Presence, distribution and physiological function of adrenergic and muscarinic receptor subtypes in the human heart

The sympathetic and parasympathetic nervous system play a powerful role in controlling cardiac function by activating adrenergic and muscarinic receptors. In the human heart there exist alpha (1)-, beta (1)- and beta (2)-adrenoceptors and M-2-muscarinic receptors and possibly also (prejunctional) alpha (2)-adrenoceptors. beta (1)- and beta (2)-adrenoceptors are quite evenly distributed in the human heart while M-2-receptors are heterogeneously distributed (more receptors in atria than in ventricles). Stimulation of beta (1)- and beta (2)-adrenoceptors causes increases in heart rate and force of contraction while stimulation of M-2- receptors decreases heart rate (directly in atria) and force of contraction (indirectly in ventricles). Pathological situations (such as heart failure) or pharmacological interventions (for example, P-blocker treatment) can alter the distribution of beta (1)- and beta (2)-adrenoceptors in the human heart, while M2-receptors are only marginally affected. on the other hand, relatively little is known on distribution and functional role of alpha (1)- and alpha (2)-adrenoceptor subtypes in the human heart.