EFFECTS OF EXTRACELLULAR CA2- ON EPIDERMAL GROWTH FACTOR-INDUCED DNA-SYNTHESIS IN CULTURED RAT HEPATOCYTES( AND HCO3)

Background/Aims: The elevation of cytosolic free calcium concentration ([Ca2+](i)) and intracellular pH mediate the growth factor-initiated proliferation of many cells, but it is not known if they trigger mitos is in resting hepatocytes. The maintenance of [Ca2+](i) and intracellu lar pH depends partly on extracellular calcium concentration ([Ca2+](e )) and extracellular bicarbonate concentration ([HCO3-](e)). Therefore , the effects of [Ca2+](e) and [HCO3-](e) on hepatocyte proliferation were examined. Methods: Epidermal growth factor induced proliferation in primary cultures of rat hepatocytes. [H-3]thymidine incorporation i nto DNA and nuclear labeling indices were measured. Results: Between 0 .2 and 0.9 mmol/L of [Ca2+](e), the proliferative response to epiderma l growth factor increased, and total hepatocellular Ca2+ content was i ncreased. Increasing [HCO3-](e) also stimulated DNA synthesis in a con centration-dependent manner, maximal at 35 mmol/L, Using optimal [Ca2](e) (0.9 mmol/L) and [HCO3-](e) (35 mmol/L), a synergistic stimulatio n of hepatocellular DNA synthesis was shown. Voltage-dependent Ca2+ ch annel blockers failed to inhibit hepatocyte proliferation when adminis tered in concentrations that inhibit proliferation in other cell types . Conclusions: [Ca2+](e) and [HCO3-](e) are both essential for hepatoc yte proliferation, and their effects are synergistic. The entry of ext racellular Ca2+ is criticai for epidermal growth factor-induced DNA sy nthesis in hepatocytes, but this is not mediated by voltage-dependent Ca2+ channels.