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Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06510, U.S.A.

1. Cells were dissociated from rat pituitary pars intermedia, maintained in culture, and subjected to electrophysiological study.

2. Recorded membrane potentials varied widely (range — 18 to about — 80 mV). They were relatively insensitive to changes in external Na but were rapidly and reversibly lowered by excess K.

3. Action potentials were elicited by passing current through intracellular recording electrodes. They were reversibly blocked by tetrodotoxin (TTX, 2 x 10^-6 M) or by removal of Na from the recording solution and thus appeared to be Na spikes. Cells yielding action potentials in response to depolarization had relatively high membrane potentials (about — 65 mV) which may be representative of the true resting membrane potential of pars intermedia cells.

4. Spontaneous action potentials were recorded extracellularly from nearly all the many isolated pars intermedia cells studied with a microsuction electrode. Their amplitude was reduced by TTX (0·1-2 x 10^-6 M). Electron microscopic examination of cells producing action potentials showed them to be hormone-containing parenchymal (gland) cells.

5. Dopamine (10^-6 M), a presumed physiological inhibitor of secretion of melanocyte stimulating hormone (MSH) from pars intermedia cells, decreased the frequency of spontaneous action potentials but not their amplitude. Similar effects were seen with noradrenaline (10^-6 M), another inhibitor of MSH secretion, whereas isoprenaline and 5-hydroxytryptamine (5-HT), which do not inhibit MSH secretion, had no effect.

6. Action potentials may be involved in stimulus-secretion coupling in pars intermedia cells.

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