Proinsulin C-peptide elicits disaggregation of insulin resulting in enhanced physiological insulin effects.

Using surface plasmon resonance (SPR) and electrospray mass spectrometry (ESI-MS), proinsulin C-peptide was found to influence insulin-insulin interactions. In SPR with chip-bound insulin, C-peptide mixed with analyte insulin increased the binding, while alone C-peptide did not. A control peptide with the same residues in random sequence had little effect. In ESI-MS, C-peptide lowered the presence of insulin hexamer. The data suggest that C-peptide promotes insulin disaggregation. Insulin/insulin oligomer muM dissociation constants were determined. Compatible with these findings, type 1 diabetic patients receiving insulin and C-peptide developed 66% more stimulation of glucose metabolism than when given insulin alone. A role of C-peptide in promoting insulin disaggregation may be important physiologically during exocytosis of pancreatic beta-cell secretory granulae and pharmacologically at insulin injection sites. It is compatible with the normal co-release of C-peptide and insulin and may contribute to the beneficial effect of C-peptide and insulin replacement in type 1 diabetics.

Exercise-induced platelet and leucocyte activation is not enhanced in well-controlled Type 1 diabetes, despite increased activity at rest.

AIMS/HYPOTHESIS: Stress can evoke a prothrombotic state with activated platelets and leucocytes, and increased platelet activation at rest has been reported for diabetic subjects. Thus, prothrombotic responses to stress may be enhanced in diabetes mellitus. We therefore evaluated platelet and leucocyte responses to exercise in Type 1 diabetic patients. METHODS: Type 1 diabetes mellitus patients with good metabolic control and healthy subjects matched for age and body mass index ( n=15 for both) performed a maximal exercise test. Platelet and leucocyte activation and their heterotypic aggregation were monitored by whole blood flow cytometry. RESULTS: Diabetic platelets did not show higher basal levels of P-selectin expression, but were more reactive to ADP and thrombin stimulation. Diabetic patients had increased lymphocyte and monocyte CD11b expression, and increased circulating platelet-monocyte aggregates. Exercise evoked thrombocytosis, increased circulating platelet P-selectin expression, enhanced platelet sensitivity to ADP and thrombin, and elevated plasma levels of soluble P-selectin to a similar degree in diabetic patients and healthy subjects. Exercise induced marked leucocytosis and elevated plasma elastase, but only slightly increased leucocyte CD11b expression and leucocyte reactivity to stimulation by N-formyl-methionyl-leucyl-phenylalanine. In all of these there was no difference between diabetic patients and healthy subjects. The numbers, but not percentages of circulating platelet-leucocyte aggregates were markedly increased by exercise, but the increase was less prominent among diabetic patients. CONCLUSIONS/INTERPRETATION: Strenuous exercise evokes platelet and leucocyte activation in Type 1 diabetic patients and healthy subjects. Platelet and monocyte hyperactivity were found at rest, but responses to stress were not augmented in metabolically well-controlled Type 1 diabetes mellitus patients.

Human saphenous vein and coronary bypass surgery: ultrastructural aspects of conventional and "no-touch" vein graft preparations.

Coronary artery bypass graft surgery (CABG) is routinely used to restore blood flow to diseased cardiac muscle due to coronary artery disease. The patency of conventional grafts decreases with time, which is due to thrombosis and formation of neointima. A primary cause of graft failure is the mechanical damage inflicted to the graft during harvesting, including removal of surrounding tissue accompanied by high pressure saline distension to overcome vasospasm (both causing considerable mechanical trauma). The aim of this study was to compare the ultrastructural features of human saphenous vein (SV) grafts harvested conventionally and grafts prepared using an atraumatic 'no-touch' harvesting technique introduced by Souza (1996). The results of this study showed a better preservation of the lumenal endothelium and medial vascular smooth muscle (SM) in 'no-touch' versus conventional grafts. A 'fast' (within 30 min) response of SM cells to conventional harvesting was noted where features of both SM cell division and apoptosis were observed. It is concluded that the 'preserved' nature of the 'no-touch' aortocoronary SV grafts renders them less susceptible to thrombotic and atherosclerotic factors than grafts harvested conventionally. These features are suggested to contribute to the improved early patency rate described using the no-touch technique of SV harvesting.

Effects of C-peptide on glomerular and renal size and renal function in diabetic rats.

