Pertussis toxin unmasks stimulatory myocardial A2-adenosine receptors on ventricular cardiomyocytes

Pertussis toxin-pretreatment abolished the contractility- and cAMP-decreasing effects of the A1-adenosine receptor agonist (-)-N6-phenylisopropyladenosine (R-PIA) in the presence of isoprenaline in isolated ventricular cardiomyocytes from guinea-pigs, indicating that these stimulatory effects of A1-adenosine receptors are mediated via pertussis toxin-sensitive G-proteins. Furthermore, the decrease in contractile response by the A1/A2-adenosine receptor agonist 5'-N-ethylcarboxamidadenosine (NECA) was abolished. Moreover, NECA increased cAMP content in pertussis toxin-pretreated cells. Thus, pertussis toxin unmasked cAMP-augmenting effects of NECA, indicating that NECA can stimulate A2-adenosine receptors on cardiomyocytes. Thereby, the present study provides evidence that besides cAMP- and contractility-decreasing A1-adenosine receptors, cAMP-increasing A2-adenosine receptors coexist on ventricular cardiomyocytes, which do not influence contractile response.