Effects of restraint stress on the daily rhythm of hydrolysis of adenine nucleotides in rat serum

Adult male Wistar rats were divided into 4 groups: ZT0, ZT6, ZT12 and ZT18. Each group was subdivided in 4 groups: control, immediately, 6 h and 24 h after one hour of restraint stress. ATP, ADP and AMP hydrolysis were assayed in the serum.All stressed groups showed significant decreases in all enzyme activities at ZT 12 and ZT 18 when compared with control.Acute stress provokes a decrease in nucleotidase activities dependent on the time that this stress occurs and this effect appears to persist for at least 24 hours. Stress can change levels of nucleotides, related to increased frequency of cardiovascular events during the activity phase. Altered levels of nucleotides in serum may be involved in cardiovascular events more frequent during the activity phase in mammals, and with their etiology linked to stress.Extracellular adenosine 5'-triphosphate (ATP) and its breakdown products, adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP) and adenosine, can act as extracellular messengers in a range of biological processes through binding to purinergic receptors. In addition, they have been shown to have pronounced effects on a variety of biological processes such as neurotransmission, regulation of cardiac function and platelet aggregation [1], as well as pathological events including neurodegenerative and cardiovascular diseases [2]. ATP can be released via stimulation of sympathetic nerves [3] promoting vasoconstriction or vasodilatation, and contributes to platelet aggregation [4]. Additionally, its breakdown produces the nucleotide diphosphate (ADP), which is the most important platelet aggregator and also promotes vasoconstriction [5]. The nucleoside adenosine, also generated by ATP breakdown, is able to act as a vasodilator, inhibitor of platelet aggregation, and it may act as an endogenous cardioprotective substance [4].Extracellular nucleotides can be hydrolyzed by a variety of enzymes that are located on the surface of cells, or by soluble forms in