Progressive glomerular sclerosis develops in a large proportion of patients with insulin-dependent diabetes mellitus. In diabetic rats glomerular sclerosis can be prevented if glomerular capillary pressures are kept within the normal range even if moderate hyperglycemia persists. Thus, renal vasodilatation appears to play a central role in this development of diabetic glomerular sclerosis. The cellular mechanisms that lead to renal vasodilatation and the factors that stimulate glomerular mesangial cell hyperplasia and hypertrophy in diabetes mellitus are the focus of this proposal. Chronic hyperglycemia is known to be associated with a reduction in glomerular myo-inositol content. Plasma membrane phosphotidylinositol metabolism is triggered by the interaction of vasoconstictor hormones with cell surface receptors, thus liberating inositol trisphosphate (IP3), which in turn releases calcium from intracellular stores, the latter required for smooth muscle cell contraction. This proposal aims to determine whether glomerular and glomerular mesangial cell myo-inositol depletion in diabetes results in a decrease of 1P3 release from membrane phospholipids in response to stimulation with the vasoconstrictor hormones, and whether the increment in cytosolic calcium concentration that is normally triggered by 1P3 is reduced. An attenuation of post-receptor signalling mechanisms that could result from inositol depletion may well underlie the reduced responsiveness of vascular smooth muscle and mesangial cells to vasoconstrictor hormones in diabetes and may therefore explain, in part, renal vasodilation early in diabetes mellitus. The second question that will be addressed concerns the hyperplastic and hypertrophic response of mesangial cell to the diabetic environment. It is reasoned that glomerular endothelial cells, like endothelial cells from other tissue may produced growth factors that stimulate mesangial cell proliferation and/or hypertrophy. In a system of glomerular mesangial-endothelial cell co-culture, it will be determined whether endothelial cells produced factors that enhance mesangial cell proliferation or hypertrophy and whether the release of such factors is influenced by insulin deprivation, exposure to extracellular high glucose concentrations, or by enhanced shear stresses on endothelial cells. In this fashion the mechanisms that initiate glomerular mesangial expansion in diabetes mellitus may be elucidated.
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