Alpha-adrenergic receptors on rat ventricular myocytes: characteristics and linkage to cAMP metabolism

When incubated with purified cardiomyocytes from adult rat ventricle, the alpha 1-antagonist [3H]prazosin binds to a single class of sites (8 X 10(4) per cell) with high affinity [dissociation constant (KD) = 82 pM]. Competition for [3H]prazosin binding by the alpha 2-selective antagonist yohimbine [inhibitor dissociation constant (KI) = 714 nM] and the nonselective alpha-antagonist phentolamine (KI = 168 nM) demonstrates that these receptors are of the alpha 1-subtype. In addition, incubation of myocyte membranes with [3H]yohimbine results in no measurable specific binding. Agonist competition for [3H]prazosin binding to membranes prepared from purified myocytes demonstrates the presence of two components of binding: 28% of alpha 1-receptors interact with norepinephrine with high affinity (KD = 36 nM), whereas the majority of receptors (72%) have a low affinity for agonist (KD = 2.2 microM). After addition of 10 microM GTP, norepinephrine competes for [3H]prazosin binding to a single class of sites with lower affinity (KD = 2.2 microM). Incubation of intact myocytes for 2 min with 1 microM norepinephrine leads to significantly less cyclic AMP (cAMP) accumulation (13.6 pmol/mg) than stimulation with either norepinephrine plus prazosin or isoproterenol (18 pmol/mg). Likewise, incubation of intact myocytes with 10(-6) M norepinephrine leads to significantly less activation of cAMP-dependent protein kinase (71 +/- 4%) than when myocytes are stimulated by both norepinephrine and the alpha 1-adrenergic antagonist, prazosin (95 +/- 1%), or the beta-adrenergic agonist, isoproterenol (100%).(ABSTRACT TRUNCATED AT 250 WORDS)