BACKGROUND: Strict glycemic control and antihypertensive treatment may decrease but not eliminate the risk of progressive nephropathy in diabetic patients. C-peptide has been shown to exert beneficial effects on complications, including incipient nephropathy, in type 1 diabetes. METHODS: Renal effects of 14 days of intravenous infusion of C-peptide or NaCl (placebo) were studied in three groups of rats: one nondiabetic NaCl-treated (normal, N = 7), one streptozotocin diabetic NaCl-treated (D-placebo, N = 7), and one streptozotocin diabetic C-peptide-treated group (D-C-p, N = 7). Metabolic data and albuminuria were measured in metabolic cages every fourth day. After 14 days, the glomerular filtration rate (GFR) was measured by inulin clearance and available renal functional reserve (RFR) by glycine infusion, whereupon one kidney was perfusion fixed for morphological studies. RESULTS: Glucose levels were 36.7 +/- 1.3 and 34.0 +/- 1.7 mmol/L in the D-placebo and D-C-p groups, respectively. The D-placebo group presented a 32% (P < 0.001) larger glomerular volume than the D-C-p group. The D-placebo group also presented a significantly larger renal weight than the normal group in contrast to the D-C-p group. Urinary albumin excretion increased in the D-placebo group in contrast to the other groups. GFR was 1.72 +/- 0.12 mL/min (normal), 3.73 +/- 0.19 mL/min (D-placebo, P < 0.001 vs. normal) and 2.16 +/- 0.16 mL/min (D-C-p, nonsignificant vs. normal). Available RFR was 93 +/- 25% (normal), 10 +/- 4% (D-placebo, P < 0.05 vs. normal) and 57 +/- 13% (D-C-p, nonsignificant vs. normal) of basal GFR. CONCLUSIONS: Physiological doses of homologous C-peptide prevent the development of glomerular hypertrophy, albuminuria, and glomerular hyperfiltration in rats with experimentally induced diabetes.

Effects of C-peptide on forearm blood flow and brachial artery dilatation in patients with type 1 diabetes mellitus.

Recent studies suggest that C-peptide increases blood flow in both exercising and resting forearm in patients with type 1 diabetes. Now we have studied the effect of C-peptide administration on endothelial-mediated and non-endothelial-mediated arterial responses as well as central haemodynamics in 10 patients with type 1 diabetes in a placebo-controlled double-blind study. Euglycaemia was maintained with an i.v. insulin infusion before and during the study. A high-resolution ultrasound technique and Doppler echocardiography were used to assess haemodynamic functions. Brachial artery blood flow and brachial artery diameter were measured in the basal state, 1 and 10 min after reactive hyperaemia and 4 min after sublingual glyceryl trinitrate administration (GTN; endothelial-independent vasodilatation), both before and after the end of 60-min C-peptide (6 pmol kg-1 min-1) or saline infusion periods. Echocardiographic measurements were also performed before and at the end of the infusion periods. Seven healthy age-matched males served as controls for vascular studies. The patients showed a blunted brachial dilatation after reactive hyperaemia in comparison with the healthy controls (2.1 +/- 0.5% vs. 9.3 +/- 0.3%, P < 0.001), indicating a disturbed endothelial function. C-peptide infusion compared with saline resulted in increased basal blood flow (33 +/- 6%, P < 0.001) and brachial arterial dilatation (4 +/- 1%, P < 0.05). Left ventricular ejection fraction seemed to be improved (5 +/- 2%, P < 0.05) at the end of C-peptide infusion compared with placebo. The vascular response to reactive hyperaemia and GTN was not affected by C-peptide infusion. Our results demonstrate that physiological concentrations of C-peptide increase resting forearm blood flow, brachial artery diameter and left ventricular systolic function in patients with type 1 diabetes.

Diminished skin blood flow in Type I diabetes: evidence for non-endothelium-dependent dysfunction.

The purpose of this study was to quantify the extent to which skin blood flow (SBF) responses to application of endothelium-dependent and -independent vasodilating agents differ between Type I diabetic patients and healthy subjects. Patients and matched controls were studied after an overnight fast. SBF was determined with laser Doppler perfusion imaging before and after iontophoresis of acetylcholine (Ach; endothelium-dependent) and sodium nitroprusside (SNP; endothelium-independent). Basal SBF did not differ significantly between groups. Iontophoresis of ACh and SNP increased SBF 20-fold in controls. In the patients, the increases in SBF following iontophoresis of ACh and SNP were reduced by 18% and 19%, respectively, versus controls (P<0.05 for both). These data demonstrate that Type I diabetic patients have similar diminished SBF responses to iontophoresis of ACh and SNP, which suggests that non-endothelial-dependent factors are primarily responsible for the diminished SBF responses.

