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Sodium- and Calcium-Dependent Mechanisms in the Action Potential of the Secretory Epithelium of a Clam Mantle
1 Departamento de Bioquimica e Imunologia, Institute de Ciências Biológicas, UFMG, Caixa Postal 2486, 30.161 Belo Horizonte, MG, Brazil
2 Departamento de Biologia Animal, Universidade de Viçosa, 36.570 Viçosa, MG, Brazil
The secretory epithehum of the mantle of the clam Anomalocardia brasiliana is excitable. The ionic dependence of its action potentials was investigated. Two distinct phases could be recognized by their ionic dependences. The early spike phase, that appeared in all action potentials, was dependent on the Na+ concentration of the solution in the interstitial space and was insensitive to tetrodotoxin (TTX) at concentrations as high as 36µmol l-1. It was inhibited by local anesthetics, and its repolarization was inhibited by veratrine. The data show this electrogenesis is caused by TTX-insensitive sodium channels located at the basolateral membrane of this epithelium. Cardiac-like action potentials were recorded in several specimens: the rapid Na+-dependent spike was followed by a slower repolarization phase that formed a plateau and increased the action potential duration. The plateau amplitude was markedly increased when the external Ca2+ concentration was increased to (60 mmol l-1 and it was inhibited by the addition of inorganic calcium channel blockers such as Mn2+ and Cd2+. These observations suggest that inward Ca2+ currents cause the sustained depolarization during the plateau.
Key words: clam mantle, epithelial excitability, TTX-insensitive sodium channel, calcium channel, secretory epithelium
Accepted on March 31, 1989
