Ionic effects on intrinsic gill muscles in the freshwater bivalve, Dreissena polymorpha

The intrinsic gill muscles in Dreissena polymorpha are arranged to affect i nterfilament distance and ostial dimension. This orientation suggests a rol e in the general control of gill function including the regulation of water flow. Muscle contraction in the gill results in a decrease in overall surf ace area and we used this change in dimension as an indirect measure of mus cle contraction. The Ringer's solution bathing the gill was experimentally manipulated to assess the effects of ionic composition on muscle activity e licited by acetylcholine application. Eliminating CaCl2 in the Ringer's and adding 1 mM EGTA, or replacing the CaCl2 with CoCl2 or MnCl2 abolished the contractile response. A proper balance between NaCl and KCl was also criti cal for the maintenance of muscle response. The optimal KCI concentration w as between 1 and 2 mM and became most important at higher NaCl concentratio ns. Acclimation to hyperosmotic conditions was dependent in part on the oua bain-sensitive activity of a Na+/K+ ATPase. Overall, these muscles respond to their ionic environment as one might expect from typical molluscan smoot h muscles. An alteration of muscle function experienced during changing env ironmental conditions may limit the distribution of this species. (C) 1999 Elsevier Science Inc. All rights reserved.