Responsiveness of RNA degradation to amino acids in cultured rat hepatocytes: comparison with isolated rat hepatocytes.

The role of amino acids in the regulation of RNA degradation was investigated in cultured hepatocytes from fed rats previously labeled in vivo with 6-14C]orotic acid. Rates of RNA degradation were determined between 42 and 48 h of culture from the release of radioactive cytidine in the presence of 0.5 mM unlabeled cytidine. The fractional rate was about 4.4 +/- 0.4%/h in the absence of amino acids (0x). The catabolism of RNA was decreased to basal level (1.5 +/- 0.3%/h) by the addition of amino acids at 10 times normal plasma concentration (10x). The inhibition of RNA degradation, expressed as percentage of maximal deprivation-induced response (0x minus 10x), averaged 60% at normal plasma levels of amino acids. The degree of responsiveness was greatly improved as compared to freshly isolated hepatocytes (20%) and was similar to the sensitivity previously observed with perfused livers. In cultured hepatocytes, the sensitivity of RNA degradation to amino acids was not affected by varying the volume of medium from 1 to 4 ml per dish. In freshly isolated hepatocytes, the inhibitory effect of amino acids was not modified by changing the cell density from 0.5 to 5 x 10(6) cells per ml. In the range of normal plasma concentration of amino acids, the low sensitivity of RNA degradation in isolated hepatocytes persisted with inhibition ranging from 10 to 20%. These findings suggest that the control of RNA degradation in both cultured and isolated hepatocytes is not affected by the total quantity of amino acids available in the medium, but their concentration is crucial. Electron microscopy observations and the inhibitory effect of 3-methyl-adenine in cultured rat hepatocytes partially confirmed the role of the lysosomal system in the increase of RNA degradation and its regulation by amino acids.