Human cardiac beta-adrenergic receptors: subtype heterogeneity delineated by direct radioligand binding.

Human myocardial beta-adrenergic receptors were directly identified and characterized using the high affinity antagonist radioligand [125I]iodocyanopindolol. Beta 1 and beta 2 adrenergic receptors were found to coexist in both the left ventricle and right atrium. The relative proportions of the two receptor subtypes were determined by the use of competition radioligand binding and computer modelling techniques employing the subtype selective agents atenolol (beta 1 selective) and zinterol (beta 2 selective). The left ventricle contains 86 +/- 1% beta 1 and 14 +/- 1% beta 2 adrenergic receptors while the right atrium contains 74 +/- 6% beta 1 and 26 +/- 6% beta 2 adrenergic receptors. The direct demonstration of beta 2 adrenergic receptors in the human heart, with a higher proportion in the right atrium agrees with pharmacologic data and supports the notion that chronotropic effects of adrenergic agonists in man may be mediated by both beta 1 and beta 2 adrenergic receptors.