The tubular heart of the sea potato is composed of a single layer of myoepithelial cells interconnected near the extraluminal surface by specialized junctions. If these junctions are used as the border which separates the luminal from extraluminal membrane, the surface area ratio, luminal:extraluminal, is approximately 12:1. A single myofibril is located near the luminal surface in each cell. Current passed across the heart wall in the direction that depolarizes the luminal membrane and hyperpolarizes the extraluminal membrane immediately produces "all-or-none" action potentials and contractions. Current passed in the opposite direction fails to produce action potentials until after the break of the stimulus, suggesting anodal break excitation of the hyperpolarized luminal membrane. High potassium solutions depolarized the myoepithelium and produced contractions only when applied to the luminal surface of the heart. [Ca]0 increases and [Mg]0 decreases twitch tension only on the luminal surface of the heart. The transwall resistivity is low (50-100 omega/cm2) due to an extracellular shunt. Because of this shunt and the larger surface area of the luminal membrane, the extraluminal membrane is effectively clamped to the potential of the luminal membrane and is not capable of directly influencing excitation-contraction coupling. These findings suggest that only the luminal membrane of the sea potato myoepithelium is capable of generating an action potential and triggering contraction.

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