It has been postulated that endothelin-1 and nitric oxide (NO) could participate in the modulation of aqueous humor dynamic in the eye. This study investigates whether endothelin-1 can reduce the production of nitrite (a stable metabolite of NO) in isolated porcine ciliary processes. Nitrite production was measured (Griess reaction) in the medium surrounding isolated ciliary processes before and after exposure to different drugs. Results are expressed in percent of basal nitrite production. In a concentration-dependent manner (0.1 nM to 1 microM), endothelin-1 significantly decreased basal nitrite production (1 microM: 81.7+/-3.5%; P<0.001). This effect was prevented by the endothelin type A (ETA)-receptor antagonist BQ123 (1 microM: 95.6+/-4.3%; P<0.05), but not by the endothelin type B (ETB)-receptor antagonist BQ788 (1 microM: 83.3+/-4.4%; P=0.86) or by the prostaglandin F2alpha analog unoprostone (30 microM: 86.7+/-3.7%; P=0.74). The adenylcyclase activator forskolin significantly increased basal nitrite production (1 microM: 143.4+/-3.9%, P<0.001). This effect was prevented both by endothelin-1 (0.1 microM; 113.9+/-6.7%: P<0.01) or the nitric oxide synthase inhibitor L-NAME (0.5 mM: 103.3+/-8.4%: P<0.001). The inhibitory effect of endothelin-1 on forskolin-induced nitrite production was significantly reversed by BQ123 (1 microM: 132.4+/-5.1%; P<0.01), but not by BQ788 (1 microM: 112.7+/-4.1%; P=0.64) or unoprostone (30 microM: 109.3+/-4.8%; P=0.98). These results suggest that endothelin-1, through an ETA receptor activation, can reduce both basal and forskolin-induced nitrite production in isolated porcine ciliary processes.