The M muscarinic receptors are located at many places in the body. They are located in the smooth muscles of the blood vessels, as well as in the lungs. Because the M receptor is G-coupled and mediates an increase in intracellular calcium, it typically causes contraction of smooth muscle, such as that observed during bronchoconstriction and bladder voiding.   However, with respect to vasculature, activation of M on vascular endothelial cells causes increased synthesis of nitric oxide, which diffuses to adjacent vascular smooth muscle cells and causes their relaxation, thereby explaining the paradoxical effect of parasympathomimetics on vascular tone and bronchiolar tone.  Indeed, direct stimulation of vascular smooth muscle, M mediates vasoconstriction in diseases wherein the vascular endothelium is disrupted.
The M receptors are also located in many glands, which help to stimulate secretion in, for example, the salivary glands, as well as other glands of the body.
Like the M muscarinic receptor, M receptors are G proteins of class G that upregulate phospholipase C and, therefore, inositol trisphosphate and intracellular calcium as a signaling pathway.