ENDOTHELIUM-DEPENDENT REGULATION OF VASCULAR TONE OF THE PORCINE OPHTHALMIC ARTERY

The endothelium produces relaxing and contracting substances, among them nitric oxide and endothelin. The role of the endothelium was investigated in the regulation of vascular tone in isolated porcine ophthalmic arteries suspended in a myograph system for isometric tension recording. In quiescent arteries with endothelium, the inhibitor of nitric oxide formation from L-arginine, L-N(G)-monomethyl arginine (L-NMMA), evoked endothelium-dependent contractions which were reversed by L-, but not D-arginine. In arteries contracted with serotonin, bradykinin evoked potent endothelium-dependent relaxations; L-NMMA markedly reduced the sensitivity but not the maximal response. Acetylcholine caused relaxations in preparations with endothelium but contractions in those without endothelium; L-NMMA prevented the relaxations and unmasked the contractions. The relaxations to the nitrovasodilator SIN-1 were more pronounced in preparations without than with endothelium. In quiescent arteries, quick stretching evoked endothelium-dependent contractions which were prevented by indomethacin. Endothelin-1, serotonin, and norepinephrine evoked concentration-dependent contractions with pD2 values of 8.1 +/- 0.1, 6.9 +/- 0.1, and 5.9 +/- 0.1, respectively. Removal of the endothelium markedly augmented the contractions induced by serotonin but not endothelin and norepinephrine. Thus, the endothelium profoundly affects vascular tone of the porcine ophthalmic artery. Endothelium-derived nitric oxide is released both under basal conditions and after stimulation with acetylcholine and bradykinin. It may play an important protective role in the circulation against vasospasm. In contrast, stretching can evoke endothelium-dependent contractions, and endothelin-1 has potent vasoconstrictor properties. Endothelium-dependent regulatory mechanisms may provide new insights into the pathogenesis of diseases of the ocular circulation.