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First published online August 25, 2003

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The Journal of Experimental Biology 206, 3507-3520 (2003)
doi: 10.1242/jeb.00573

Nitric oxide mediates seasonal muscle potentiation in clam gills

Louis F. Gainey, Jr^1,* and Michael J. Greenberg²

¹ Department of Biological Sciences, University of Southern Maine, Portland, Maine, 04104, USA
² The Whitney Laboratory, The University of Florida, 9505 Ocean Shore Boulevard, St Augustine, Florida, 32086, USA

^* Author for correspondence (e-mail: gainey{at}usm.maine.edu)

Accepted 27 June 2003

The physiology and timing of gill muscle potentiation were explored in the clam Mercenaria mercenaria. When isolated demibranchs were exposed twice (with an intervening wash) to the same concentration of 5-hydroxytryptamine, the second contraction was larger than the first. This potentiation was seasonal: it was present from November through June, and absent from July through October. Potentiation was not affected by the geographic origin of the clams, nor by their acclimation temperature. Potentiation was inhibited by the nitric oxide synthase (NOS) inhibitor L-NAME and mimicked by the nitric oxide (NO) donor DEANO. During the season of potentiation, immunoreactive NOS appeared in the gill muscles and the gill filament epithelium, but during the off-season, the enzyme occurred at the base of the gill filaments. Potentiation was inhibited by ODQ, which inhibits soluble guanylate cyclase (sGC), and it was mimicked by dibutyryl-cGMP, an analog of cyclic GMP (cGMP). Moreover, potentiation was inhibited by the protein kinase G (PKG) inhibitor Rp-8-CPT-cGMPS. During the season of potentiation, immunoreactive sGC was concentrated in the gill muscles and the gill filament epithelium; but during the off-season, immunoreactive sGC was found in the gill filament epithelium. These data suggest that the potentiation of gill muscle is mediated by a NO/cGMP/PKG signaling pathway.

Key words: clam, Mercenaria mercenaria, gill, muscle, nitric oxide (NO), potentiation, season, signaling pathway

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