The vascular endothelial cell mediates insulin transport into skeletal muscle

Abstract

The pathways by which insulin exits the vasculature to muscle interstitium have not been characterized. In the present study, we infused FITC-labeled insulin to trace morphologically (using confocal immunohistochemical methods) insulin transport into rat skeletal muscle. We biopsied rectus muscle at 0, 10, 30, and 60 min after beginning a continuous (10 mU·min−1·kg−1), intravenous FITC-insulin infusion (with euglycemia maintained). The FITC-insulin distribution was compared with that of insulin receptors (IR), IGF-I receptors (IGF-IR), and caveolin-1 (a protein marker for caveolae) in skeletal muscle vasculature. We observed that muscle endothelium stained strongly for FITC-insulin within 10 min, and this persisted to 60 min. Endothelium stained more strongly for FITC-insulin than any other cellular elements in muscle. IR, IGF-IR, and caveolin-1 were also detected immunohistochemically in muscle endothelial cells. We further compared their intracellular distribution with that of FITC-insulin in cultured bovine aortic endothelial cells (bAECs). Considerable colocalization of IR or IGF-IR with FITC-insulin was noted. There was some but less overlap of IR or IGF-IR or FITC-insulin with caveolin-1. Immunoprecipitation of IR coprecipitated caveolin-1, and conversely the precipitation of caveolin-1 brought down IR. Furthermore, insulin increased the tyrosine phosphorylation of caveolin-1, and filipin (which inhibits caveolae formation) blocked insulin uptake. Finally, the ability of insulin, IGF-I, and IGF-I-blocking antibody to diminish insulin transport across bAECs grown on transwell plates suggested that IGF-IR, in addition to IR, can also mediate transendothelial insulin transit. We conclude that in vivo endothelial cells rapidly take up and concentrate insulin relative to plasma and muscle interstitium and that IGF-IR, like IR, may mediate insulin transit through endothelial cells in a process involving caveolae.

Direct Administration of Insulin Into Skeletal Muscle Reveals That the Transport of Insulin Across the Capillary Endothelium Limits the Time Course of Insulin to Activate Glucose Disposal

“…idly enters the vascular endothelial cell of skeletal muscle (within 10 min) and is then concentrated in the cell (60 min) to be slowly released from the abluminal side (19). Their results demonstrated a slow traversal of insulin across the vascular wall, which supports the delayed transendothelial transport hypothesis.…”

“…Our laboratory and others have reported that the transcapillary transport of insulin is not saturable in vivo (15,18). Barrett et al (19) have recently imaged the endothelial transport process and showed that fluorescently labeled insulin is rapidly taken up and con-centrated in endothelial cells, which suggests that transit to the interstitium may be rate limiting. However, Clark and colleagues have suggested that insulin mediates multiple hemodynamic changes in the vasculature before acting on skeletal muscle, which could also account for the delay in insulin action (20,21).…”

Endothelial cells and the IGF system

“…Capillary endothelium contains gaps between cells, and there is evidence that IGFs utilise this paracellular pathway (Bastian et al 1997). Insulin traverses the endothelium by a transcytotic pathway that involves insulin and IGF1 receptors (Wang et al 2006), so it remains possible that IGFs also utilise this pathway. Binary complexes containing IGFs and IGFBPs can traverse the endothelium and the latter have a role in regulating this process (Boes et al 2003).…”

Mechanism of Insulin Uptake in Rat Alveolar Type II and Type I-Like Epithelial Cells

“…Furthermore, the reported K d value of insulin for its receptor in primary rat alveolar epithelial cells is approximately 2.0ϫ10 Ϫ3 mg/ml, 20) which is quite different from the K m value observed in this study. Wang et al 21) suggested that insulin-like growth factor-I (IGF-I) receptor as well as insulin receptor may mediate insulin transport in endothelial cells in a process involving caveolae. However, no effect of nystatin, a caveolae-mediated endocytosis inhibitor, was observed in the present study.…”