Molecular pharmacology
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Molecular pharmacology · Jan 1985
Receptors for beta-adrenergic agonists in cultured chick ventricular cells. Relationship between agonist binding and physiologic effect.
To determine if the two-state, guanine nucleotide-modulated beta-adrenergic receptor model elucidated in erythrocyte membranes accurately describes hormone binding to intact heart cells, and to determine the relationship of agonist binding to physiologic contractile response, we studied beta-adrenergic antagonist and agonist binding to intact cultured heart cells and homogenates of these cells from embryonic chick ventricle and related the binding observations to alterations in amplitude of contraction of intact cells under identical conditions. The levo isomer of the beta-adrenergic antagonist pindolol was radioiodinated, purified, and utilized to characterize the beta-adrenergic receptor in intact, beating heart cells under physiologic conditions. Computer analysis of iodopindolol-binding isotherms revealed a KD = 22 +/- 3 pM with Bmax = 10.3 fmol/mg of protein in intact cells; in homogenates of cells, the KD was 39 +/- 12 pM in the absence of exogenous guanine nucleotides and 19 +/- 7 pM in their presence. ⋯ At the isoproterenol concentration causing 50% maximal inotropic response, 67% occupancy of high affinity receptors occurs. Thus, there is a close relationship between high affinity receptor occupancy and augmentation of contractility in intact cells. These findings support the view that agonist interaction with the guanine nucleotide-sensitive, high affinity receptor state initiates the physiologic response of myocardial tissue to beta-adrenergic agonists.