Adenosine A2a and A2b receptors in cultured fetal chick heart cells. High- and low-affinity coupling to stimulation of myocyte contractility and cAMP accumulation.

Adenosine exerts pronounced biological effects in the heart cell. The role of multiple adenosine receptor subtypes in regulating the heart cell function is not known. Ventricular cells cultured from chick embryos 14 days in ovo were used to study a novel feature of heart cell regulation by the stimulatory adenosine receptors. The inhibitory adenosine A1 receptor pathway was first inactivated by pertussis toxin treatment of the cultures, and the effects of adenosine agonists and antagonists on the heart cell contractile amplitude, measured via an opticovideo motion detection system, and on the modulation of cAMP level were determined. Adenosine and N-ethyladenosine-5'-uronic acid (NECA), capable of activating both the adenosine A2a and A2b receptors, caused a greater increase in the contractile amplitude than did the A2a-selective agonist 2-[4-(2-carboxythyl)phenylethylamino]-5'-N-ethylcarboxamidoa denosine (CGS21680). NECA caused a biphasic increase in cAMP, which became monophasic in the presence of the A2a receptor-selective antagonist 8-(3-chlorostyryl)caffeine, whereas the CGS21680-induced cAMP response was monophasic. Blocking with 8-(3-chlorostyryl)caffeine abolished most of the CGS21680-elicited contractile or cAMP response while attenuating only part of the adenosine- or NECA-stimulated responses. Blocking with the A2b-selective antagonists 1,3-diethyl-8-phenylxanthine or alloxazine caused a more pronounced inhibititon of the contractile or cAMP response by adenosine or NECA than by CGS21680. Affinity of the A2a receptor was 60-fold higher than that of the A2b receptor. These data demonstrate that a functional A2b receptor is expressed on the heart cell and is capable of mediating augmentation of cardiac myocyte contractility and that adenosine A2a and A2b receptors, with greatly different affinity, coexist and are coupled to the same functional responses. Taken together, the data suggest a novel feature of heart cell regulation, where the high-affinity A2a receptor can play an important modulatory role in the presence of a low level of adenosine, whereas the low-affinity A2b receptor becomes functionally important when the adenosine level is high.