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Journal of Experimental Biology 121,179-195 (1986)
Published by Company of Biologists 1986

Two Types of Action Potentials in a Secretory Epithelium of a Clam Mantle

PAULO S. BEIRÃO ¹ and A. L. SORENSON ²

¹ Departamento de Bioquímica e Imunologia, Institute de Ciências Biológicas, UFMG, Caixa Postal 2486, 30000 Belo Horizonte, Minos Gerais, Brazil
² Departamento de Bioquímica e Imunologia, Institute de Ciências Biológicas, UFMG, Caixa Postal 2486, 30000 Belo Horizonte, Minos Gerais, Brazil; Instituto de Bioffsica, UFRJ, Ilha do Fundão, 21910 Rio de Janeiro, RJ, Brazil

The secretory epithelium that faces the shell of the clam Anomalocardia brasiliana is electrically excitable. It was found that two types of action potentials could be elicited by electrical stimulation. In mantles showing signs of being in the phase of shell secretion, cardiac-like action potentials were observed. They had an overshoot of 48.4 ± 8.8 mV (±S.D.), a rate of rise of 54.7 ± 5.6 V s^-1 and three phases of repolarization of 9.3 ± 4.8, 1.44 ± 0.70 and 4.7 ± 1.1 V s^-1. Their duration averaged 38.4 ± 13.3 ms. Mantles of clams resorbing the shell had spike-like action potentials with an overshoot of 38.7 ± 5.5 mV, a rate of rise of 52.7 ± 16.8 Vs^-1, and a single phase of repolarization of 12.4 ± 2.5 Vs^-1. Their duration was 11.1 ± l.4ms. It is suggested that the presence of a plateau may be related to shell secretion. Significant differences were also found in the resting potentials: -54.8±5.4mV in cells of mantles with cardiac-like responses and -62.5 ±4.9 mV in cells of mantles with spikes. In both types of mantles the action potentials were propagated throughout the epithelium and both exhibited a refractory period. Lowered Ca or increased frequency of stimulation reduced the plateau of cardiac-like action potentials without any marked alteration in the resting potential, rate of rise, or overshoot. The action potential was elicited by outward (depolarizing) current across the basolateral membrane, and not by similarly-directed current across the apical membrane, so we conclude that the basolateral membrane is the excitable one.

Key words: clam mantle, epithelial excitability, frequency-dependent action potentials, secretory epithelium

Accepted on September 26, 1985