Endurance training in dogs increases vascular responsiveness to an alpha 1-agonist

The effects of endurance training on vascular responsiveness to an alpha 1-agonist and the associated changes in baroreflex modulation of heart rate and vascular resistance were studied. Graded dosages of phenylephrine were given to eight treadmill-trained dogs and to eight untrained dogs; both groups were chronically instrumented and were sedated and resting when tested. These dosages were repeated after ganglionic blockade. Aortic pressure, cardiac output, central venous pressure, peripheral resistance, and heart rate were each averaged over 30 s before injection and 90 s after injection. The slope of the peripheral resistance-dose relationship was significantly increased in trained compared with untrained dogs in both the unblocked and blocked cases [unblocked: trained 0.89, untrained 0.47; blocked: trained 4.30, untrained 2.05 (mmHg.l-1.min)/(microgram.kg-1)]. The unblocked resistance slopes were reduced with respect to the blocked slopes by 77 (untrained) and 79% (trained). The slope of the heart rate-aortic pressure response was reduced, but not significantly, by endurance training. We conclude that 6 wk of endurance training in dogs resulted in a doubling of the vascular responsiveness to an alpha 1-agonist, with no significant change in the baroreflex regulation of resistance or heart rate.