ALPHA-ADRENERGIC-MEDIATED REFLEX RESPONSES TO INDUCED MUSCULAR-CONTRACTION ARE CHANGED WITH AGE IN DOGS

Aging significantly affects reflex cardiovascular (CV) responses to in duced muscular contraction in anesthetized dogs. To further investigat e whether age-related changes in alpha-adrenergic-mediated responses t o muscular contraction contribute to these previously reported age-rel ated changes in CV responses associated with advanced age, hemodynamic and regional blood flow (BF) responses at baseline and during hind-li mb contraction (HLC) were evaluated both before and after alpha-blocka de (alpha-AB) in older (8-14 yr old) and in younger (2-3 yr old) beagl es during alpha-chloralose anesthesia. Alpha-AB with phentolamine resu lted in significant (P < 0.05) reductions in mean arterial pressure be fore and during HLC, regardless of age. However, age-related differenc es in the systemic vascular resistance, cardiac output, and stroke vol ume responses to HLC, observed before alpha-AB, were eliminated after phentolamine as the result of an age-related difference in each of the se responses to alpha-AB. Baseline BF (microspheres) was unchanged aft er alpha-AB in seven of eight abdominal organs, regardless of age. How ever, reductions in BF during HLC were attenuated in seven of eight ab dominal organs in the younger dogs after alpha-AB, but in none of thes e organs in the older dogs, indicative of diminished alpha-mediated va soconstriction in these organs in the older dogs during HLC. Furthermo re, the age-related difference in the combined BF reduction to all eig ht abdominal organs before alpha-AB was eliminated after alpha-AB. Fin ally, 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 demonstrat e that alpha-blockade eliminates many of the age-related differences i n CV responses to HLC observed before alpha-AB and suggest that alpha- adrenergic-mediated responses to HLC change with age in beagles.