Some enzyme and hormonal attributes of hepatoma cell membranes.

Adenylate cyclase activity was measured in plasma membranes isolated form Morris Hepatomas (44 and 47C) and from their host livers. We found that the enzyme activity in the tumours was very low, approx. 5% of the level in control and host livers. The amount of cAMP and cGMP in the tumours was also lower than in the host livers but the ratio of cGMP to cAMP in the tumours was increased by a factor of 4-5. The membrane binding capacities for the pancreatic hormones insulin and glucagon were measured. Hepatoma membranes bound less glucagon than those of livers. A decrease in the number of the glucagon receptors was found but there were no changes in the affinity constant. For insulin, we found the same binding capacity as the host and control livers; thus there was an increase in the ratio of insulin bound/glucagon bound in tumours as compared to controls. The plasma levels of insulin in the tumour bearing animals were approximately half of those in control, whereas the glucagon levels in plasma were 60-62% higher in tumour bearing animals. These results are discussed in terms of the characterization of normal, foetal and regenerating liver, in comparison with slow growing hepatomas. The levels of cAMP and cGMP are discussed with respect to control mechanisms of cell proliferation.