Mediators of coronary reactive hyperaemia in isolated mouse heart

Mechanisms regulating coronary tone under basal conditions and during reactive hyperaemia following transient ischaemia were assessed in isolated mouse hearts.Blockade of NO-synthase (50a??I?M L-NAME), KATP channels (5a??I?M glibenclamide), A2A adenosine receptors (A2AARs; 100a??nM SCH58261), prostanoid synthesis (100a??I?M indomethacin), and EDHF (100a??nM apamin+100a??nM charybdotoxin) all reduced basal flow a??40 . Effects of L-NAME, glibenclamide, and apamin+charybdotoxin were additive, whereas coadministration of SCH58261 and indomethacin with these inhibitors failed to further limit flow.Substantial hyperaemia was observed after 5a??40a??s occlusions, with flow increasing to a peak of 48A?1a??mla??mina??1a??ga??1. Glibenclamide most effectively inhibited peak flows (up to 50 ) while L-NAME was ineffective.With longer occlusions (20a??40a??s), glibenclamide alone was increasingly ineffective, reducing peak flows by a??15 after 20a??s occlusion, and not altering peak flow after 40a??s occlusion. However, cotreatment with L-NAME+glibenclamide inhibited peak hyperaemia by 70 and 25 following 20 and 40a??s occlusions, respectively.In contrast to peak flow changes, sustained dilation and flow repayment over 60a??s was almost entirely KATP channel and NO dependent (each contributing equally) with all occlusion durations.Antagonism of A2AARs with SCH58261 reduced hyperaemia 20a??30 whereas inhibition of prostanoid synthesis was ineffective. Effects of A2AAR antagonism were absent in hearts treated with L-NAME and glibenclamide, supporting NO and KATP-channel-dependent effects of A2AARs.EDHF inhibition alone exerted minor effects on hyperaemia and only with longer occlusions. However, residual hyperaemia after 40a??s occlusion in hearts treated with L-NAME+glibenclamide+SCH58261+indomethacin was abrogated by cotreatment with apamin+charybdotoxin.Data support a primary role for KATP channels and NO in mediating sustained dilation after coronary occlusion. While KATP channels (and not NO) are also important in mediating initial peak flow adjustments after brief 5a??10a??s occlusions, their contribution declines with longer 20a??40a??s occlusions. Intrinsic activation of A2AARs is important in triggering KATP channel/NO-dependent hyperaemia. Synergistic effects of combined inhibitors implicate interplay between mediators, with compensatory changes occurring in KATP channel, NO, and/or EDHF responses when one is individually blocked.