C-peptide: a new potential in the treatment of diabetic nephropathy.

C-peptide is formed in the biosynthesis of insulin and the two peptides are subsequently released in equimolar amounts to the circulation. C-peptide has long been considered to be without physiologic effects. Recent data now demonstrate that C-peptide in the nanomolar concentration range binds specifically to cell surfaces, probably to G protein-coupled receptors, with subsequent activation of Ca(2+)-dependent intracellular signaling pathways and stimulation of Na+, K(+)-ATPase activities. C-peptide replacement in animal models of type 1 diabetes results in diminished hyperfiltration, improved functional reserve, reduction of urinary albumin excretion, and prevention of glomerular and renal hypertrophy. Administration of C-peptide to physiologic concentrations in patients with type 1 diabetes and incipient nephropathy for periods of 3 hours to 3 months is accompanied by reduced glomerular hyperfiltration and filtration fraction, and diminished urinary albumin excretion. C-peptide replacement together with insulin therapy may be beneficial in type 1 diabetes patients with nephropathy.

Enhanced vasoconstrictor response to endothelin-B-receptor stimulation in patients with atherosclerosis.

The vascular responses to endothelin-1 [ET-1; nonselective endothelin-A and -B (ET(A) and ET(B) agonist)] and sarafotoxin 6c (S6c; ET(B) agonist) were investigated in patients with atherosclerosis. ET-1 and S6c (3, 10 and 30 pmol/min) were infused into the brachial artery while forearm blood flow (FBF) was measured by venous occlusion plethysmography in seven male patients with atherosclerosis and six age-matched healthy male controls. S6c evoked an initial increase followed by a dose-dependent reduction in FBF. The initial dilator component did not differ between the two groups. The vasoconstrictor component of the two lower doses of S6c was significantly larger in the atherosclerotic patients than in controls. The reduction in FBF induced by 3 and 10 pmol/min S6c was 18 +/- 2% and 27 +/- 6% in the control group compared to 29 +/- 3% (p < 0.02) and 42 +/- 2% (p < 0.05) in patients with atherosclerosis. The vasoconstrictor response to S6c correlated with low-density lipoprotein (LDL) cholesterol levels (r = 0.47, p < 0.05). The vasoconstrictor response to ET-1 was similar in the two groups. It is concluded that the forearm vasoconstrictor response to S6c but not that to ET-1 is enhanced in patients with atherosclerosis as compared with healthy controls. This finding suggests an upregulation of vascular smooth muscle ET(B)-receptors in atherosclerosis.

Beneficial effects of C-peptide on incipient nephropathy and neuropathy in patients with Type 1 diabetes mellitus.

AIMS: Recent studies have indicated that proinsulin C-peptide shows specific binding to cell membrane binding sites and may exert biological effects when administered to patients with Type 1 diabetes mellitus. This study was undertaken to determine if combined treatment with C-peptide and insulin might reduce the level of microalbuminuria in patients with Type 1 diabetes and incipient nephropathy. METHODS: Twenty-one normotensive patients with microalbuminuria were studied for 6 months in a double-blind, randomized, cross-over design. The patients received s.c. injections of either human C-peptide (600 nmol/24 h) or placebo plus their regular insulin regimen for 3 months. RESULTS: Glycaemic control improved slightly during the study and to a similar extent in both treatment groups. Blood pressure was unaltered throughout the study. During the C-peptide treatment period, urinary albumin excretion decreased progressively on average from 58 microg/min (basal) to 34 microg/min (3 months, P < 0.01) and it tended to increase, but not significantly so, during the placebo period. The difference between the two treatment periods was statistically significant (P < 0.01). In the 12 patients with signs of autonomic neuropathy prior to the study, respiratory heart rate variability increased by 21 +/- 9% (P < 0.05) during treatment with C-peptide but was unaltered during placebo. Thermal thresholds were significantly improved during C-peptide treatment in comparison to placebo (n = 6, P < 0.05). CONCLUSION: These results indicate that combined treatment with C-peptide and insulin for 3 months may improve renal function by diminishing urinary albumin excretion and ameliorate autonomic and sensory nerve dysfunction in patients with Type 1 diabetes mellitus.

