Gluconeogenesis in the tumor-influenced rat hepatocyte: importance of tumor burden, lactate, insulin, and glucagon.

In an attempt to define the relationship between tumor burden (cachexia) and host hepatocyte gluconeogenesis, the following experiments were performed with the use of an F344 male rat bearing a transplantable sarcoma. Food intake of tumor-bearing (TB) rats was constant until day 24 following implant and a tumor burden of 18 +/- 5.2% (mean +/- SD), at which time food intake progressively declined daily. Tumor burden was arbitrarily divided at 12.8% to determine if any measured changes occurred prior to or following the approximate time when a significant decline in food intake occurred. Plasma glucose levels decreased with tumor burden. Whole-blood lactate levels increased with tumor burden. Fasting plasma alanine levels decreased with tumor burden. Plasma 3-methylhistidine levels increased with tumor burden. Plasma glucagon levels increased with tumor burden, whereas plasma insulin levels decreased. Hormone changes were noted at small tumor burdens prior to a decline in food intake. Viable hepatocytes were isolated from 4 groups: non-tumor-bearing (NTB), small tumor burden [(STB) 3.5% total body weight (TBW)], moderate tumor burden [(MTB) 14% TBW], and large tumor burden [(LTB) 23% TBW]. As expected in NTB rats, hepatocytes produced significantly more glucose with 20 mM lactate than 20 mM alanine or than Hanks' balanced salt solution (HBSS) alone. Hepatocytes from STB rats demonstrated the same basic relationship for lactate, alanine, and HBSS, but they produced significantly more glucose from lactate and HBSS alone than NTB hepatocytes. With alanine as substrate, the rates of glucose production by hepatocytes were not affected by the presence or size of tumor. However, with lactate as substrate, hepatocytes from MTB and LTB rats produced progressively less glucose as tumor burden increased (r = -0.85, p less than .001), which may partly explain the reduction in blood glucose and elevation in blood lactate levels observed. Elevated gluconeogenesis in TB rats occurred early prior to a decline in food intake. The key precursor appeared to be lactate. The balance between glucagon and insulin appeared to promote the abnormal host carbohydrate metabolism observed.