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Protein kinase C (PKC) may contribute to enhanced contractile responses of arteries from streptozotocin-diabetic rats to stimulation of G-protein coupled receptors. This was investigated by comparing the effects of PKC inhibitors on contractile responses of mesenteric arteries from diabetic and age-matched control rats to noradrenaline (NA) and endothelin-1 (ET-1). The effects of NA and ET-1 on the distribution of three isoforms of PKC implicated in contraction were also determined. In addition, the effect of NA on phosphorylation of CPI-17, a substrate for PKC, was investigated. Contractile responses of endothelium-denuded arteries from diabetic rats to NA were enhanced, but were normalized by PKC inhibition. In contrast, contractile responses to ET-1 were not significantly different, and were blocked to a similar extent by PKC inhibition, in arteries from control and diabetic rats.NA produced only a small increase in particulate levels of PKCepsilon in control arteries (to 125+/-8% of levels in untreated arteries), but a significant increase in particulate PKCalpha (to 190+/-22%) and a much greater increase in particulate PKCepsilon (to 230+/-19%) in arteries from diabetic rats. ET-1 increased particulate PKCalpha and epsilon to a similar extent in arteries from control and diabetic rats.NA significantly enhanced CPI-17 phosphorylation from a basal level of 22+/-10 to 71+/-7% of total in arteries from diabetic rats, and this was prevented by PKC inhibition. NA had no detectable effect on CPI-17 phosphorylation in arteries from control rats. These data suggest that NA-induced activation of PKC and CPI-17, its downstream target, is selectively enhanced in arteries from diabetic rats, and mediates the enhanced contractile responses to this agonist. |
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