Physiology of small contractile arteries : Ca²⁺-sensitization in myogenic tone and glucose transport in endothelial cells

Recent evidence suggests the involvement of Ca²⁺ sensitization in the development of myogenic tone, possibly mediated by PKC and Rho-kinase. To investigate this, rat cerebral arteries were mounted on a pressure myograph, and pressure-induced constrictions and changes in intracellular [Ca²⁺][sub i] were recorded. The ratio of the change in diameter to change in [Ca²⁺][sub i] was greater for pressureinduced constriction compared with constriction produced by depolarization with 60 mM KPSS. Pressure-induced constriction in depolarised arteries was not associated with further increases in [Ca²⁺][sub i] but was abolished by selective inhibitors of PKC, and Rho kinase. These data suggest that in addition to increases in [Ca²⁺][sub i], enhanced myofilament Ca²⁺ sensitivity, mediated by PKC and Rho kinase activation, occurs during myogenic tone development. Although it is well established that elevated intracellular glucose concentration leads to endothelial dysfunction, how and why glucose tends to accumulate in ECs remains poorly understood. The effects of hyperglycaemia on the expression and subcellular distribution of GLUT-1 to 5 and SGLT-1 in ECs of rat microvasculature were examined. We found, through immuohistochemistry and fluorescence microscopy that all transporters except SGLT-1 were expressed preferentially at the cell-to-cell junction and on the abluminal side of these cells. Hyperglycaemia, significantly downregulated GLUT-1, 3, 4 and 5 and dramatically upregulated GLUT-2; leaving SGLT-1 unchanged. To determine the functionality of these glucose transporters in ECs, glucose uptake was monitored with a fluorescent glucose analog in live coronary arteries. The arteries were mounted in an arteriograph chamber on the stage of a confocal microscope. Results show a dense accumulation of glucose at the ECs periphery, as predicted by the subcellular distribution of the transporters. In addition, an increase in glucose uptake was observed in the presence of insulin. We conclude that the high susceptibility to glucose toxicity of ECs may be the result of the subcellular organization of their GLUTs, and the increased expression of GLUT-2. The asymmetric subcellular organisation of GLUTs may facilitate transcellular glucose exchange between the blood and the cells of the vascular wall. Finally, it was demonstrated that the endothelium of coronary arteries is insulin sensitive.Medicine, Faculty ofCellular and Physiological Sciences, Department ofGraduat