T he heart is heavily innervated with sympathetic and parasympathetic nerve fibers. Although both receptor types are present in the atria and ventricles, the cholinergic receptors are located mainly in the atria and the adrenergic receptors in the ventricles. The distribution of these receptors in the ventricles is not homogenous but follows apex-to-base and epicardium-to-endocardium gradients. The adrenergic receptors are localized in the myocardium and in the coronary arteries and are of

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... al types and subtypes. The ␣receptors constitute Ϸ15% of the total receptors and are both post-and presynaptic, whereas the ␤-receptors constitute Ϸ85% of the receptors and are postsynaptic. In the coronary arteries, ␣-1d and ␤-2 are most common, whereas in the left ventricular (LV) myocardium, ␣-1a and ␤-1 prevail. 1 Article see p 87 The stimulation of ␤-1 receptors induces positive inotropic and chronotropic effects, which ultimately lead to an increase in cardiac output, whereas ␤-2 stimulation activates antiapoptotic and cardioprotective pathways. 2 The stimulation of ␣-1 receptors (abundant in the larger epicardial arteries) in normal, unlike in diseased, arteries does not lead to vasoconstriction or reduction in myocardial blood flow (MBF), most probably due to low receptor density and the opposing effect of nitric oxide. The stimulation of ␣-2 receptors (more abundant in the microvasculature) results in a decrease in MBF in both normal and diseased arteries. 3 The ␤-receptors, on the other hand, induce coronary artery dilation, but treatment with ␤-blockers increases MBF perhaps because of increased diastolic time and a decrease in the abundance of ␣-receptors.