Role of C-peptide in human physiology.

The C-peptide of proinsulin is important for the biosynthesis of insulin but has for a long time been considered to be biologically inert. Data now indicate that C-peptide in the nanomolar concentration range binds specifically to cell surfaces, probably to a G protein-coupled surface receptor, with subsequent activation of Ca(2+)-dependent intracellular signaling pathways. The association rate constant, K(ass), for C-peptide binding to endothelial cells, renal tubular cells, and fibroblasts is approximately 3. 10(9) M(-1). The binding is stereospecific, and no cross-reaction is seen with insulin, proinsulin, insulin growth factors I and II, or neuropeptide Y. C-peptide stimulates Na(+)-K(+)-ATPase and endothelial nitric oxide synthase activities. Data also indicate that C-peptide administration is accompanied by augmented blood flow in skeletal muscle and skin, diminished glomerular hyperfiltration, reduced urinary albumin excretion, and improved nerve function, all in patients with type 1 diabetes who lack C-peptide, but not in healthy subjects. The possibility exists that C-peptide replacement, together with insulin administration, may prevent the development or retard the progression of long-term complications in type 1 diabetes.

Effects of proinsulin C-peptide on nitric oxide, microvascular blood flow and erythrocyte Na+,K+-ATPase activity in diabetes mellitus type I.

This study was conducted to evaluate the influence of proinsulin C-peptide on erythrocyte Na(+),K(+)-ATPase and endothelial nitric oxide synthase activities in patients with type I diabetes. In a randomized double-blind study design, ten patients with type I diabetes received intravenous infusions of either human C-peptide or physiological saline on two different occasions. C-peptide was infused at a rate of 3 pmol.min(-1).kg(-1) for 60 min, and thereafter at 10 pmol.min(-1).kg(-1) for 60 min. At baseline and after 60 and 120 min, laser Doppler flow (LDF) was measured following acetylcholine iontophoresis or mild thermal stimulation (44 degrees C), and venous blood samples were collected to determine plasma cGMP levels and erythrocyte membrane Na(+),K(+)-ATPase activity. The LDF response to acetylcholine increased during C-peptide infusion and decreased during saline infusion [18.6+/-19.2 and -13.2+/-9.4 arbitrary units respectively; mean+/-S.E.M.; P<0.05). No significant change in LDF was observed after thermal stimulation. The baseline plasma concentration of cGMP was 5.5+/-0.6 nmol.l(-1); this rose to 6.8+/-0.9 nmol.l(-1) during C-peptide infusion (P<0.05). Erythrocyte Na(+),K(+)-ATPase activity increased from 140+/-29 nmol of P(i).h(-1).mg(-1) in the basal state to 287+/-5 nmol of P(i). h(-1).mg(-1) during C-peptide infusion (P<0.01). There was a significant linear relationship between plasma C-peptide levels and erythrocyte Na(+),K(+)-ATPase activity during the C-peptide infusion (r=0.46, P<0.01). No significant changes in plasma cGMP levels or Na(+),K(+)-ATPase activity were observed during saline infusion. This study demonstrates an effect of human proinsulin C-peptide on microvascular function, which might be mediated by an increase in NO production and an activation of the erythrocyte Na(+),K(+)-ATPase. These mechanisms are compatible with the previous observed microvascular effects of C-peptide in patients with type I diabetes.

Integrated bioprocess for production of human proinsulin C-peptide via heat release of an intracellular heptameric fusion protein.

An integrated bioprocess has been developed suitable for production of recombinant peptides using a gene multimerization strategy and site-specific cleavage of the resulting gene product. The process has been used for production in E. coli of the human proinsulin C-peptide via a fusion protein BB-C7 containing seven copies of the 31-residues C-peptide monomer. The fusion protein BB-C7 was expressed at high level, 1.8 g l(-1), as a soluble gene product in the cytoplasm. A heat treatment procedure efficiently released the BB-C7 fusion protein into the culture medium. This step also served as an initial purification step by precipitating the majority of the host cell proteins, resulting in a 70% purity of the BB-C7 fusion protein. Following cationic polyelectrolyte precipitation of the nucleic acids and anion exchange chromatography, native C-peptide monomers were obtained by enzymatic cleavage at flanking arginine residues. The released C-peptide material was further purified by reversed-phase chromatography and size exclusion chromatography. The overall yield of native C-peptide at a purity exceeding 99% was 400 mg l(-1) culture, corresponding to an overall recovery of 56%. The suitability of this process also for the production of other recombinant proteins is discussed.

