Coronary alpha-adrenergic tone and contraction of ischemic myocardium in awake dog.
The effect of neurogenic coronary vasomotor tone upon contraction in ischemic myocardium was investigated in awake, mongrel dogs. The animals were chronically instrumented with a hydraulic occluder around the left circumflex coronary artery; a small catheter was also implanted within the vessel. Ultra-sound crystal pairs were placed distal to the occluder in myocardium perfused by the left circumflex artery. Pacing electrodes were sutured onto the right ventricular conus. During the experiment (n = 6) the occluder was inflated to stenose the vessel; the imposition of cardiac pacing (210/min) in conjunction with this stenosis resulted in depressed contraction of the myocardium distal to the occluder as assessed by the ultra-sound crystals: Segmental shortening decreased to 45.4 +/- 5.4% of unpaced control. Phentolamine, an alpha-antagonist, was then infused into the left circumflex catheter for ten minutes (0.1 mg/min) and the experiment repeated. After the alpha-blockade the combination of coronary stenosis and heart rate pacing decreased segmental shortening to only 84.6 +/- 10.1% of control, which was significantly (P less than 0.01) improved relative to the unblocked condition. In another experiment (n = 4), a less severe stenosis was imposed upon the left circumflex vessel. During pacing, muscle shortening decreased to 94 +/- 8.5% of control. Infusion of phenylephrine, an alpha-agonist, for ten minutes (0.1 mg/min) resulted in a 56.7 +/- 5.9% decrease in shortening during pacing; this was significantly greater (P less than 0.01) than the previous decrease. These data indicate that coronary alpha-adrenergic tone can significantly compromise regional myocardial function even in ischemic muscle whose coronary blood flow reserve has been exhausted.
Vallance
S R
SR
Department of Physiology and Biophysics, University of Kentucky College of Medicine, Lexington 40536-0084.
Randall
D C
DC
Yingling
J D
JD
D000818:Animals; D001794:Blood Pressure / Q000502:physiology; D002304:Cardiac Pacing, Artificial; D003243:Consciousness; D003326:Coronary Circulation / Q000187:drug effects / Q000502:physiology; D003327:Coronary Disease / Q000503:physiopathology*; D004285:Dogs; D005260:Female; D006321:Heart / Q000294:innervation* / Q000503:physiopathology; D006339:Heart Rate / Q000502:physiology; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology*; D010646:Phentolamine / Q000494:pharmacology*; D010656:Phenylephrine / Q000494:pharmacology*; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*; D014661:Vasoconstriction / Q000502:physiology*
D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine; D010646:Phentolamine