GAINEY, L.F., Jr*; GREENBERG, M.J.; Univ. of Southern Maine, Portland; C.V. Whitney Lab, Univ. of Florida, St. Augustine: Hypoxia abolishes potentiation of the gill muscles in the clam Mercenaria mercenaria
Earlier we showed that when the gill muscles of the clam Mercenaria mercenaria are stimulated to contract by 5-hydroxytryptamine (5HT), the contraction is about doubled when another identical dose of 5HT is applied after washout of the first dose. This potentiation is mediated by an NO/H2S -stimulated cGMP signaling cascade; it is also strongly seasonal, being absent from July through October. But during this period of warmer water and no potentiation, clams are also exposed to environmental hypoxia. Further, hypoxic mammalian smooth muscle has been shown to produce less NO and cGMP. We therefore hypothesized that hypoxia reduces the production of NO and cGMP by the gill muscle, thereby preventing potentiation from July through October. We tested this idea in the spring, when gill muscle is potentiated. Since each clam has four pieces of gill (demibranchs), 2 demibranchs were used as controls and exposed to normoxia, while 2 demibranchs were exposed to 30 min of hypoxia before the first and the second exposures to 5HT. The treatment demibranchs exposed to hypoxia were allowed to recover in normoxic seawater (again for 30 min), and were then treated with a third dose of 5HT. Although hypoxia had no significant effect upon the magnitude of the initial 5HT-induced contraction (two tailed t test p= 0.21; n= 27), hypoxia did abolish the potentiation of the second contraction in comparison with the contractions of the demibranchs exposed to normoxia (two tailed t test p= 0.01; n= 27). Finally, when the hypoxic demibranchs were returned to normoxia for 30 min, the next (third) contraction was potentiated; this potentiation was indistinguishable from the normoxic response (two tailed t test p = 0.28; n = 26).
