THE EFFECT OF CHANGES IN HEPATOCYTE MEMBRANE-POTENTIAL ON IMMEDIATE-EARLY PROTOONCOGENE EXPRESSION FOLLOWING PARTIAL-HEPATECTOMY IN RATS

The stimulus responsible for inducing hepatocytes to enter the cell cy cle following partial hepatectomy (PHx) remains to be identified. One suggested candidate is the change in hepatocyte membrane potential (PD ) that occurs immediately following PHx, To test this possibility, we monitored changes in hepatocyte PD and immediate-early proto-oncogene expression in rats pretreated with saline or gamma aminobutyric acid ( GABA), an aminoacid neurotransmitter that hyperpolarizes isolated hepa tocytes, Intraperitoneal injections of saline or GABA (500 mu g/g body weight) were administered to adult, male Sprague-Dawley rats 1 hour p rior to 70% PHx. Rats were sacrificed and the livers excised at variou s times until 180 minutes post-PHx for messenger RNA (mRNA) and protei n analyses. In additional groups of saline- and GABA-treated rats, PD changes were recorded continuously from -260 to 180 minutes post-PHx, Serum GABA concentrations were determined by ion-exchange chromatograp hy with orthopthaldehyde fluorescence detection, Hepatocyte PD's were recorded in situ by intracellular microelectrodes with an Axoprobe-1A amplifier, Steady-state levels of c-fos, c-jun, jun-B, and c-myc trans cripts and proteins were documented by Northern blots of poly(A)-enric hed RNA derived from resected livers and Western blots of total nuclea r protein, respectively, Serum GABA concentrations remained unchanged in saline-treated controls but increased 10- to 20-fold above baseline values in GABA-treated rats, In saline-treated controls, hepatocyte d epolarization occurred immediately and was maintained throughout the 1 80 minutes post-PHx period (PD pre-PHx, -36.8 +/- 5.1; 15 minutes post -PHx, -27.5 +/- 5.7; and 180 minutes post-PHx, -28.3 +/- 4.4 mV, mean +/- SD); whereas in GABA-treated rats, hepatocyte PD remained unchange d (-37.0 +/- 1.1; -36.4 +/- 3.1 and -39.2 +/- 2.7 mV, respectively), D espite abrogation of hepatocyte PD changes, proto-oncogene mRNA and pr otein levels in saline- and GABA-treated rats were either similar or, in the case of c-fos and c-jun, increased five- to sevenfold in GABA-t reated rats, The results of this study indicate the following: 1) hepa tocytes depolarize immediately post-PHx and remain depolarized through out the priming phase of the cell cycle; 2) elevated serum GABA concen trations prevent PHx-induced hepatocyte depolarization; and 3) depolar ization is not the stimulus responsible for priming hepatocytes into r eplicative competence.