GAINEY, L.F. , Jr.; GREENBERG, M.J.; Univ. of Southern Maine, Portland; C.V. Whitney Lab., Univ. of Florida, St. Augustine.: Potentiation of branchial muscle contraction in bivalve molluscs

Previously, we found that when isolated gills of Mercenaria mercenaria are treated with 5-hydroxytryptamine (5HT), the branchial musculature contracts. Moreover, a second application of the same concentration of 5HT produces a larger contraction. At 2×10^-5 M 5HT, the mean ratio of the second to the first contractions (contraction ratio) was 2.08, and it was significantly greater than 1. This potentiation is, furthermore, mediated by a NO/cGMP signaling cascade. Now we have examined the response of the branchial muscles in the gills of 4 other bivalves to determine whether they too are potentiated. The contraction ratios of Spisula solidissima (mean = 2.33; se = 0.25; n = 31) and Mya arenaria (mean = 1.44; se = 0.11; n = 28) are both significantly greater than 1; i.e., they were potentiated. In contrast, the contraction ratios of Artica islandica (mean = 0.99; se = 0.09; n = 16) and Crassostrea virginica (mean = 1.01; se = 0.1; n = 20), are equal to 1; i.e., they were not potentiated. Mercenaria, Spisula and Mya gills contain immunoreactive nitric oxide synthase and soluble guanylate cyclase, whereas Artica and Crassostrea gills do not . We conclude that potentiation of gill contraction is not a general characteristic of bivalves and that the uneven distribution is not phylogenetically based. We also conclude tentatively that where potentiation is present, it is mediated by an NO/cGMP signaling cascade. Supported by funds from USM to LFG.