Evidence suggests that muscarinic receptors (MAChRs) are involved in various aspects of neuronal and vascular functioning, and that there is selective oxidative stress sensitivity (OSS) among MAChR subtypes. COS-7 cells transfected with M1, M2 and M4 subtypes show greater OSS than the M1 and M3 subtypes, as seen by the decreased ability of cells to extrude or sequester calcium (Ca(2+)) following exposure to dopamine (DA) or A beta 25-35, and depolarization by oxotremorine. We sought to determine which receptor domain may be responsible for the differential vulnerability to OS between 'OS-sensitive' (M1) and 'non-sensitive' (M3) subtypes. Comparison of the amino acid sequences of each receptor has shown that the third cytoplasmic loop (i3 loop) is the domain with the most variability between the two subtypes. Therefore, mutations were made by either deleting or exchanging the i3 loop of M1 and M3 receptors. Experiments revealed that deletions of the i3 loop increased DA sensitivity (a lower percentage of cells showing recovery of [Ca(2+)](i) following depolarization) in both receptors. Chimerics of M1 in which the i3 loop of the M3 was exchanged with the i3 loop of the M1 (M1M3i3) showed that DA sensitivity was reduced (a greater percentage of cells showing increases in calcium clearance) following depolarization. The M3 chimerics containing the M1 i3 loop (M3M1i3) offered no protection against DA-induced decrements in calcium buffering. Results suggest that the longer i3 loop of the M3 may decrease OSS, possibly playing a role in targeting antioxidants to specific receptor sites that impart OSS.