Abnormal Ca2+ signalling in vascular endothelial cells from spontaneously hypertensive rats: role of free radicals

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Abstract
To test the hypothesis that the Ca2+ signal transduction process in endothelial cells from genetically hypertensive rats (SHR) is affected by an overproduction of free radicals. The Ca2+ response to the inositol 1,4,5-triphosphate (IP3) mobilizing agonist, ATP, was measured using the fluorescent probe, fura-2, in endothelial cells from Sprague–Dawley rats, and in young and age-matched genetically hypertensive rats (SHR). The effect of free radicals and reducing agents on the intracellular release of Ca2+ and IP3production was determined in resting and ATP-stimulated cells. Experiments were also performed to compare the level of expression and enzymatic activity of catalase and superoxide dismutase (SOD ) in endothelial cells from SHR and Sprague–Dawley rats. The exposure of aortic endothelial cells from Sprague–Dawley rats to the free-radical generating system, hypoxanthine + xanthine oxidase (HX/XO), caused a time- and concentration-dependent inhibition of the ATP-induced Ca2+ response. A similar HX/XO-dependent inhibition was also observed in Sprague–Dawley cells stimulated with the endoplasmic reticulum Ca2+-ATPase inhibitor, thapsigargin. Incubation with the antioxidative enzymes, catalase and SOD, had no effect on the ATP-induced Ca2+ release in Sprague–Dawley cells, but led to a strong increase in the internal release of Ca2+ in cells from adult (12 weeks old) or young (3 weeks old) SHR. The effect of antioxidants was not related either to an enhancement of the ATP-induced production of IP3, or to a lower expression and activity of SOD and catalase. The present work provides evidence that the Ca2+ signalling process in SHR endothelial cells is affected by an overproduction of free radicals, resulting in a depletion of releasable Ca2+ from IP3-sensitive and -insensitive Ca2+ pools. These results point towards a beneficial action of antioxidants on Ca2+ signalling in endothelial cells from models of hypertension.