Hypertonicity-induced alkalinization of rat hepatocytes is not involved in activation of Na+ conductance or Na+,K+-ATPase.
We investigated whether cell alkalinization via activation of Na+/H+ exchange is involved in the stimulation of Na+ conductance and Na+,K+-ATPase in rat hepatocytes under hypertonic stress. Osmolarity was increased from 300 to 400 mOsm/l at constant extracellular pH (7.4), whereas osmotically induced cell alkalinization (0.3 pH units in HCO3-free solutions) was mimicked by increasing extracellular pH from 7.4 to 7.8 in normosmotic solutions. In intracellular recordings with conventional and ion-sensitive microelectrodes, hypertonic stress led to a transient shift in the voltage response to low Na+ solutions (95% in exchange for choline) by -4.3 +/- 0.8 mV and a continuous increase in cell Na+ from 13.7 +/- 1.8 to 18.6 +/- 3.0 mmol/l within 8 min. In the presence of 10(-5) mol/l amiloride, these effects were reduced by 80 and 90%, respectively. In contrast, increasing pH did not change the voltage responses to low Na+ or cell Na+ concentrations significantly. In addition, application of 2 mmol/l Ba2+ pulses revealed that a sustained membrane hyperpolarization of 15.6 +/- 1.4 mV following intracellular alkalinization exclusively reflects an increase in K+ conductance. Increasing osmolarity at pH 7.4 augmented ouabain-sensitive 86Rb+ uptake from 5.5 +/- 1.1 to 8.5 +/- 1.6 nmol mg protein(-1) min(-1). In normosmotic solution at pH 7.8, 86Rb+ uptake equalled 4.9 +/- 1.6 nmol mg protein(-1) min(-1), which is not significantly different from control. We conclude that, in rat hepatocytes, cell alkalinization under hypertonic stress is not responsible for the activation of Na+ conductance and probably does not participate in the stimulation of Na+,K+-ATPase.
Wehner
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F
Max-Planck-Institut für molekulare Physiologie, Abteilung Epithelphysiologie, Dortmund, Germany. frank.wehner@mpi-dortmund.mpg.de
Kinne
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RK
Tinel
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D000468:Alkalies; D000818:Animals; D001692:Biological Transport; D048429:Cell Size; D004553:Electric Conductivity; D006863:Hydrogen-Ion Concentration; D008099:Liver / Q000166:cytology / Q000502:physiology*; D009997:Osmotic Pressure; D051381:Rats; D012413:Rubidium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D000468:Alkalies; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D012413:Rubidium