The present review describes the biological implications of the periodic changes of adenosine concentrations in different tissues of the rat. Adenosine is a purine molecule that could have been formed in the prebiotic chemical evolution and has been preserved. The rhythmicity of this molecule, as well as its metabolism and even the presence of specific receptors, suggests a regulatory role in eukaryotic cells and in multicellular organisms. Adenosine may be considered a chemical messenger and its action could take place at the level of the same cell (autocrine), the same tissue (paracrine), or on separate organs (endocrine). Exploration of the circadian variations of adenosine was planned considering the liver as an important tissue for purine formation, the blood as a vehicle among tissues, and the brain as the possible acceptor for hepatic adenosine or its metabolites. The rats used in these studies were adapted to a dark–light cycle of 12 h with an unrestrained feeding and drinking schedule. The metabolic control of adenosine concentration in the different tissues studied through the 24-h cycle is related to the activity of adenosine-metabolizing enzymes: 5′-nucleotidase adenosine deaminase, adenosine kinase, and S-adenosylhomocysteine hydrolase. Some possibilities of the factors modulating the activity of these enzymes are commented upon. The multiphysiological action of adenosine could be mediated by several actions: (i) by interaction with extracellular and intracellular receptors and (ii) through its metabolism modulating the methylation pathway, possibly inducing physiological lipoperoxidation, or participating in the energetic homeostasis of the cell. The physiological meaning of the circadian variations of adenosine and its metabolism was focused on: maintenance of the energetic homeostasis of the tissues, modulation of membrane structure and function, regulation of fasting and feeding metabolic pattern, and its participation in the sleep–wake cycle. From these considerations, we suggest that adenosine could be a molecular oscillator involved in the circadian pattern of biological activity in the rat.Key words: adenosine, circadian rhythm energy, membrane structure, sleep–wake cycle.

Idiopathic pulmonary fibrosis (IPF) is a devastating disease characterized by increased activation of fibroblasts/myofibroblasts. Previous reports have shown that IPF fibroblasts are resistant to apoptosis, but the mechanisms remain unclear. Since inhibition of the mitochondrial permeability transition pore (mPTP) has been implicated in the resistance to apoptosis, in this study, we analyzed the role of mitochondrial function and the mPTP on the apoptosis resistance of IPF fibroblasts under basal conditions and after mitomycin C-induced apoptosis. We measured the release of cytochrome c, mPTP opening, mitochondrial calcium release, oxygen consumption, mitochondrial membrane potential, ADP/ATP ratio, ATP concentration, and mitochondrial morphology. We found that IPF fibroblasts were resistant to mitomycin C-induced apoptosis and that calcium, a well-established activator of mPTP, is decreased as well as the release of pro-apoptotic proteins such as cytochrome c. Likewise, IPF fibroblasts showed decreased mitochondrial function, while mPTP was less sensitive to ionomycin-induced opening. Although IPF fibroblasts did not present changes in the mitochondrial membrane potential, we found a fragmented mitochondrial network with scarce, thinned, and disordered mitochondria with reduced ATP levels. Our findings demonstrate that IPF fibroblasts are resistant to mitomycin C-induced apoptosis and that altered mPTP opening contributes to this resistance. In addition, IPF fibroblasts show mitochondrial dysfunction evidenced by a decrease in respiratory parameters.