β-Blockade reduces tidal volume during heavy exercise in trained and untrained men

The effects of β-blockade on tidal volume (VT), breath cycle timing, and respiratory drive were evaluated in 14 endurance-trained [maximum O₂ uptake (V̇O₂(max)) ~65 ml·kg^-1·min^-1] and 14 untrained (V̇O₂(max) ~50 ml·kg^-1·min^-1) male subjects at 45, 60, and 75% of unblocked V̇O₂(max) and at V̇O₂(max). Propranolol (PROP, 80 mg twice daily), atenolol (ATEN, 100 mg once a day) and placebo (PLAC) were administered in a randomized double-blind design. In both subject groups both drugs attenuated the increases in VT associated with increasing work rate. CO₂ production (V̇CO₂) was not changed by either drug during submaximal exercise but was reduced in both subject groups by both drugs during maximal exercise. The relationship between minute ventilation (V̇E) and V̇CO₂ was unaltered by either drug in both subject groups due to increases in breathing frequency. In trained subjects VT was reduced during maximal exercise from 2.58 l/breath on PLAC to 2.21 l/breath on PROP and to 2.44 l/breath on ATEN. In untrained subjects VT at maximal exercise was reduced from 2.30 l/breath on PLAC to 1.99 on PROP and 2.12 on ATEN. These observations indicate that 1) since V̇E vs. V̇CO₂ was not altered by β-adrenergic blockade, the changes in VT and f did not result from a general blunting of the ventilatory response to exercise during β-adrenergic blockade; and 2) blockade of β₁- and β₂-receptors with PROP caused larger reductions in VT compared with blockade of β₁-receptors only (ATEN), suggesting that β₂-mediated bronchodilation plays a role in the VT response to heavy exercise.