Alpha-adrenergic vasoconstriction in arterial and arteriolar sections of the canine coronary circulation

Using a method which allowed the in-situ measurement of segmental coronary vascular resistances, the reaction of arterial and arteriolar sections of the coronary vascular system to alpha-receptor stimulation was studied i anaesthetized dogs. The left coronary artery was cannulated, and the perfusion pressure was kept constant. Allowance was made for extravascular and metabolic influences of alpha-stimulation on coronary vascular resistances. On an average, submaximal alpha-stimulation with xylometazoline increased the arterial resistance by about 60% and the arteriolar resistance by about 90%. The cannulation of the left coronary artery increased the sympathetic reactivity of the arterioles. Moreover, xylometazoline increased the extravascular component of the coronary vascular resistance by about 4%. Thus it can be assumed that under normal in-vivo conditions alpha-receptor vasoconstriction might be less different in coronary arteries and arterioles. Since the arterial resistance ranges from 20% to 50% of total coronary resistance, a sympathetic vasoconstriction of this vascular section might lead even to a critical limitation of coronary blood flow. On the other hand, a predominant constriction in arterioles leads to an increase in peripheral coronary pressure, i.e. to a "reverse coronary steal phenomenon".