Nonneuronal cholinergic system in human erythrocytes: biological role and clinical relevance

Acetylcholine is well known in the medical setting as one of the most exemplary neurotransmitters. Its ubiquity in nature otherwise suggests a theoretically diverse spectrum of action and an extremely early appearance in the evolutionary process. In humans, acetylcholine and its synthesizing enzyme, choline acetyltransferase, have been found in various nonneural tissues such as the epithelium, mesothelium, endothelium, muscle, immune cells and blood cells. The widespread expression of nonneuronal acetylcholine is accompanied by the ubiquitous presence of acetylcholinesterase and nicotinic/muscarinic receptors. Structural and functional dissimilarities are evident between the nonneuronal and neuronal cholinergic systems. An increasing body of evidence throughout the last few years has placed acetylcholine as a major cellular signaling molecule in many pathways. Furthermore, numerous erythrocyte physiological events in the microcirculation are strongly regulated by acetylcholine. Thus, it is time to revise our understanding of the role of vascular acetylcholine in humans. Its biological and pathobiological roles must be evaluated in more detail to eventually achieve novel therapeutical targets. The present article reviews recent findings about nonneuronal acetylcholine in red blood cells, with special regard to (1) red cell rheology, (2) plasma ion concentrations, (3) nitric oxide intracellular translocation and metabolism and (4) band 3 protein phosphorylation.