The effect of noradrenaline on the ion permeability of isolated mammalian hepatocytes, studied by intracellular recording.

Abstract

1. The influence of noradrenaline on the membrane potential and conductance of isolated guinea-pig and rabbit hepatocytes in short-term (2-8 h) tissue culture has been studied by intracellular recording. 2. Resting hepatocytes had linear current-voltage relationships, with input resistances of 166 and 216 M.OMEGA. in guinea-pig and rabbit cells respectively. The recorded membrane potential was -18 mV in each species, though the true resting potential is likely to have been up to 10 mV greater. 3. The hepatocytes sometimes slowly hyperpolarized during intracellular recording, and this was associated with a fall in membrane resistance, and an increase followed by a decrease in membrane potential noise. These effects were abolished by quinine (200 .mu.M) but not by apamin (50 nM), and are attributable to a K+ conductance activated by cell swelling. 4. Noradrenaline (2 .mu.M, in the presence of propranolol at 1 .mu.M) was applied to individual hepatocytes by pressure ejection (puffer pipette technique). After a short latency, the cells hyperpolarized by a mean of 18 mV in both guinea-pig and rabbit preparations. This was associated with a large rise in membrane conductance (50 nS in guinea-pig, 54 nS in rabbit cells). The reversal potential for this action was -38 mV. 5. The experiments were repeated in the presence of apamin (50 nM) to block the Ca2+-dependent K+ permeability which noradrenaline activates in these cells. Noradrenaline still caused some hyperpolarization and a substantial increase (.apprx. 40 nS) in conductance, with a reversal potential (Er) or -31 mV. This can be attributed to an increase in Cl- conductance. 6. In keeping with this interpretation, noradrenaline applied in the absence of Cl- (replaced by isethionate or gluconate) caused a much greater hyperpolarization (58 mV in guinea-pig, 40 mV in rabbit cells) associated with a smaller rise in conductance (.apprx. 12 nS). Er for this action was -95 mV (guinea-pig) and -68 mV (rabbit), suggesting that the conductance increase was now mainly to K+. 7. The magnitude of the conductance changes produced by noradrenaline under the various experimental conditions suggest that the increase in the conductance to Cl- (.DELTA.GCl) is 3-fold greater than that to K+ (.DELTA.GK). 8. The activation of .DELTA.GCl occurs either at the same time as .DELTA.GK, or (in ca, one cell in ten) a few seconds later. 9. The membrane actions of noradrenaline could be mimicked by the divalent cation ionophore A23187 (100 nM-10 .mu.M), both in the presence and absence of apamin. This suggests that .DELTA.GCl, like .DELTA.GK, is triggered by a rise in cytosolic Ca2+. 10. The noradrenaline response occurred with a latency of at least 1 s whereas the action of A23187 often began in less than 0.4 s, suggesting that the latency with noradrenaline was not attributable to a delay in the activation of .DELTA.GCl and .DELTA.GK by the rise in [Ca2+]i. 11. The increases in membrane potential and conductance initiated by noradrenaline were not maintained during prolonged applications. Unexpectedly, recovery occurred in a series of transients of increasing amplitude. 12. We conclude that noradrenaline acting through .alpha.-adrenoceptors causes a large increase in the permeability of guinea-pig and rabbit hepatocytes to both Cl- and K+. These effects are mediated by a rise in cytosolic Ca2+.