Increased alphaCGRP potency and CGRP-receptor antagonist affinity in isolated hypoxic porcine intramyocardial arteries.

1. This study describes the effects of hypoxia on relaxing responses and cAMP production induced by the known vasodilator peptides: alphaCGRP, amylin (AMY) and adrenomedullin (AM) on isolated pig coronary arteries in vitro. 2. Hypoxic incubation increased the vasorelaxant effect of alphaCGRP (four-fold; P<0.05), AMY (3.2-fold; P<0.05), but not significantly for AM (two-fold; NS). 3. Whereas hypoxia had no effect on arterial cAMP levels, it significantly potentiated the production of cAMP stimulated of alphaCGRP and AMY, but not of AM. 4. The antagonist alphaCGRP(8-37) also exerted an increased effect in hypoxia. The Schild plot-derived pK(B) values revealed an increase in the apparent affinity of the antagonist for the CGRP(1) receptor from 7.0 to 7.2 under control conditions versus 8.0 in hypoxia. 5. Removal of endothelium, peptidase inhibitors, preincubation with the adenosine A(2A) receptor antagonist CSC (10(-3) M), the ATP-sensitive K-channel inhibitor glibenclamide (10(-5) M), the cyclooxygenase inhibitor indomethacin (10(-3) M) or NG-monomethyl-L-arginine (10(-4) M) had no effect on the alphaCGRP-induced vasorelaxation in hypoxia; neither did hypoxia influence the levels of CGRP and AM receptor mRNA. 6. We conclude that hypoxic incubation increases the relaxation and cAMP production induced by alphaCGRP and AMY in rings of porcine coronary arteries in vitro. A concomitant release of adenosine, a cyclooxygenase product, an endothelium-derived substance, activation of vascular ATP-sensitive K-channels, peptidase inhibitors or changes in CGRP and AM receptor mRNA cannot account for the changes observed in hypoxia. Moreover, alphaCGRP(8-37) showed increased affinity at the CGRP(1) receptor during hypoxia, possibly due to a conformational change at the CGRP(1) receptor site.