Hyperpolarization by Noradrenaline in Guinea pig Liver Cells: Effbcts of Ouabain and External Ca²⁺

Abstract

The mechanism of hyperpolarization of the membrane caused by noradrenaline (NA) was investigated in guinea pig liver cells.The hyperpolarization produced by NA was accompanied by a reduction of membrane resistance.Both hyperpolarization and reduction of membrane resistance were suppressed by ouabain (5×10^-6M).However, when the external Na⁺ was reduced to 38mm, the NA response reappeared even in the presence of ouabain.The hyperpolarizing NA response may be divided into two phases, an early phase and a late phase. In a Ca²⁺-free solution, the NA response gradually disappeared.However, the early phase of hyperpolarizing response was more resistant to a removal of external Ca²⁺than the late phase.Manganese ion also suppressed the late phase more strongly than the early phase.It was concluded that the hyperpolarization induced by noradrenaline is due to an increase in K+conductance and that this may be a result of an increase in intracellular Ca²⁺ concentration.The early phase of hyperpolarizing NA response is probably due to release of bound Ca²⁺ and the late phase to influx of Ca²⁺ from the external solution.Suppression of the NA response by ouabain may be explained by assuming that the Cat²⁺ activated K⁺conductance is blocked by an increase in intracellular Na⁺ concentration.