Alpha-adrenergic-mediated reflex responses to induced muscular contraction are changed with age in dogs.

Aging significantly affects reflex cardiovascular (CV) responses to induced muscular contraction in anesthetized dogs. To further investigate whether age-related changes in alpha-adrenergic-mediated responses to muscular contraction contribute to these previously reported age-related changes in CV responses associated with advanced age, hemodynamic and regional blood flow (BF) responses at baseline and during hindlimb contraction (HLC) were evaluated both before and after alpha-blockade (alpha-AB) in older (8-14 yr old) and in younger (2-3 yr old) beagles during alpha-chloralose anesthesia. alpha-AB with phentolamine resulted in significant (P < 0.05) reductions in mean arterial pressure before and during HLC, regardless of age. However, age-related differences in the systemic vascular resistance, cardiac output, and stroke volume responses to HLC, observed before alpha-AB, were eliminated after phentolamine as the result of an age-related difference in each of these responses to alpha-AB. Baseline BF (microspheres) was unchanged after alpha-AB in seven of eight abdominal organs, regardless of age. However, reductions in BF during HLC were attenuated in seven of eight abdominal organs in the younger dogs after alpha-AB, but in none of these organs in the older dogs, indicative of diminished alpha-mediated vasoconstriction in these organs in the older dogs during HLC. Furthermore, the age-related difference in the combined BF reduction to all eight abdominal organs before alpha-AB was eliminated after alpha-AB. Finally, BF increases to two of four contracting muscles, as well as the combined increase in blood flow to all four contracting muscles, were attenuated after alpha-AB, regardless of age. These results demonstrate that alpha-blockade eliminates many of the age-related differences in CV responses to HLC observed before alpha-AB and suggest that alpha-adrenergic-mediated responses to HLC change with age in beagles.