Immunological detection of muscarinic receptor subtype proteins (m1-m5) in rabbit peripheral tissues

Employing subtype-specific antisera, we have measured the relative levels of five muscarinic receptor subtype proteins (m1-m5) in rabbit peripheral tissues. Immunoprecipitation assays demonstrated the presence of four distinct receptor proteins (m1-m4), which showed notable differences in their tissue distribution. Significant amounts of m5 receptors were not detected in any of the tissues examined. High levels of m1 receptors were found in sympathetic ganglia and submaxillary gland. The m1 receptor was the predominant subtype in vas deferens. Muscarinic receptors of the m2 type were present in varying proportions in all tissues examined, being the major subtype in sympathetic ganglia, ileum, and uterus and the only subtype detected in rabbit atrium. The m3 receptor protein was abundant in submaxillary gland but was not detected at appreciable levels in other tissues studied. Receptors of the m4 type represented a major subtype in peripheral lung and were also present, although at lower levels, in uterine and ileal tissues.