L-Carnitine is an essential component of mitochondrial fatty acid ␤-oxidation and plays a pivotal role in the maturation of spermatozoa within the male reproductive tract. Epididymal plasma contains the highest levels of L-carnitine found in the human body, and initiation of sperm motility occurs in parallel to L-carnitine increase in the epididymal lumen. Using a specific carrier, epididymal epithelium secretes L-carnitine into the lumen by an active transport mechanism; however, the structure-activity relationship comprising the carnitine-permeation pathway is poorly understood. We discovered a novel carnitine transporter (CT2) specifically located in human testis. Analyzing the primary structure of CT2 revealed that it is phylogenetically located between the organic cation transporter (OCT/OCTN) and anion transporter (OAT) families. Hence, CT2 represents a novel transporter family. When expressed in Xenopus oocytes, CT2 mediates the high affinity transport of L-carnitine but does not accept mainstream OCT/ OCTN cationic or OAT anionic substrates. We synthesized and tested various carnitine-related compounds and investigated the physicochemical properties of substrate recognition by semi-empirical computational chemistry. The data suggest that the quaternary ammonium cation bulkiness and relative hydrophobicity be the most important factors that trigger CT2-substrate interactions. Immunohistochemistry showed that the CT2 protein is located in the luminal membrane of epididymal epithelium and within the Sertoli cells of the testis. The identification of CT2 represents an interesting evolutionary link between OCT/OCTNs and OATs, as well as provides us with an important insight into the maturation of human spermatozoa.

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The effect of metformin or rosiglitazone monotherapy versus placebo on insulin signaling and gene expression in skeletal muscle of patients with newly diagnosed type 2 diabetes was determined. A euglycemic-hyperinsulinemic clamp, combined with skeletal muscle biopsies and glucose uptake measurements over rested and exercised muscle, was performed before and after 26 weeks of metformin (n ‫؍‬ 9), rosiglitazone (n ‫؍‬ 10), or placebo (n ‫؍‬ 11) treatment. Insulin-mediated whole-body and leg muscle glucose uptake was enhanced 36 and 32%, respectively, after rosiglitazone (P < 0.01) but not after metformin or placebo treatment. Insulin increased insulin receptor substrate 1 (IRS-1) tyrosine phosphorylation, IRS-1-associated phosphatidylinositol (PI) 3-kinase activity, and phosphorylation of Akt Ser 473 and AS160, a newly described Akt substrate that plays a role in GLUT4 exocytosis, ϳ2.3 fold before treatment. These insulin signaling parameters were unaltered after metformin, rosiglitazone, or placebo treatment. Expression of selected genes involved in glucose and fatty acid metabolism in skeletal muscle was unchanged between the treatment groups. Low-intensity acute exercise increased insulin-mediated glucose uptake but was without effect on insulin signaling. In conclusion, the insulin-sensitizing effects of rosiglitazone are independent of enhanced signaling of IRS-1/PI 3-kinase/Akt/ AS160 in patients with newly diagnosed type 2 diabetes.

We investigated whether insulin resistance in patients with chronic heart failure (CHF) is associated with impaired insulin signalling in skeletal muscle and whether exercise training would lead to an improvement in insulin signalling, concomitant with enhanced insulin action. Fourteen men with CHF due to idiopathic dilated cardiomyopathy, with mild‐to‐moderate limitation of physical activity and a left‐ventricular ejection fraction of less than 45 %, were studied before and after either a 5 month exercise training programme (n= 7) or standard care (n= 7). Seven healthy men participated as controls. Whole‐body insulin‐stimulated glucose uptake was determined by the euglycaemic hyperinsulinaemic clamp technique and skeletal muscle biopsy samples were obtained before and after the insulin infusion for insulin signalling measurements. Insulin‐stimulated glucose uptake was 20 % lower in CHF patients versus healthy subjects. Physiological hyperinsulinaemia increased tyrosine phosphorylation of insulin receptor substrate (IRS)‐1 by ≈2.5‐fold, IRS‐1‐associated phosphatidylinositol 3‐kinase (PI‐3‐kinase) activity by ≈2‐fold and Akt (protein kinase B) phosphorylation by ≈3‐fold, with similar responses between healthy subjects and CHF patients. Insulin‐mediated glucose uptake was not altered in patients after standard care, whereas exercise training elicited a 25 % increase in glucose uptake. Neither standard care nor exercise training altered insulin‐stimulated tyrosine phosphorylation of IRS‐1, IRS‐1‐associated PI‐3‐kinase activity or Akt phosphorylation. In conclusion, the CHF patients demonstrated impaired insulin‐stimulated glucose uptake, despite normal signal transduction in skeletal muscle at the level of IRS‐1, PI‐3‐kinase and Akt. Of clinical relevance is the finding that exercise training improves glucose uptake. However, these changes in insulin action after exercise training appear to be independent of enhanced insulin signalling at the level of IRS‐1, PI‐3‐kinase or Akt.