Abstract
Background: Hypertension is characterised by endothelial dysfunction and reduced nitric oxide (NO) bioavailability. Approximately 30% of hypertensive patients are unresponsive to current clinical treatments. We hypothesised that this is due to changes in the calcium channels mediating vascular tone. Methods: Male C57BL/6 mice (8-12weeks) were treated with the NO inhibitor L-NAME (0.5 mg/ml) for two weeks. Blood pressure was measured by tail-cuff plethysmography. L- (1 μmol/L nifedipine) and T- (3 μmol/L NNC 55-0396) type calcium channel function was measured in cremaster muscle arterioles of anaesthetised (fentanyl 0.002, medetomidine 0.02, midazolam 0.2 mg/hr, i.v.) mice, before and after acute inhibition of reactive oxygen species (ROS) with the NADPH oxidase inhibitors, apocynin (500 μmol/L) or DPI (5 μmol/L), or the superoxide dismutase mimetic, tempol (1mmol/L). Results: Chronic L-NAME treatment significantly elevated blood pressure (120 ± 1mmHg), compared to control (104 ± 2mmHg, n = 5-6, P<0.05). Although vascular tone of cremaster muscle arterioles in vivo was not significantly altered (L-NAME: 45 ± 2%, n = 23 vs. Control: 51 ± 4% maximal diameter, n = 8, P>0.05). Chronic L-NAME treatment resulted in a significant increase in the contribution of T-type calcium channels (37 ± 2% vs. 18 ± 2%, P<0.05). Acute treatment of arterioles of L-NAME treated mice with apocynin, DPI or tempol significantly reduced vascular tone (18 ± 2%, 12 ± 6% and 24 ± 4% maximal diameter, respectively, n = 5-7, P<0.05) and the contribution of T-type calcium channels (12 ± 3%, 15 ± 4% and 7 ± 2%, respectively, P<0.05). Conclusions: T-type calcium channels contribute minimally to tone under physiological conditions in vivo, however chronic reduction in NO bioavailability, as occurs during cardiovascular disease, significantly increases T-type channel function and this is reversible by inhibition of ROS.
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