Bullfrog Atrial Myocytes

Platelet-activating factor (PAF), a potent signaling lipid implicated as a mediator of pathological responses, has both negative chronotropic and inotropic effects on the heart, although the mechanism(s) involved is not well defined. Because activation of the muscarinic acetylcholine-activated K' current (IK(ACh)) also produces a negative chronotropic and inotropic response in myocardium, this study examines whether PAF has effects on IK(ACh) in isolated bullfrog atrial myocytes under whole-cell voltage-clamp conditions. We find that 2 ,uM PAF increases the rate of GTP-Y-S-mediated IK(ACh) activation (from 0.30±0.01 min' [n=201 to 0.73±0.07 min' [n=12],p<0.005, in the absence of acetylcholine). This effect of 2 ,M PAF was blocked by the PAF antagonist CV-3988 (5 ,M, 0.33±0.14 min' [n=12]), suggesting the presence of specific PAF receptors coupled to IK(ACh) activation. Further support for mediation by specific G protein-coupled PAF receptors derives from the inability of PAF to modulate IK(ACh) after maximal activation in the presence of GTP-y-S. Eicosatetraynoic acid (ETYA, an inhibitor of 5and 12-lipoxygenases) did not prevent the PAF-mediated increase in the rate of IK(ACh) activation (10 ,M ETYA, 0.28+0.03 min' [n=7]; 10 jAM ETYA plus 2 ,M PAF, 0.58±0.13 min'1 [n=81; p<0.05), suggesting that the observed PAF effect is not mediated by increases in arachidonic acid metabolism. These results suggest that PAF receptors are present in bullfrog atrial myocytes and that activation of IK(ACh) by PAF results from receptor-mediated increases in G protein activation. (Circulation Research 1993;72:786-794) KEY WoRDs * platelet-activating factor * muscarinic K' channels * atrium * negative inotrope