Abstract
Beta-adrenoreceptor antagonists (β blockers) reduce systemic O(2) delivery and blood pressure (BP) during exercise, but the subsequent effects on O(2) extraction within the active limb muscles are unknown. In this study, we examined the effects of the fast-acting, β(1) selective blocker esmolol on systemic hemodynamics and leg muscle O(2) saturation (near infrared spectroscopy, NIRS) during submaximal leg ergometry. Our main hypothesis was that esmolol would augment exercise-induced reductions in leg muscle O(2) saturation. Eight healthy adults (6 men, 2 women; 23-67 year) performed light and moderate intensity bouts of recumbent leg cycling before (PRE), during (β(1) -blocked), and 45 min following (POST) intravenous infusion of esmolol. Oxygen uptake, heart rate (HR), BP, and O(2) saturation (SmO(2) ) of the vastus lateralis (VL) and medial gastrocnemius (MG) muscles were measured continuously. Esmolol attenuated the increases in HR and systolic BP during light (-12 ± 9 bpm and -26 ± 12 mmHg vs. PRE) and moderate intensity (-20 ± 10 bpm and -40 ± 18 mmHg vs. PRE) cycling (all P < 0.01). Exercise-induced reductions in SmO(2) occurred to a greater extent during the β(1) -blockade trial in both the VL (P = 0.001 vs. PRE) and MG muscles (P = 0.022 vs. PRE). HR, SBP and SmO(2) were restored during POST (all P < 0.01 vs. β(1) -blocked). In conclusion, esmolol rapidly and reversibly increases O(2) extraction within exercising muscles of healthy humans.