Rat C peptide I and II stimulate glucose utilization in STZ-induced diabetic rats.

AIMS: To study the effects of physiological concentrations of rat proinsulin C peptide I and II, respectively, on whole body glucose utilization in streptozotocin diabetic and healthy rats. METHODS: A sequential insulin clamp procedure was used (insulin infusion rates 3.0 and 30.0 mU.kg-1.min-1) in awake animals. C-peptide infusion rates were 0.05 and 0.5 nmol.kg-1.min-1. Blood glucose was clamped at 7.7 +/- 0.3 mmol/l in the diabetic rats and at 3.9 +/- 0.1 mmol/l in the healthy rats. RESULTS: In diabetic rats infused at lower rates of C peptide and insulin, glucose utilization increased by 79-90% (p < 0.001) compared with diabetic animals infused with saline and insulin. Increasing the rate of C-peptide infusion tenfold did not elicit a statistically significant further increase in glucose utilization. C peptide I and II exerted similar effects. The metabolic clearance rate for glucose in the diabetic animals infused with C peptide was not different from that of the healthy rats. During high-dose insulin infusion (30.0 mU.kg-1.min-1) glucose utilization increased considerably and no statistically significant C-peptide effects were observed. About 85% of the increase in glucose utilization induced by C peptide could be blocked by treatment with N-monomethyl-L-arginine. CONCLUSIONS/INTERPRETATION: Physiological concentrations of homologous C peptide stimulate whole body glucose utilization in diabetic but not in healthy rats. C peptide I and II elicit similar effects. The influence of C peptide on glucose utilization may be mediated by nitric oxide.

C-peptide potentiates the vasoconstrictor effect of neuropeptide Y in insulin-dependent diabetic patients.

Recent findings suggest that proinsulin C-peptide improves renal and nerve function as well as microcirculation in patients with insulin-dependent diabetes possibly by stimulating Na-K+-ATPase activity. Furthermore, in vitro studies on proximal rat renal tubule cells show that the effect of C-peptide on Na+, K+-ATPase activity is potentiated in the presence of the vasoconstrictor peptide neuropeptide Y. The aim of the present study was to examine whether the effects of neuropeptide Y on resting forearm blood flow in insulin-dependent patients is altered in the presence of C-peptide. Forearm blood flow was measured by a plethysmographic method in eight insulin-dependent patients and six healthy control subjects. Neuropeptide Y (20, 200 and 2000 pmol min(-1)) was infused into the brachial artery before and during an i.v. infusion of C-peptide (5 pmol kg(-1) min(-1)). Basal blood flow was 36.7 +/- 2.2 mL min(-1) L(-1) tissue. It decreased in a dose dependent manner by 11 +/- 2, 18 +/- 3 and 25 +/- 3%, respectively, during infusion of neuropeptide Y. Administration of C-peptide increased basal blood flow by 25 +/- 6%, to 46.3 +/- 3.5 mL min(-1) L(-1) tissue (P < 0.01) and forearm glucose uptake by 76 +/- 34% (P < 0.05). Infusion of the three doses of neuropeptide Y during administration of C-peptide decreased forearm blood flow by 14 +/- 4, 22 +/- 3 and 42 +/- 4%. There was a significant difference (43%, P < 0.001) between the reduction in blood flow evoked by the high dose (2000 pmol min(-1)) of neuropeptide Y before and during C-peptide infusion. Similar differences were also obtained when data were calculated as changes in vascular resistance. C-peptide did not affect resting forearm blood flow or the response to neuropeptide Y in healthy controls. In conclusion, the present data demonstrate that C-peptide increases resting forearm blood flow and augments the vasoconstrictor effects of neuropeptide Y in insulin-dependent patients.

Effects of C-peptide on renal function at the early stage of experimental diabetes.

