Adrenomedullin-induced endothelium-dependent relaxation in porcine ciliary arteries

PURPOSE. To investigate adrenomedullin-induced relaxation in isolated porcine ciliary arteries. METHODS. In a myograph system (isometric force measurement), precontracted vessels (similar to0.1 muM U46619; thromboxane A(2) analogue or similar to10 nM endothelin-1) were exposed, in a cumulative manner, to increasing concentrations of adrenomedullin (1 nM to 1 muM) in the presence or absence of different drugs. Some experiments were conducted in vessels with nonfunctional (intentionally mechanically damaged) endothelium. RESULTS. Adrenomedullin evoked marked relaxation [maximum relaxation (Rel(max)): 85.5 +/- 3.0%; negative log M concentration inducing 50% of Rel(max) (pD(2)): 7.4 +/- 0.1] in comparison to time-controls (Rel(max): 19.2 +/- 4.8%; P < 0.001). Relaxation was inhibited by 3 mu M CGRP[8-37] (CGRP(1) receptor antagonist; Rel(max): 27.2% +/- 5.3%; P < 0.001) but not by 3 muM adrenomedullin[22-52] (presumed adrenomedullin receptor antagonist; P = 0.75). Adrenomedullin-induced relaxation was less pronounced in nonfunctional endothelium. vessels (Rel(max): 67.6% +/- 3.1%; pD(2): 6.9 +/- 0.1; P < 0.01). In vessels with functional endothelium, relaxation was not significantly influenced by 0.1 mM N-G-nitro-L-arginine methyl ester (L-NAME; a nitric oxide synthesis inhibitor), 10 mu M indomethacin (a cyclooxygenase inhibitor), or 10 mu M 17-octadecynoic acid (a cytochrome P-450 inhibitor). In contrast, relaxation was significantly inhibited by either 10 mM tetraethylammonium (nonselective potassium channel inhibitor; P < 0.01) or 50 nM apamin (small conductance potassium channel inhibitor), together with 50 nM charybdotoxin (large and intermediate potassium channel inhibitor; P < 0.01). In the presence of these potassium channel inhibitors, the amount of relaxation was not significantly different (P > 0.50) from that observed in vessels with nonfunctional endothelium. CONCLUSIONS. In isolated porcine ciliary arteries, adrenomedullin induces relaxation that involves CGRP, receptors and is in part endothelium dependent. Endothelium-dependent relaxation was blocked by some potassium channel inhibitors, suggesting the possible release of an endothelium-derived hyperpolarizing factor (EDHF).