Functional voltage-gated Ca2+ channels in muscle fibers of the platyhelminth Dugesia tigrina |
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Authors: | Cobbett Peter Day Timothy A |
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Institution: | Department of Pharmacology and Toxicology, and the Neuroscience Program, Michigan State University, East Lansing, MI 48824-1317, USA. cobbett@msu.edu |
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Abstract: | The presence and function of voltage-gated Ca(2+) channels were examined in individual muscle fibers freshly dispersed from the triclad turbellarian Dugesia tigrina. Individual muscle fibers contracted in response to elevated extracellular K(+) in a concentration-dependent fashion. These depolarization-induced contractions were blocked by extracellular Co(2+) (2.5 mM), suggesting that they were dependent on depolarization-induced Ca(2+) influx across the sarcolemma. A voltage-gated inward current was apparent in whole cell recordings when the outward K(+) current was abolished by replacement of intracellular K(+) by Cs(+). This inward current was amplified with increasing concentration (=10 mM) of extracellular Ba(2+) and was independent of extracellular Na(+) concentration suggesting the current was mediated by voltage-gated Ca(2+) channels. Further, and supporting the hypothesis that the inward current was mediated by these Ca(2+) channels, the Ba(2+) current was blocked by extracellular Co(2+) (2.5 mM) but not by tetrodotoxin (5 microM). Action potentials were generated by the muscle fibers in the presence of, but not in the absence of, extracellular Ba(2+) (5 mM). These data are the first clear demonstration of a voltage-gated Ca(2+) channel current in platyhelminth muscle, and they demonstrate a role for Ca(2+) influx in depolarization-induced contractions of muscle in these organisms. |
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