Electrophysiological and Morphological Identification of Action Potential Generating Secretory Cell Types Isolated from the Salivary Gland of Ariolimax

Abstract

The salivary glands of the terrestrial slug, Ariolimax, are composed of several morphologically distinct cell types which are observable in thin pieces of living glands and in sectioned tissue viewed with the light or electron microscope. ‘Blind’ penetration via microelectrode into the gland reveals different classes of electrical activity including: (1) cells with fast action potentials; (2) cells with slow action potentials; and (3) cells with large resting potentials but which display only delayed rectification. After isolation of cells by enzymatic dissociation we could electrophysiologically characterize individual cell types and relate these to cell types identified morphologically in the living gland and under the light and electron microscopes. Microelectrode recordings from identified cell types have demonstrated: (1) the large granule cell type (granule diameter = 8–12μm) displays a characteristic long duration, slow rise time action potential whose inward current is primarily carried by Ca2+; (2) the medium granule cell type (granule diameter = 3--6 μm) displays only delayed rectification and (3) the small granule cell type (granule diameter = 1–2μm) displays fast rise time, short duration action potentials whose inward current is also carried primarily by Ca2+ ions. The finding that morphologically distinct cell types display distinct electrical activity may indicate that different secretory products are released only under a specific set of conditions associated with changes in membrane potential.