Expression of muscarinic receptor subtypes in salivary glands of rats, sheep and man

Abstract In rat parotid, submandibular and sublingual glands and in ovine parotid and in human labial glands, the expression of muscarinic receptor subtypes was examined by immunoblotting and immunohistochemistry. Functional correlates were searched for in rat salivary glands. In the rat submandibular and sublingual glandular tissues clear signals of muscarinic M1 and M5 receptors could be detected in the immunoblotting and vague bands for muscarinic M3 and, in particular for, M4 receptors. The rat parotid gland differed. In this gland, the signal was less obvious for the muscarinic M1 receptor, and further, muscarinic M4 receptors appeared more strongly marked than in the submandibular glands. The results from the immunohistochemistry could be interpreted as the muscarinic M4 receptors are located on nerve fibres, since the outer layer of lobuli were densely stained. Intraglandular vessels in the rat submandibular and parotid glands showed expression of M3 receptors. In contrast to the parotid gland, the submandibular vessels also expressed M1 and M2 receptors. Occasionally M5 receptors appeared in the arteries and veins also. The functional studies in the rat confirmed muscarinic M1 receptor mediated secretion in the submandibular gland. Since the M1 receptor blockade did not affect submandibular blood flow, indirect vascular effects could not in total explain the secretory inhibition. Also in the human labial glands, muscarinic M1, M3 and M5 receptors occurred. No or low amounts of muscarinic M2 and M4 receptors could be detected. In patients with Sjögren-like symptoms an up-regulation of M3, M4 and M5 receptors was apparent in the labial glands. In ovine parotid glands all receptors could be detected, but constantly with vague bands for muscarinic M2 receptors. In conclusion, muscarinic M1 receptors seem to be expressed in seromucous/mucous glands. A secretory effect by muscarinic M5 receptors is not to be excluded, since they were expressed in all the glands examined. However, other functions, such as promotion of inflammation, cell growth and proliferation are possible as well.