Cycles in anilmals

Importance of circadian rhythms in animal cell cultures

Circadian rhythms are a characteristic feature of many cell and organ cultures. Such rhythms may be important in the interpretation of data from cells in culture. More examples of circadian rhythms in tissue culture are badly needed to understand this phenomena. However, they will only be expressed under optimum and well-understood conditions.

Different levels of gene realization in the diurnal control of rat liver enzymes

Diurnal rhythms are demonstrated in five rat liver enzymes : argininosuccinate synthetase, ATP : citrate lyase, glutamate dehydrogenase, phosphoenolpyruvate carboxykinase, and succinate dehydrogenase. In a 12 : 12 h light-dark cycle, maxima of enzyme activities occur at the beginning of the dark phase in the case of phosphoenolpyruvate carboxykinase, at the end of the dark phase in ATP : citrate lyase, and in the middle of the dark phase in the other three enzymes. The diurnal increase of phosphoenolpyruvate carboxykinase is blocked by cycloheximide, cordycepin, alpha-amanitin, and 5-azacytidine. The maximum of ATP : citrate lyase is likewise suppressed at the levels of both translation and transcription, as shown by administration of cycloheximide and 5-azacytidine, respectively. Hence, these two enzymes appear to be regulated transcriptionally. The diurnal rise of argininosuccinate synthetase an glutamate dehydrogenase is also totally inhibited by cycloheximide, whereas cordycepin, alpha-amanitin, and 5-azacytidine are ineffective in the first phase of enzyme accumulation. In a later phase, however, alpha-amanitin and 5-azacytidine become inhibitory. The two enzymes therefore seem to be regulated sequentially by post-transcriptional and transcriptional mechanisms. The diurnal increase of succinate dehydrogenase is nearly insensitive to alpha-amanitin and 5-azacytidine; cycloheximide is only partially inhibitory and, in particular, almost ineffective during the late rise. Thus, the rhythm of this enzyme might be controlled mainly by an activation and, perhaps, by a transitory post-transcriptional mechanism.