Pancreatic acinar cells: acetylcholine‐evoked electrical uncoupling and its ionic dependency

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Abstract
Isolated segments of mouse or rat pancreas were placed in a Perspex bath through which physiological saline solutions were circulated. Intracellular recordings from surface acini were made by inserting up to 3 separate micro-electrodes into neighboring cells within the same acinus. Through 1 of these electrodes current pulses could be injected. Electrical coupling between the cells could be continuously assessed. The acinus under investigation was stimulated by iontophoresis of acetylcholine (ACh) from an extracellular micropipette. During resting conditions all cells within an acinus were fully electrically coupled. A 5-10 s pulse of a relatively large iontophoretic ACh ejecting current, causing initially the well characterized surface cell membrane depolarization and resistance reduction, was followed by a marked electrical uncoupling of neighboring cells. In some cases 1 cell was clearly being isolated equally from the 2 other cells impaled in other cases the cell into which current pulses were injected was becoming more isolated from 1 of the neighboring cells than from the other. The ACh-evoked electrical uncoupling was completely reversible and could be repeatedly observed in the same cell. The electrical uncoupling in response to short pulses of ACh (5-10 s) was not affected by even prolonged (up to 50 min) exposure to Ca-free solution containing EGTA [ethylene glycol bis (.beta. aminoethyl ether) tetraacetic acid]. ACh-evoked uncoupling (short pulses) was not reduced about 5 min after introduction of Mn (2 mM), Ni or Co (5 mM). Min thereafter gradually reduced and finally (after about 1/2 h) abolished the ACh-evoked uncoupling. Ni and Co gradually enhanced the uncoupling response. The blocking effect of Mn on the ACh-evoked uncoupling occurred much faster in the absence of superfusion fluid Ca. During a period of sustained ACh stimulation sustained electrical uncoupling was observed. If Ni or Co (5 mM) was introduced during such a stimulatory period the cells recoupled. This effect of Ni and Co was immediate and fully reversible. The uncoupling responses to short pulses of ACh was not reduced immediately after replacing superfusion fluid Na by Tris at a time when the surface cell membrane depolarization and resistance reduction was already severely reduced. Thereafter a gradual slow reduction of the ACh-evoked uncoupling was observed. This did not occur if Na was replaced by Li. The Ca needed to raise [Ca2+]i sufficiently to cause electrical uncoupling apparently comes from the cells in cases of uncoupling evoked by short pulses of ACh, but from the external solution in cases of sustained uncoupling evoked by sustained stimulation of ACh.