During exercise, β-adrenergic receptors are activated throughout the body. In healthy humans, the net effect of β-adrenergic stimulation is an increase in coronary blood flow. However, the role of vascular β1 versus β2 receptors in coronary exercise hyperemia is not clear. In this study we simultaneously measured noninvasive indices of myocardial oxygen supply (i.e., blood velocity in the left anterior descending coronary artery; Doppler echocardiography) and demand (i.e., rate pressure product; RPP = heart rate x systolic blood pressure) and tested the hypothesis that β1 blockade with esmolol improves coronary exercise hyperemia compared to nonselective β-blockade with propranolol. Eight healthy young men received intravenous infusions of esmolol, propranolol, and saline on three separate days in a single-blind, randomized, crossover design. During each infusion, subjects performed isometric handgrip exercise until fatigue. Blood pressure, heart rate, and coronary blood velocity (CBV) were measured continuously and RPP was calculated. Changes in parameters from baseline were compared with paired t-tests. Esmolol (=3296 ± 1204) and propranolol (=2997 ± 699) caused similar reductions in peak RPP compared to saline (=5384 ± 1865). In support of our hypothesis, CBV with esmolol was significantly greater than with propranolol (7.3 ± 2.4 versus 4.5 ± 1.6 cm/sec, P = 0.002). This effect was also evident when normalizing CBV to RPP. In summary, not only does selective β1 blockade reduce myocardial oxygen demand during exercise but it also unveils β2 receptor mediated coronary exercise hyperemia.
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