GAINEY, L.F., Jr. ; GREENBERG, M.J.; Univ. of Southern Maine, Portland ; C.V. Whitney Lab, Univ. of Florida, St. Augustine: Protein synthesis is necessary for potentiation of muscle contraction in clam gills

When isolated gills of Mercenaria mercenaria are exposed twice (with an intervening wash) to the same concentration of 5-hydroxytryptamine (5HT), the second contraction is larger than the first. This muscle potentiation persists for 24 h and has a strong seasonal component. Potentiation is present from November through June and is absent from July through October. Moreover, potentiation is mediated by an NO/cGMP signaling cascade. Because NO and cGMP are produced during both the first and second exposures to 5HT, we hypothesized that concentrations of the enzymes in the signaling cascade would be larger after the initial exposure to 5HT. To test this hypothesis, we exposed isolated gills to the protein translation inhibitor anisomycin (10^-5 M) for 60 min, either before the first or second contraction. In both cases, anisomycin inhibited potentiation (p=0.01, n= 12). As a control, we exposed isolated gills to anisomycin during the summer when potentiation is absent, and anisomycin had no effect upon the contractions (p = 0.14, n= 8). We also exposed gills to the mRNA transcription inhibitor DRB (10^-4 M) for 60 min, either before the first or second contraction. DRB blocked potentiation when it was applied before the first contraction (p= 0.03; n= 8), but had no effect if applied before the second contraction (p = 0.4; n= 8). These data imply that the initial exposure to 5HT initiates transcription and translation of the enzymes in the signaling cascade. Supported by grants from The University of Southern Maine (LFG).