EFFECTS OF AGE ON BETA-ADRENERGIC-MEDIATED REFLEX RESPONSES TO INDUCED MUSCULAR-CONTRACTION IN BEAGLES

Aging significantly affects reflex cardiovascular (CV) responses to in duced muscular contraction in anesthetized dogs. To further investigat e whether age-related changes in beta-adrenergic responsiveness contri bute to these previously reported changes in reflex CV responses assoc iated with advanced age, hemodynamic and regional blood flow (BF) resp onses to static hindlimb contraction (HLC) were evaluated during alpha -chloralose anesthesia before and after beta-adrenergic blockade (AB) in younger (2-3 years) and older (8-14 years) beagles. AB with propran olol resulted in significant (P less-than-or-equal-to 0.05) reductions in cardiac output, heart rate and mean arterial pressure at baseline, regardless of age. However, baseline stroke volume and systemic vascu lar resistance were only changed by AB in the older dogs. During HLC, AB had a similar effect (or lack of effect) on each of the five hemody namic responses evaluated in both age groups. AB did not significantly change baseline BF (microspheres) to any of the individual abdominal organs evaluated in the younger dogs, but resulted in a reduction in b aseline BF to five of eight abdominal organs in the older dogs. By con trast, AB resulted in attenuated, rather than accentuated, BF reductio ns during HLC to six of eight abdominal organs in the younger dogs but in only one of eight abdominal organs in the older dogs. Likewise, th e combined BF reduction to all eight abdominal regions during HLC was attenuated only in the younger dogs, probably reflecting age-related c hanges in reflex sympathetic nerve responses to HLC rather than the ef fects of age on beta-adrenergic mediated CV responses. Baseline BF was reduced in three of eight muscles after AB only in the older beagles. However, the BF response to three of four contracting muscles was unc hanged after AB, regardless of age. These results demonstrate that bas eline differences in the level of beta-adrenergic stimulation and/or f unction in younger and older dogs are generally not associated with ag e-related differences in CV responses to HLC initiated in contracting muscles and mediated by reflex beta-adrenergic CV stimulation.