Abstract
Heart rate and blood and muscle lactate concentrations were studied in 15 physically active males during submaximal exercise before and after oral administration of 80 mg propranolol (Inderal®). First, a cycle exercise protocol, with a stepwise increased exercise intensity (30 W every 4 min until voluntary exhaustion) was performed before and after β-blockade. Blood samples for lactate determination were obtained before each exercise intensity increase. In a second set of experiments, the procedure was repeated before and after β-blockade, but exercise was terminated at the exercise corresponding to a blood lactate concentration of 4 mmol·l-1 l-1, as determined beforehand without β-blockade (i.e., the same absolute work load was compared with and without β-blockade), and a muscle biopsy was taken from m. vastus lateralis for subsequent lactate analyses. Following β-blockade at low and high work loads, the heart rate decreased similarly by 22%-26%. Blood lactate concentration was unchanged at exercise intensities up to 150 W. At higher exercise intensities (180-240 W), a significant increase (P < 0.05 - P < 0.01) in blood lactate concentration occurred after β-blockade. Muscle lactate concentration was unaffected by β-blockade at a work load (130-230 W) where blood lactate was higher with than without blockade. Assuming a reduced blood flow to the exercising muscle, there are reasons to speculate that lactate production was diminished following β-blockade even though blood lactate increased probably as a result of reduced lactate uptake by other organs.
Key words
exercise - heart rate - muscle lactate - blood lactate - β-blockade
