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  μ M L ‐NAME), K ATP channels (5  μ M glibenclamide), A 2A adenosine receptors (A 2A ARs; 100 n M SCH58261), prostanoid synthesis (100  μ M indomethacin), and EDHF (100 n M apamin+100 n M charybdotoxin) all reduced basal flow ∼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. 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 ∼15% 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 A 2A ARs 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 A 2A ARs. 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+glibenclamide+SCH58261+indomethacin was abrogated by cotreatment with apamin+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 A 2A ARs 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.British Journal of Pharmacology (2005) 144 , 576–587. doi: 10.1038/sj.bjp.0706099