BACKGROUND: C-peptide has been reported to have biological effects on renal function early in the course of diabetes. This investigation was initiated to elucidate and amplify these findings with respect to protein leakage, hyperfiltration and renal functional reserve in diabetic rats. METHODS: Acute effects of 140 minutes i.v. infusion of human C-peptide (0.5 nmol x min(-1) x kg(-1) body wt) on renal function and urinary protein leakage were studied in anesthetized diabetic male rats two weeks after streptozotocin injection without insulin treatment. Streptozotocin-induced diabetic rats were studied with (N = 6) and without C-peptide (N = 11) treatment. The two groups showed a similarly elevated blood glucose concentration during the study. Age-matched normal rats served as controls (N = 5). Glomerular filtration rate (GFR) was measured by inulin clearance in the basal state and during a 60-minute glycine infusion (0.22 mmol x min(-1) x kg(-1) body wt)-resembling a protein load challenge-to test the renal functional reserve in all three groups. RESULTS: In the basal state, the non-C-peptide-treated diabetic rats displayed increased GFR and increased total protein leakage compared with normal rats. Whereas normal rats responded to glycine infusion with an increase in GFR, no increase occurred in diabetic rats not treated with C-peptide. In diabetic rats given C-peptide, this reduced the initial glomerular hyperfiltration prior to glycine infusion. This indicates a specific effect, since a control peptide with the same amino acid composition as C-peptide, but in a randomized sequence, had no such effect. C-peptide also restored half of the normal renal functional reserve and resulted in 70% lower (P < 0.05) total protein leakage compared with that in rats not given C-peptide. CONCLUSION: Thus, short-term infusion of C-peptide had beneficial effects on protein leakage and hyperfiltration and improved the renal functional reserve in rats with experimental diabetes.

Gene fragment polymerization gives increased yields of recombinant human proinsulin C-peptide.

A multimerization strategy to improve yields upon recombinant production of the 31-aa human proinsulin C-peptide is presented. Gene fragments encoding the C-peptide were assembled using specific head-to-tail multimerization. DNA constructs encoding one, three or seven copies of the C-peptide gene, fused to a serum albumin binding affinity tag, were expressed intracellularly in Escherichia coli. The three fusion proteins were produced at similar levels (approximately 50 mg/l) and were proteolytically stable during production. Enzymatic digestion by trypsin-carboxypeptidase B treatment of the fusion proteins was shown to efficiently release native C-peptide, as determined by mass spectrometry, reverse-phase chromatography and a radioimmunoassay. The quantitative yields of C-peptide obtained from the three different fusion proteins suggest that this multimerization strategy could provide a cost-efficient production scheme for the C-peptide, and that this strategy could be useful also for production of other recombinant peptides.

Differential effects of proinsulin C-peptide fragments on Na+, K+-ATPase activity of renal tubule segments.

Proinsulin C-peptide has been shown to stimulate the activity of Na+ K+ ATPase of rat renal tubule segments. Thirty-six peptides and amino acids, corresponding to parts of the intact rat C-peptide and suitable controls were screened for capacity to stimulate Na+, K+-ATPase in an attempt to determine potential active sites in the C-peptide molecule. The carboxy-terminal tetra and penta peptides were found to elicit 92-103% of the intact molecule's activity, and the remaining segment, des-(27-31) C-peptide, did not possess stimulatory activity. Peptides from the middle C-peptide segment, however, centering around a GGPEAG sequence, stimulated Na+, K+-ATPase activity (36-80% of the intact molecule's effect) but this effect was not balanced by corresponding inactivity of other parts. Furthermore, it was paralleled by activity of a non-native dipeptide D-form. It is concluded that the latter effect and that of the middle segment may represent complex interactions other than the apparently specific effects of the C-terminal segment.

Ernst-Friedrich-Pfeiffer Memorial Lecture. New aspects of C-peptide physiology.

C-peptide is co-secreted with insulin and has generally been considered not to possess biological activity. However, several recent studies during the last five years have demonstrated that administration of C-peptide in physiological amounts to type 1 diabetes (IDDM) patients on a short term basis (1-3h) results in decreased glomerular hyperfiltration, augmented glucose utilization and improved autonomic nerve function. More prolonged administration (1-3 months) of C-peptide to IDDM patients is accompanied by improvements in both renal function (diminished microalbuminuria) and autonomic and sensory nerve function. Both in vitro and in vivo data indicate that C-peptide may have a role in the regulation of insulin secretion. C-peptide's mechanism of action is not known but it may be related to its ability to stimulate Na+, K(+)-ATPase, activity, probably by activating a receptor coupled to a pertussis toxin-sensitive G-protein with subsequent activation of Ca2(+)-dependent intracellular signaling pathways. In conclusion, the combined findings indicate that C-peptide is a biologically active hormone. The possibility that C-peptide therapy in IDDM patients may be beneficial should be considered.