Seasonal potentiation of gill muscle contraction in four species of bivalve molluscs

When isolated gills of Mercenaria mercenaria are treated with dopamine (DA), the branchial musculature contracts. Moreover, a second application of the same concentration of DA produces a larger contraction. The mean ratio of the second to the first contractions (contraction ratio) was 2.18, and it is significantly greater than 1. This potentiation was seasonal and was mediated by a nitric oxide/cyclic GMP (NO/cGMP) signaling cascade. The responses of the branchial muscles in the gills of four other bivalves were examined to determine whether they too are potentiated. The contraction ratios of Mya arenaria (mean = 1.48; 95% CI of mean: 1.28–1.72; n = 39) and Spisula solidissima (mean = 2.04; 95% CI of mean: 1.71–2.43; n = 31) are both significantly greater than 1; i.e., they were potentiated. In contrast, the contraction ratios of Arctica islandica (mean = 0.94; 95%CI of mean: 0.79–1.14; n = 16) and Crassostrea virginica (mean = 0.91; 95% CI of mean: 0.73–1.13; n = 20), are equal to 1; i.e., they were not potentiated. The potentiation of the gill muscles in both Mya and Spisula was seasonal like that in Mercenaria: potentiation was present during the winter but absent during the summer. Spisula and Mya gills contain immunoreactive nitric oxide synthase and soluble guanylate cyclase during the winter but not the summer, whereas Arctica and Crassostrea gills do not contain either enzyme. While potentiation in Mya and Spisula was abolished by the inhibition of nitric oxide synthase, inhibition of guanylate cyclase had no effect on potentiation. These results are consistent with the location of the enzymes: both are present in the gill filaments but not the gill muscles. Thus, NO must diffuse from the filaments to the muscles to induce potentiation.