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Objective: Although an increasing number of reports suggest that physiological concentrations of C-peptide protect against the development of diabetic nephropathy, possibly through the modulation of Na -K pump activity, the intracellular pathways controlled by C-peptide are still unrecognized. C-peptide and vasopressin share similar intracellular effects including the activation of calcium influx and endothelial nitric oxide synthase. Both hormones stimulate also the activity of Na-K pump activity. Whether the activity of C-peptide is mediated by the recently identified vasopressinactivated calcium-mobilizing receptor (VACM-1) has never been previously investigated. Design and methods: To clarify this issue, we evaluated the effect of C-peptide on VACM-1 RNA (measured by semiquantitative RT-PCR) and protein expression (measured by immunoblotting) in human skin fibroblasts (where a specific binding of C-peptide was demonstrated) and in human mesangial cells, the cellular target of diabetic nephropathy. Results: C-peptide-induced activation of VACM-1 was demonstrated in fibroblasts from six healthy individuals (0.51^0.1 vs 1.48^0.4, arbitrary units^S.E., P ¼ 0.025). This finding was paralleled by an increased VACM-1 protein expression (5.64^1.0 vs 8.47^1.2, arbitrary units^S.E., P ¼ 0.043). Similar results were confirmed in three independent cultures of human mesangial cells. VACM-1 activation in fibroblasts was insensitive to phosphatidylinositol-3-kinase inhibitor LY294002, but was inhibited by pertussis toxin, suggesting that activation of VACM-1 could be mediated by a G protein-coupled receptor. Conclusions: This study demonstrates for the first time that C-peptide activates VACM-1, possibly through a G protein-coupled receptor. Further studies are needed to clarify whether VACM-1 is involved in the protective effect of C-peptide against the development of diabetic nephropathy.

OBJECTIVE -This study was performed to ascertain whether insulin resistance with respect to protein metabolism is an additional primary metabolic abnormality affecting insulin-resistant offspring of type 2 diabetic parents, along with insulin resistance with respect to glucose and lipid metabolism.RESEARCH DESIGN AND METHODS -We studied 18 young, nonobese offspring of type 2 diabetic parents and 27 healthy matched (by means of dual-energy X-ray absorption) individuals with the bolus plus continuous infusion of [6,6-2 H 2 ]glucose and [1-13 C]leucine in combination with the insulin clamp (40 mU ⅐ m -2 ⅐ min Ϫ1 ).RESULTS -Fasting plasma leucine, phenylalanine, alanine, and glutamine concentrations, as well as the glucose and leucine turnover (reciprocal pool model: 155 Ϯ 10 vs. 165 Ϯ 5 mol ⅐ kg lean body mass -1 ⅐ h Ϫ1 in offspring of type 2 diabetic patients and healthy matched individuals, respectively), were also not different. During the clamp, glucose turnover rates were significantly reduced in offspring of type 2 diabetic patients (7.1 Ϯ 0.5) in comparison with healthy matched individuals (9.9 Ϯ 0.6 mg ⅐ kg lean body mass -1 ⅐ min Ϫ1 ; P Ͻ 0.01). Also, the suppression of leucine turnover was impaired in offspring of type 2 diabetic patients (12 Ϯ 1%) in comparison with healthy matched individuals (17 Ϯ 1%; P ϭ 0.04) and correlated with the degree of the impairment of insulin-stimulated glucose metabolism (R 2 ϭ 0.13; P ϭ 0.02).CONCLUSIONS -Nonobese, nondiabetic, insulin-resistant offspring of type 2 diabetic patients were characterized by an impairment of insulin-dependent suppression of protein breakdown, which was proportional to the impairment of glucose metabolism. These results demonstrate that in humans, a primary in vivo impairment of insulin action affects glucose and fatty acid metabolism as previously shown and also protein/amino acid metabolism. Diabetes Care 27:2716 -2722, 2004T ype 2 diabetic patients are characterized by an impairment of in vivo insulin action with respect to glucose metabolism involving glucose transport, glycogen synthesis, glycolysis, and glucose oxidation (1). Multiple abnormalities in insulin-stimulated lipid and fatty acid metabolism were also found in type 2 diabetes (2). In contrast, a clear-cut dissociation between the effects of insulin on glucose/fatty acid metabolism and amino acid metabolism in type 2 diabetes was reported (3,4). Leucine turnover assessed by means of radioactive or stable isotopes dilution techniques was found to be normal in the postabsorptive condition and in the insulin-stimulated state in normal-weight and obese patients with type 2 diabetes (3-5). The normal insulin sensitivity with respect to leucine metabolism in type 2 diabetic patients was challenged by the observation that in other nondiabetic human model of impaired insulinstimulated glucose and fatty acid metabolism (obesity), insulin resistance with respect to amino acid metabolism was detected (6), and by the report of a defect of whole-body protein breakdown estimated by means of th...

Abstract. An increased activity of sodium-lithium countertransport (SLC) is a common finding in patients who have essential hypertension. The evidence that a similar dysfunction is shared also by patients with type 1 diabetes and nephropathy has suggested the hypothesis that a predisposition to essential hypertension may be the factor that, along with hyperglycemia, underlies the development of diabetic nephropathy. Despite the initial enthusiasm surrounding the potential use of SLC activity as a marker for the early detection and treatment of individuals who are predisposed to hypertension and diabetic nephropathy, its use has been so far restricted to epidemiologic studies, as specificity and sensitivity of the test are still too low to justify any clinical use. The recent finding, however, that the measurement of kinetic parameters of SLC can significantly increase the power to discriminate among individuals with and without hypertension or diabetic nephropathy could be of help toward a future clinical use of the measurement of this membrane transport. A second major point relates to the possibility that SLC per se might be directly involved in the pathogenesis of essential hypertension and diabetic nephropathy. This case has never been fully tested, as the gene responsible for this membrane transport has been, until recently, unknown. The recent identification of an alternative splicing of the first isoform of Na-H exchange that mediates SLC activity should allow for a rapid comprehension of the role of this transport in the pathophysiology of essential hypertension and diabetic nephropathy.The presence in the erythrocyte membrane of a transport mediating the exchange of intracellular lithium for extracellular sodium was first suggested after the finding that, in lithiumtreated patients who had affective disorders, lithium concentration was systematically lower in the erythrocytes than in the plasma (1,2). Since the first identification, sodium-lithium countertransport (SLC)-so called because sodium, at opposite with the sodium pump, is moved toward the intracellular compartment-was confirmed in the erythrocyte of humans and of a number of animal species (3,4).SLC gained widespread reputation in 1980 after the demonstration by Canessa et al. (5) that elevated activity rates of this transport are a consistent concomitant of essential hypertension. It is interesting that although SLC activity rate can be affected by several environmental factors (6,7), family and genetic epidemiology studies have shown that the effect of polygenic inheritance and/or of recessive major gene is nonetheless a major determinant in the interindividual variability of SLC (8 -10).Since the original finding, an increased activity of SLC has always been considered an established marker of essential hypertension. For this reason, after the demonstration that hyperglycemia by itself is not sufficient to explain the development of diabetic nephropathy (11) and the discovery that arterial BP is higher in parents of patients with type 1 dia...