Changes in cytosolic Ca2+ in response to noradrenaline in diabetic rat mesenteric artery

Diabetic patients are at greater risk of developing cardiovascular disease such as hypertension and atherosclerosis than the non-diabetic population. One contributing factor that has been suggested to lead to the development of the hypertensive state in diabetes is an enhanced reactivity of the vasculature. We have previously observed increased maximum contractile responses of arteries from rats with streptozotocin (STZ)- induced diabetes to ai-adrenoceptor stimulation. Additionally, this increased contractile response was still evident despite the absence of extracellular Ca and the presence of dihydropyridine (DHP) Ca²⁺ channel antagonists. The purpose of this investigation was to determine if the increase in vessel response to a i-adrenoceptor stimulation is associated with increased cytosolic Ca²⁺. The effects of a maximal concentration (30 µM) of noradrenaline (NA) and of 80 mM KC1 on cytoplasmic Ca²⁺ (measured with fura 2-AM) and contractile tension were determined simultaneously in endothelium-denuded mesenteric arteries from male Wistar rats with 12-14 week STZ-induced diabetes and age-matched controls. Contractile responses and cytoplasmic Ca²⁺ elicited by 80 mM KC1 challenge were not different between control and diabetic arteries. The NA-induced contractile responses of the diabetic arteries were significantly greater than those of the controls. This difference was still evident when the NA response was expressed as a percentage of the maximal response to 80 mM KC1. However, no difference in the maximum cytosolic Ca²⁺ concentration following NA stimulation was detected between control and diabetic arteries. Interestingly, following the addition of NA, both the rate of change in cytosolic Ca²⁺ and the rate of increase in contractile tension were significantly greater in diabetic vessels than in the controls. The influence of the endothelium on the peak cytosolic Ca²⁺ change in response to NA was also investigated. In endothelium intact control and diabetic vessels, the changes in intracellular calcium following NA stimulation were not different from the changes observed in the endothelium-denuded control and diabetic vessels. Moreover, in contrast to our previous findings, the removal of the endothelium did not appear to have any effect on the contractile responses of either the control or the diabetic arteries to NA. The peak cytosolic Ca²⁺ changes in response to maximal NA challenge in both control and diabetic tissues were not found to be significantly different. Moreover, in disagreement with our previous results, there were no differences evident between the contractile responses of control and diabetic arteries in the absence of extracellular Ca²⁺. Similarly, treatment with verapamil did not have any effect on the contractions or changes in [Ca²⁺]j in either the control or diabetic vessels. These findings indicate that although both the rates of change in cytosolic Ca²⁺ and tension development are different between the control and diabetic tissues, the enhanced maximal contractile response of diabetic rat mesenteric arteries to NA is not associated with an enhancement in the peak cytosolic Ca²⁺. Therefore other changes must contribute to the increased diabetic vessel reactivity. There may be changes in the expression and function of PKC isoforms in the diabetic state that may contribute to the enhanced contractile response seen in diabetic vessels.Medicine, Faculty ofAnesthesiology, Pharmacology and Therapeutics, Department ofGraduat