Intracellular recordings of membrane potential and input resistance have been made in vivo and in vitro from the exocrine acinar cells of rat pancreas using indwelling glass micro-electrodes. 2. The resting cell membrane potential and input resistance in the in vivo experiments were not markedly different from the values obtained in the in vitro experiments. The effect of both acetylcholine (ACh) and pancreozymin (CCK-Pz) on the pancreas in vivo as well as in vitro was to reduce both the acinar cell membrane potential and the input resistance narkedly. The amplitude of the evoked depolarization and the change in input resistance evoked by supramaximal stimuli were of the same magnitude in both types of preparations. 3. Gastrin had an effect on the acinar cell potential and resistance which was indistinguishable from that of CCK-Pz or ACh. The effect of gastrin or CCK-Pz was, in contrast to that of ACh, not influenced by the presence of atropine. The reversal potential for the gastrin evoked potential change was about -20 mV. 4. Secretin in doses producing maximal volume secretion in vivo had no effect on acinar cell membrane potential and input resistance. 5. Dibutyryl cyclic AMP (5mM) and cyclic GMP (1mM) had no effect on cell membrane potential or resistance. 6. It is concluded that the in vitro superfused pancreas segment preparation is a useful model system in electrophysiological studies since it functions essentially as the in vivo preparation. In contrast to both gastrin and CCK-Pz, secretin has no effect on the bioelectrical properties of the acinar cells, indicating that there are no physiologically important secretin receptors in rat acinar cells.
