Characterization of adenosine receptors in guinea‐pig isolated left atria

1 The effects of purinergic stimulation on action potential, force of contraction, 86Rb efflux and 45Ca uptake were investigated in guinea‐pig left atria. 2 Adenosine exerted a negative inotropic effect which was antagonized by adenosine deaminase but enhanced by dipyridamole. 3 The negative inotropic effect of adenosine was mimicked by 5′‐(N‐ethyl)‐carboxamido‐adenosine (NECA) and the isomers of N6‐(phenyl‐isopropyl)‐adenosine, R‐PIA and S‐PIA. NECA and R‐PIA were about 1000 times more potent than adenosine, whereas R‐PIA was about 100 times more potent than S‐PIA. 4 The inotropic effects of adenosine (in the presence of dipyridamole), NECA, R‐PIA and S‐PIA were competitively antagonized either by theophylline (pA2 about 4.5) or 8‐phenyltheophylline (pA2 about 6.3). 5 NECA and R‐PIA shortened the action potential duration and increased the rate constant of the efflux of 86Rb in a concentration‐dependent manner with no differences in potency; the effects were competitively antagonized by 8‐phenyltheophylline. 6 Barium ions reduced the efflux of 86Rb under control conditions and antagonized the increase induced by NECA and R‐PIA. 7 NECA and R‐PIA significantly reduced 45Ca uptake in beating preparations. 8 It is concluded that adenosine, NECA and R‐PIA activate a common receptor population (P1 or A3) on the outside of the cell membrane of atrial heart muscle to increase the potassium conductance and to reduce the action potential and, thereby, calcium influx and force of contraction.