Effects of age on beta-adrenergic-mediated reflex responses to induced muscular contraction in beagles

Aging significantly affects reflex cardiovascular (CV) responses to induced muscular contraction in anesthetized dogs. To further investigate whether age-related changes in beta-adrenergic responsiveness contribute to these previously reported changes in reflex CV responses associated with advanced age, hemodynamic and regional blood flow (BF) responses 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 propranolol resulted in significant (P < or = 0.05) reductions in cardiac output, heart rate and mean arterial pressure at baseline, regardless of age. However, baseline stroke volume and systemic vascular 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 hemodynamic 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 baseline BF to five of eight abdominal organs in the older dogs. By contrast, AB resulted in attenuated, rather than accentuated, BF reductions 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, the combined BF reduction to all eight abdominal regions during HLC was attenuated only in the younger dogs, probably reflecting age-related changes in reflex sympathetic nerve responses to HLC rather than the effects 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 unchanged after AB, regardless of age. These results demonstrate that baseline differences in the level of beta-adrenergic stimulation and/or function in younger and older dogs are generally not associated with age-related differences in CV responses to HLC initiated in contracting muscles and mediated by reflex beta-adrenergic CV stimulation.