Baroreceptor activity affects coronary vascular resistance reflexly. With carotid occlusion, baroreceptor hypotension leads to reflex adrenergic stimulation, increased metabolic activity and secondary vasodilation. When this is prevented by beta blockade, reflex coronary vasoconstriction secondary to carotid hypotension is unmasked. Alpha1 adrenoreceptor stimulation is capable of causing epicardial vasoconstriction in ischaemic myocardium, thereby influencing favourably the transmural distribution of blood flow to the subendocardium during exercise. Activation of chemoreceptors initially causes coronary dilation, a reflex that is mediated by vagi and can be abolished by atropine e.g. Bezold Jarisch reflex.