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

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Abstract
The mechanism of hyperpolarization of the membrane caused by noradrenaline (norepinephrine, NA) was investigated in guinea-pig liver cells. The hyperpolarization produced by NA was accompanied by a reduction of membrane resistance were suppressed by ouabain (5 .times. 10-6 M). When the external Na+ was reduced to 38 mM the NA response reappeared even in the presence of ouabain. The hyperpolarizing NA response may be divided into 2 phases, an early phase and a late phase. In a Ca2+-free solution, the NA response gradually disappeared. The early phase of hyperpolarizing response was more resistant to a removal of external Ca2+ than the late phase. Mn2+ also suppressed the late phase more strongly than the early phase. The hyperpolarization induced by Na apparently is due to an increase in K+ conductance; 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 Ca2+-activated K+ conductance is blocked by an increase in intracellular Na+ concentration.

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