Urea Synthesis in Novikoff and Morris Hepatomas

Summary

The rates of urea synthesis in rat liver and in a series of rat liver neoplasms with widely different growth rates and degree of differentiation were investigated using tissue slices incubated in a Krebs-Ringer bicarbonate buffer. Urea synthesis did not occur in fast-growing, poorly differentiated Novikoff and Morris 3924A hepatomas, but it did occur in slow-growing, well- and highly differentiated hepatomas; however, there was no correlation with growth rate or degree of differentiation. Urea synthesis was comparable with normal liver, at about 32 µmoles/hr/g tissue, in the slow-growing Morris hepatomas 21, 28A, 47C, and 44; but it was very low in two other slow-growing, highly differentiated hepatomas, 9618A and 20. The well-differentiated Morris hepatoma 5123C had intermediate levels of urea synthesis. This pattern of urea synthesis closely paralleled the previously reported activity of carbamyl phosphate synthetase in these tumors.

The rate of urea synthesis was normal in livers of Buffalo rats bearing fast- or slow-growing hepatomas with low urea synthesis rates, but it was markedly lowered in the livers of rats bearing large, slow-growing tumors with high urea synthesis rates. Urea synthesis in liver declined as the tumors increased in size.

The total rate of urea synthesis in liver and tumor, as well as the concentrations of urea in the serum and urine of tumor-bearing animals, remained remarkably constant throughout the period of tumor growth, suggesting the existence of a homeostatic mechanism that controls the urea cycle activity in accordance with the synthetic activity of the tumor.

In parabiotic animals, carbamyl phosphate synthetase activity and urea synthesis were lowered in the host livers of partners bearing tumors with high carbamyl phosphate synthetase- and urea-synthetic activity, but there was no significant effect on urea cycle activity in the normal partners. This result discounts the likelihood of a circulating humoral factor that controls hepatic urea cycle activity.