Are there ultradian rhythms at the molecular level?

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Abstract
The molecular mechanisms of short periodicities (from a few seconds to 15 min), such as glycolytic oscillations, are well known. The essential components of periodic systems in lower organisms, having a period of one to a few days, concern the main cellular compartments. Several clock genes have been identified, but the role of the post‐translational processes or gene expression at the translational level in the control of circadian enzyme activities is questioned. In non‐circadian systems the master element of rhythm regulation, a main pathway of intermediary metabolism where the reversible phosphorylation of a regulatory enzyme plays an essential part, has been identified. Ultradian ( 1 to 4 h) endogenous rhythms have been detected in cell suspensions and in embryonic or differentiated tissues or in isolated organs. Many cellular functions (e.g. protein synthesis, respiration, enzyme activities, electric properties) present the same oscillation frequencies, which cannot be related to mitotic cycles. A cellular ultradian periodic mechanism could then control these activities. In most cases the oscillations observed are irregular i.e. the rhythms most probably represent a spectrum of oscillations. The properties of these rhythms (e.g. phase‐response curve) have to be determined. The regulation of ultradian rhythms remains mostly hypothetical. Their period may be different in different specialized cells of a given organ and for the different physiological functions of an organism. The hypothesis of a central ultradian pacemaker which regulates these rhythms does not appear to be relevant. Ultradian rhythms also seem not to be related to the working of the circadian clock.