes place in the kidney we performed an intravenous glucose load in 32 patients with kidney diseases. The following results were obtained: 1. In patients with a healthy carbohydrate metabolism a clear correlation exists between the concentration of creatinine on the one hand, the creatinine-clearance and the fasting C-peptide concentration respectively the measured amount of C-peptide on the other hand. 2. The more advanced the renal insufficiency the better is the correlation between the parameter of the kidney function and the C-peptide concentration. 3. In diabetic patients there shows to be no clear correlation between the C-peptide levels and the kidney function. --In insulin-dependent diabetics the amount of C-peptide is only of diagnostic use if the renal function is well known.

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...

Hyperinsulinism of hepatic cirrhosis: Diminished degradation or hypersecretion?

The breakdown of proinsulin in the pancreatic beta cell yields insulin and C-peptide which are secreted in equimolar amounts. Unlike insulin, C-peptide is not degraded significantly by the liver, so that its measurement should give a better assessment of insulin secretion than estimation of peripheral insulin levels alone; particularly in the presence of hepatic dysfunction. Plasma C-peptide and insulin response to an oral glucose load have therefore been assessed in 14 cirrhotic and 7 normal subjects. Cirrhotic patients were divided into hyperinsulinaemic and normoinsulinaemic groups based on fasting plasma-insulin concentrations. Fasting blood-blucose and plasma-C-peptide concentrations were the same in normal and cirrhotic subjects, suggesting that basal pancreatic insulin secretion was the same in all subjects. Thus the C-peptide/insulin ratio was significantly decreased in hyperinsulinaemic subjects (2-13 +/- 0-31, compared with 4-63 +/- 0-48 in controls). After oral glucose, the two groups of cirrhotic patients showed the same glucose intolerance. C-peptide concentrations were also the same but insulin concentrations were markedly increased in the hyperinsulinaemic group. It is suggested that pancreatic insulin secretion is not increased in cirrhosis and that the peripheral hyperinsulinism is due solely to decreased hepatic insulin degradation secondary to either spontaneous portal-systemic shunting or to parenchymal damage.

Pathogenesis and characterization of hyperglucagonemia in the uremic rat

The pathogenesis of hyperglucagonemia and of the alterations in the pattern of circulating immunoreactive glucagon (IRG) associated with renal insufficiency was studied in rats in which a comparable degree of uremia was induced by three different methods, i.e., bilateral nephrectomy, bilateral ureteral ligation, and urine autoinfusion. Nephrectomized and ureteral-ligated rats were markedly hyperglucagonemic (575 +/- 95 pg/ml and 492 +/- 54 pg/ml, respectively), while IRG levels of urine autoinfused animals (208 +/- 35 pg/ml) were similar to those of control rats (180 +/- 26 pg/ml), indicating that uremia per se does not account for the hyperglucagonemia observed in renal failure. Similarly, plasma IRG composition in this group of animals was indistinguishable from that of controls, in which 88.2 +/- 5.9% of total IRG consisted of the 3,500-mol wt fraction. The same component was almost entirely responsible (82.6 +/- 4.1%) for the hyperglucagonemia observed in ligated rats, while it accounted for only 57.6 +/- 5.0% of the circulating IRG in nephrectomized animals. In the latter group, 36.8 +/- 6.6% of total IRG had a mol wt of approximately 9,000, consistent with a glucagon precursor. This peak was present in samples obtained as early as 2 h after renal ablation and its concentration continued to increase with time reaching maximal levels at 24 h. These results confirm that the kidney is a major site of glucagon metabolism and provide evidence that the renal handling of the various circulating IRG components may involve different mechanisms. Thus, the metabolism of the 3,500-mol wt fraction is dependent upon glomerular filtration, while the uptake of the 9,000-mol wt material can proceed in its absence, as long as renal tissue remains adequately perfused. This finding suggests that the 9,000-mol wt component may be handled by peritubular uptake.