TRANSIENT INHIBITION BY OROTIC ACID DOES NOT ABOLISH THE IN-VIVO RESPONSE OF RAT HEPATOCYTES TO A DIRECT MITOGEN, LEAD NITRATE

Background: Orotic acid (OA) is able to inhibit hepatocyte proliferati on in vivo induced by 2/3 partial hepatectomy. The present studies wer e aimed at establishing: (i) whether OA also inhibits hepatocyte proli feration induced by a direct mitogen and, if so (ii) whether the stimu lus provided by the mitogen is still expressed following transient inh ibition by OA. Methods/Results: In the first experiment male Wistar ra ts were injected with either lead nitrate (100 mu mol/kg, i.v.) or sal ine and 20 h later some animals receiving the mitogen were also implan ted with a 400-mg OA tablet (as OA-methyl ester. i.p.). Multiple injec tions of H-3-thymidine were given to each rat (50 mu Ci each, 6 h apar t, i.p.) until 2 h before killing. All groups were killed 3 days after the initial treatment. Results indicated that OA almost completely in hibited hepatocyte DNA synthesis and labelling induced by lead nitrate (e.g. labelling index was 1.9+/-0.5% in the saline-treated group, 44. 7+/-4.0% in the lead nitrate group and 1.4+/-0.3% in the group receivi ng lead nitrate + OA). Based on the above results, in a second experim ent rats were given a similar dose of lead nitrate and a subset of ani mals was implanted 20 h later with a 400-mg OA tablet, as previously d escribed. Multiple doses of H-3-thymidine were again given to each rat (20 mu Ci each, 6 h apart) until 2 h before killing. Animals from bot h groups were killed at 3, 6 or 8 days after lead nitrate. Results ind icated that, while at day 3 lead nitrate-induced DNA synthesis was eff ectively inhibited by OA, at day 6 the proliferative response was resu med in the group receiving OA. Cumulative labelling index over 6 days was 30.3+/-1.4 in rats given the mitogen alone and 52.1+/-2.2 in the g roup exposed to lead nitrate + OA. Conclusions: These data indicate th at: (i) OA is also able to inhibit hepatocyte proliferation induced by a direct mitogen such as lead nitrate; this, in turn, suggests that i ts inhibitory effect is not unique to the stimulus elicited by partial hepatectomy. (ii) The proliferative response triggered by the mitogen is not abolished by the transient (3-4 days) inhibitory phase imposed by OA. Possible mechanisms underlying these effects are considered in the discussion.