Positive inotropic effect of Bay K 8644: cAMP-independence and lack of inhibitory effect of adenosine.

The aim of the present study was to characterize the positive inotropic effect of the Ca2+ channel activator Bay K 8644. In isolated guinea-pig papillary muscles we investigated whether adenosine and the R site adenosine receptor agonist (-)-N6-phenylisopropyladenosine (PIA) were able to antagonize the positive inotropic effect of Bay K 8644. The effect of Bay K 8644 and adenosine or PIA on myocardial cAMP content was also measured. The influence of adenosine and PIA on the positive inotropic effect of the beta-adrenoceptor agonist isoprenaline and of the phosphodiesterase inhibitor amrinone was studied for comparison. Adenosine and PIA antagonized the positive inotropic effects of isoprenaline and amrinone in a concentration-dependent manner. In contrast, adenosine or PIA did not affect the positive inotropic effect of Bay K 8644. The positive inotropic effect of Bay K 8644 was not accompanied by a change in the cAMP content of the papillary muscles. Additionally applied adenosine or PIA also failed to affect the cAMP content. It is concluded that an increased myocardial cAMP content is not involved in the positive inotropic effect of Bay K 8644. Moreover, the results support the view that adenosine and PIA only antagonize the positive inotropic effects of drugs known to increase myocardial cAMP content and that an increased myocardial cAMP content is a prerequisite for the manifestation of a negative inotropic effect of the nucleosides in ventricular cardiac muscle.