Effect of beta 1-receptor blockade on coronary resistance in partially trained dogs

Selective beta 1-receptor blockade with atenolol (1 mg X kg-1, i.v.) was used to determine the effect of partial exercise training on diastolic coronary resistance (DCR) during sub-maximal exercise. Seven conscious dogs were studied in the untrained (UT) and partially trained (PT) conditions. The conditioning regime consisted of treadmill running 5 d X wk-1 for 4 to 5 wk. Left circumflex coronary flow, aortic pressure, and heart rate were measured, and DCR and myocardial oxygen consumption were calculated. During sub-maximal exercise, DCR in the UT dogs decreased from a resting value of 4.08 +/- 0.18 mm Hg X ml-1 X min-1 to 1.91 +/- 0.17 mm Hg X ml-1 X min-1 at a workload of 6.4 kph (speed)/16% (grade). During exercise, DCR was significantly greater in UT than in PT dogs. Atenolol significantly increased DCR at all levels of sub-maximal workload in both UT and PT dogs. However, the percent increase in DCR with atenolol was significantly greater in the PT dogs compared to UT dogs. These findings suggest that daily exercise for 4 to 5 wk may decrease beta 2-receptor activity of the coronary vasculature during sub-maximal exercise.