title: O2 consumption during exercise in dogs--roles of splenic contraction and alpha-adrenergic vasoconstriction.
abstract: To examine the influence of alpha-adrenergic vasoconstriction on the aerobic capacity of dogs, we calculated O2 consumption (VO2) by the Fick method during submaximal and maximal exertion before and during alpha-adrenergic blockade with phentolamine. Regional blood flow was measured with radioactive microspheres. alpha-Adrenergic receptor blockade reduced VO2 by 12.9% during submaximal and 17.9% during maximal exercise. Arterial and venous lactic acid approximately doubled during both levels of stress in the presence of alpha-adrenergic receptor blockade. Calculated VO2 decreased because arteriovenous O2 (A-V)O2 extraction was reduced by 11.6% during submaximal exercise. During maximal exercise a 16.7% decrease in (A-V)O2 extraction and a 5.7% decrease in cardiac output contributed to the decrease in maximal VO2. During both levels of stress, (A-V)O2 extraction was reduced because arterial O2 content was decreased. Since circulating hematocrits during exercise were reduced by alpha-adrenergic receptor blockade (43-38%), we postulate that splenic contraction likely was inhibited. Additionally, distribution of blood flow to skeletal muscle and visceral organs was unaltered by alpha-blockade. To examine the importance of splenic contraction during maximal exercise, we examined hemodynamic and metabolic responses before and after splenectomy. Compared with the spleen-intact condition, splenectomized dogs demonstrated a 12.6% reduction in VO2 as a result of 7.7 and 5.5% reductions in (A-V)O2 extraction and cardiac output, respectively. (A-V)O2 extraction was reduced because arterial O2 content and circulating hematocrit during exercise were decreased. Therefore, in the exercising dog, alpha-adrenergic receptor blockade reduces O2 consumption and causes a shift to anaerobic metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology*; D000818:Animals; D004285:Dogs; D006439:Hemodynamics / Q000187:drug effects; D007773:Lactates / Q000097:blood; D019344:Lactic Acid; D010101:Oxygen Consumption; D010646:Phentolamine / Q000494:pharmacology*; D005082:Physical Exertion; D012039:Regional Blood Flow / Q000187:drug effects; D012146:Rest; D013154:Spleen / Q000502:physiology*; D013156:Splenectomy; D014661:Vasoconstriction
chemical_list: D000317:Adrenergic alpha-Antagonists; D007773:Lactates; D019344:Lactic Acid; D010646:Phentolamine