Mediators of coronary reactive hyperaemia in isolated mouse heart

1 Mechanisms regulating coronary tone under basal conditions and during reactive hyperaemia following transient ischaemia were assessed in isolated mouse hearts. 2 Blockade of NO-synthase (50 muM L-NAME), K-ATP channels (5 muM glibenclamide), A(2A) adenosine receptors (A(2A)ARs; 100 nM SCH58261), prostanoid synthesis (100 mM indomethacin), and EDHF (100 nM apamin+100 nM charybdotoxin) all reduced basal flow similar to40%. Effects of L-NAME, glibenclamide, and apamin+charybdotoxin were additive, whereas coadministration of SCH58261 and indomethacin with these inhibitors failed to further limit flow. 3 Substantial hyperaemia was observed after 5-40 s occlusions, with flow increasing to a peak of 48+/-1 ml min(-1) g(-1). Glibenclamide most effectively inhibited peak flows (up to 50%) while L-NAME was ineffective. 4 With longer occlusions (20-40 s), glibenclamide alone was increasingly ineffective, reducing peak flows by similar to15% after 20 s occlusion, and not altering peak flow after 40 s occlusion. However, cotreatment with L-NAME+glibenclamide inhibited peak hyperaemia by 70 and 25% following 20 and 40 s occlusions, respectively. 5 In contrast to peak flow changes, sustained dilation and flow repayment over 60 s was almost entirely K-ATP channel and NO dependent (each contributing equally) with all occlusion durations. 6 Antagonism of A2AARs with SCH58261 reduced hyperaemia 20-30% whereas inhibition of prostanoid synthesis was ineffective. Effects of A(2A)AR antagonism were absent in hearts treated with L-NAME and glibenclamide, supporting NO and K-ATP-channel-dependent effects of A2AARs. 7 EDHF inhibition alone exerted minor effects on hyperaemia and only with longer occlusions. However, residual hyperaemia after 40 s occlusion in hearts treated with L-NAME+glibenclami-glibenclamide+SCH58261+indomethacin was abrogated by cotreatment with apamin+charybdo-charybdotoxin. 8 Data support a primary role for K-ATP channels and NO in mediating sustained dilation after coronary occlusion. While K-ATP channels (and not NO) are also important in mediating initial peak flow adjustments after brief 5-10 s occlusions, their contribution declines with longer 20-40 s occlusions. Intrinsic activation of A2AARs is important in triggering K-ATP channel/NO-dependent hyperaemia. Synergistic effects of combined inhibitors implicate interplay between mediators, with compensatory changes occurring in K-ATP channel, NO, and/or EDHF responses when one is individually blocked.