GAINEY, JR., L.F. *; CHICOINE, D.; WALTON, J.; GREENBERG, M.J.: Hydrogen sulfide potentiates muscle contraction in clam gills
When isolated gills of Mercenaria mercenaria are treated with 5-hydroxytryptamine (5HT), the branchial musculature contracts in a dose dependent manner. Moreover, a second application of 5HT at the same concentration produces a larger contraction; this effect is mediated by nitric oxide (NO) synthesized in the muscles. Since NO and hydrogen sulfide (H2S) both modulate muscle contraction in mammalian vascular smooth muscle and because Mercenaria is exposed to H2S in the environment, we investigated the effects of H2S on gill muscle contraction. When gills were pretreated with 10-6 M H2S and then exposed to increasing concentrations of 5HT, the dose response curve was shifted to the left and the maximal contraction increased in comparison with untreated controls. With 10-6 M H2S, the concentration of 5HT giving a half maximal contraction (EC50) was 1.1 x 10-5 M and the maximal contraction was 32% of the resting length at 10-4 M 5HT. In untreated gills, in contrast, the EC50 was 2.8×10-5 M and the maximal contraction was only 12%. Finally, when gills were pretreated with increasing concentrations of H2S and then exposed to 2×10-5 M 5HT, the contractions were potentiated in a dose dependent manner. The threshold for potentiation was 10-10 M H2S, and the effect became maximal between 10-8 and 10-5 M. Supported by grants from the Maine Science and Technology Foundation and the University of Southern Maine (LFG).