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Department of Autonomic Physiology, Tokyo Medical and Dental University, Japan.

1. The effects of ouabain (1 nM-100 microM) on the membrane properties of rabbit visceral primary afferent neurones (nodose ganglion cells) were studied with intracellular recordings and voltage-clamp techniques in vitro. 2. Ouabain (greater than or equal to 1 microM) often produced a membrane hyperpolarization associated with a fall of membrane resistance in type C neurones. The ouabain-induced hyperpolarization reversed in polarity at about -90 mV. These suggest that the ouabain-induced hyperpolarization is due to an increase in potassium conductance. 3. Both the peak amplitude and the duration of the after-hyperpolarization following an action potential were reversibly increased with increasing concentration of ouabain. In tetraethylammonium (TEA, 10-20 mM) and tetrodotoxin (TTX, 1-10 microM), the duration of both the calcium-dependent action potential and the after-hyperpolarization following the action potential was prolonged by ouabain (greater than or equal to 10 nM). 4. A depolarizing command pulse evoked a slow outward current in TEA (10-20 mM) and TTX (1-10 microM). This was increased in amplitude and prolonged in duration by ouabain (100 nM-1 microM). Such augmentation of the slow outward current by ouabain was usually associated with an increase in a slow inward current during the period of the depolarizing command pulse. 5. An outward current produced by the calcium ionophore A23187 was reversibly augmented by ouabain (greater than or equal to 10 nM). 6. An outward current caused by exchanging a potassium-free superfusion solution for one containing 4.7 or 10 mM-potassium was completely abolished by ouabain (greater than or equal to 10 nM). 7. The hyperpolarization elicited by intracellular injection of calcium was reversibly prolonged by either ouabain (1 microM) or caffeine (10 nM). 8. These results suggest that ouabain augments the after-hyperpolarization both by an increase in calcium influx across the cellular membrane and by an increase in intracellular calcium concentration.

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