Hydrogen sulfide as an endogenous regulator of vascular smooth muscle tone in trout

Hydrogen sulfide (H₂S) is an endogenous vasodilator in mammals, but its presence and function in other vertebrates is unknown. We generated H₂S from NaHS and examined the effects on isolated efferent branchial arteries from steelhead (stEBA) or rainbow (rtEBA) trout. H ₂S concentration was measured colorimetrically (CM) and with ion-selective electrodes (ISE) in rainbow trout plasma. NaHS produced a triphasic response consisting of a relaxation (phase 1), constriction (phase 2), and relaxation (phase 3) in both unstimulated vessels and in stEBA precontracted with carbachol (Carb). Phase 1 and phase 3 in stEBA were decreased and phase 2 increased in unstimulated vessels by K⁺ _ATP channel inhibition (glibenclamide), or a cocktail of inhibitors of cyclooxygenase, lipoxygenase, and cytochrome P-450 (indomethacin, esculetin, and clotrimazole). Inhibition of soluble guanylate cyclase with ODQ or NS-2028 inhibited phase 3 in stEBA, although NaHS decreased cGMP production by rtEBA. stEBA phase 2 contractions were partially inhibited by the myosin light chain kinase inhibitor, ML-9, but unaffected by L-type calcium channel inhibition (methoxyverapamil), whereas contraction in rtEBA was partially inhibited by nifedipine or removal of extracellular calcium. Phase 3 relaxations were more pronounced in stEBA precontracted with Carb and norepinephrine (NE) than those contracted by KC1 or K₂SO₄. stEBA phase 2 and phase 3 responses were dose dependent (EC₅₀ = 1.1 ± 1.2 × 10^-3 M and 6.7 ± 0.9 × 10^-5 M, respectively; n = 7). NaHS was also vasoactive in steelhead bulbus arteriosus, celiacomesenteric arteries, and anterior cardinal veins. Rainbow trout plasma sulfide concentration was 4.0 ± 0.3 × 10-5 M, n = 4 (CM) and 3.8 ± 0.4 × 10^-5 M, n = 9 (ISE); similar to phase 3 EC ₅₀. Because NaHS has substantial vasoactive effects at physiological plasma concentrations, we propose that its soluble derivative, H₂S, is a tonically active endogenous vasoregulator in trout.