Dorzolamide-induced relaxation of isolated rabbit ciliary arteries mediated by inhibition of extracellular calcium influx

The carbonic anhydrase inhibitor dorzolamide can increase optic nerve blood flow. The aim of the study reported here was to investigate the effect of dorzolamide on isolated rabbit ciliary arteries that supply the optic nerve. Changes in ciliary artery isometric tension and intracellular Ca2+ concentration ([Ca2+](i)) were recorded to elucidate the underlying pharmacologic mechanisms by which dorzolamide regulates blood flow to the optic nerve. Dorzolamide induced concentration-dependent relaxation of rabbit ciliary arteries that had been precontracted by exposure to a high potassium (high-K) solution. Neither pretreatment with 10 A mu M KB-R 7943, an Na+/Ca2+ exchanger inhibitor, nor alkalinization of the high-K solution had an effect on the dorzolamide-induced relaxation. Pretreatment with 100 A mu M N-G-nitro-l-arginine methylester, a nitric oxide synthase inhibitor (n = 10), 10 A mu M indomethacin, a prostacyclin inhibitor (n = 9), or 0.1 A mu M iberiotoxin, an inhibitor of endothelium-derived hyperpolarizing factor (n = 7), did not change the concentration-dependent relaxation induced by dorzolamide. Incubation with 3 mM dorzolamide in a Ca2+-free solution did not change the transient contractions of the rabbit ciliary arteries induced by 1 A mu M histamine (n = 9). However, 3 mM dorzolamide significantly suppressed the increase in [Ca2+](i) induced by the reintroduction of Ca2+ to a calcium-free extracellular medium (P < 0.05). Furthermore, 3 mM dorzolamide significantly suppressed the [Ca2+](i) increase induced by the high-K solution (P < 0.05). Taken together, our results reveal a novel role for dorzolamide in relaxing the ciliary arteries. Our data support the hypothesis that the vasodilatory action of dorzolamide is mediated by inhibition of Ca2+ entry through voltage-dependent Ca2+ channels.