Effect of exercise training volume on arterial contractility and BK_Ca channel activity in rat thoracic aorta smooth muscle cells

Large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels play a critical role in regulating cellular excitability and vascular tone. Exercise training showed reversible beneficial effects on cardiovascular systems with an improvement of vascular functions. This study investigated the effects of exercise training volume on vascular function and BK_Ca channel activity in thoracic aorta smooth muscle cells (SMCs) in 20 sedentary (SED) and 40 training rats, submitted to a treadmill training protocol (20 m/min, 60 min/day, 12 weeks). Training rats were divided into two groups, exercising 3 days/week (EX1) and 5 days/week (EX2). Since intensity and duration of exercise were identical between training groups, the training volume was higher in EX2 than in EX1. Exercise training not only decreased heart rate, but also attenuated pressor responses induced by angiotensin II or norepinephrine (NE). The maximal vascular contraction induced by 10⁻⁵ M NE was significantly decreased after training. In precontracted thoracic aorta with NE (10⁻⁵ M), activation of the BK_Ca channels by NS1619 significantly decreased the tension. The sensitivity of tissue to NS619 (pD2) was significantly correlated with volume of training (SED < EX1 < EX2). Inside-out patch clamp recording on aortic SMCs showed that exercise training significantly increased the open probability, decreased the mean closed time and increased the mean open time of BK_Ca channels. This effect was more significant in the EX2 group than in the EX1 group. These data suggest that there is a dose effect for exercise training volume for the activation of BK_Ca channels in vascular SMCs, which contributes to improvement of the arterial function in thoracic aortas.