oxadiazoles and Acidosis

The carbonic anhydrase inhibitor dorzolamide can induce relaxation of retinal arterioles with a consequent increase in blood flow and oxygenation of the retina. It has been shown that the mechanisms underlying this relaxation are independent of extracellular acidosis and CO2. The purpose of the present study was to investigate the possible involvement of nitric oxide (NO) and intracellular acidosis in dorzolamide-induced relaxation of retinal arterioles. Porcine retinal arterioles were mounted in a wire myograph and dorzolamide induced relaxation was studied after 1) the addition of the NO synthase inhibitor l-NAME (3 × 10(-4) M) or the guanylyl cyclase inhibitor ODQ (3 × 10(-6) M), and 2) after loading the smooth muscle cells with the pH sensitive fluorophore SNARF-1-AM and studying changes in vascular tone and intracellular fluorescence after the induction of hypoxia, addition of lactate (10(-2) M), and extracellular acidification (pH = 7.0) alone and in the presence of dorzolamide (10(-3) M). Dorzolamide significantly relaxed retinal arterioles (p < 0.03), and the effect was significantly higher in the presence of perivascular tissue than in isolated vessels at the highest concentration (p < 0.01). In the presence of perivascular tissue dorzolamide-induced relaxation could be reduced by NO inhibition (p < 0.02). Dorzolamide increased intracellular acidification (p < 0.02) during extracellular acidosis, but there was no relation between relaxation and intracellular acidosis. In conclusion, dorzolamide-induced vasorelaxation depends on NO and the perivascular retinal tissue, but is independent of acidification in the extracellular and the intracellular space of retinal vascular smooth muscle cells. Other factors than NO and acidification are involved in dorzolamide-induced relaxation of retinal arterioles.

Topics: Acidosis; Animals; Arterioles; Benzopyrans; Bradykinin; Carbonic Anhydrase Inhibitors; Endothelium, Vascular; Enzyme Inhibitors; Fluorescent Dyes; Hydrogen-Ion Concentration; Lactates; Muscle, Smooth, Vascular; Myography; NG-Nitroarginine Methyl Ester; Nitric Oxide; Oxadiazoles; Quinoxalines; Retinal Artery; Sulfonamides; Swine; Thiophenes; Vasodilation

The fundamental, yet poorly understood, physiological mechanism known as 'acidic-metabolic' vasodilation, contributes to local blood flow regulation during hypoxia/ischaemia and increased metabolic activity. The vasodilator nitric oxide (NO) has been suggested to be involved in this event. Besides enzymatic production by NO synthases, a novel mechanism for generation of this gas in vivo was recently described. This involves non-enzymatic reduction of inorganic nitrite to NO, a reaction that takes place predominantly during acidic/reducing conditions. We have studied the effects of physiological amounts of nitrite on NO generation and relaxation of rat aorta in vitro in a situation where environmental pH was reduced to levels seen in tissues during hypoxia/ischaemia. The relaxatory effect of nitrite was increased in an acidic buffer solution (pH 6.6) compared with neutral pH; EC50 for nitrite was reduced from 200 to 40 microM. Nitrite-evoked relaxation was effectively prevented by coadministration of an inhibitor of soluble guanylyl cyclase. The relaxation was further potentiated by the addition of ascorbic acid. In parallel, NO was generated from nitrite in a pH dependent manner with even larger amounts seen after addition of ascorbic acid. NO generation from nitrite correlated to the the degree of relaxation of rat aorta. These results illustrate non-enzymatic release of NO from nitrite at physiological concentrations. This may be an important auto-regulated physiological mechanism involved in the regulation of vascular tone during hypoxia/ischaemia.

Topics: Acidosis; Animals; Aorta; Dose-Response Relationship, Drug; Enzyme Inhibitors; Hypoxia; In Vitro Techniques; Indicators and Reagents; Muscle, Smooth, Vascular; Nitric Oxide; Nitric Oxide Donors; Oxadiazoles; Penicillamine; Quinoxalines; Rats; Rats, Wistar; Regional Blood Flow; Sodium Nitrite; Vasodilation