The muscarinic receptor subtypes in human blood vessels.

Acetylcholine (ACh) may induce the relaxation and the contraction of human blood vessels. These effects involve the activation of muscarinic receptors located on endothelial or smooth muscle cells. In humans and animals, five subtypes of muscarinic receptors (M1-M5) have been identified. In the particular case of human blood vessels, the M3 subtype seems to be prevalent on endothelial cells as well as on smooth muscle cells. However, the M1 subtype may be specific for the pulmonary vascular endothelium. In contrast, the M4 subtype has not been described in human vessels. These conclusions are, however, based on a limited number of studies and further work is needed in this area of research. Although muscarinic receptors may not be the only receptors involved in hypertension, the development of specific agonists for M1 receptors may be useful to counteract any elevation of blood pressure in the pulmonary circulation.