Jong Chul Rhee1, Poong Lyul Rhee1, Myoung Kyu Park2,
Insuk So3, Dae Yong Uhm2,
Ki Whan Kim3 and Tong Mook Kang2,*
1Department╩of Medicine and 2Department of Physiology, Sungkyunkwan University School of Medicine, Suwon 440-746, Korea
3Department╩of Physiology & Biophysics, Seoul National University College of Medicine, Seoul 110-799, Korea
*╩To whom correspondence should be addressed.
Abstract: Muscarinic receptor subtypes controlling the nonselective cationic current in response to carbachol (ICCh) were studied in circular smooth muscle cells of the guinea pig gastric antrum using putative muscarinic agonists and antagonists. Both oxotremorine-M (an M2-selective agonist) and CCh dose-dependently activated the cationic current with EC50 values of 0.21╩▒╩0.01╩mm and 0.97╩▒╩0.06╩mM, respectively. In contrast, pilocarpine and McN-A╩343 (an M1-selective and a putative M4╩agonist) were weak partial agonists. In response to 10╩mM CCh, 4-DAMP, methoctramine and pirenzepine dose-dependently inhibited ICCh and had IC50 values of 1.91╩▒╩0.2╩nM, 0.46╩▒╩0.07╩mM and 8.33╩▒╩0.4╩mM, respectively. 4-DAMP, methoctramine and pirenzepine shifted the concentration-response curves of ICCh to the right without significantly reducing the maximal current. Values of the apparent dissociation constant pA2 obtained from Schild plot analysis were 9.24, 7.72 and 6.62 for 4-DAMP, methoctramine and pirenzepine, respectively. Also, pertussis toxin completely blocked ICCh generation. These results suggest that the M2-subtype plays a crucial role in the activation of the ICCh, and a block of the M3-subtype reduces the sensitivity of the M2-mediated response with no significant reduction of maximum response.
Keywords: Muscarinic receptor subtype, Nonselective cationic current, Carbachol, Smooth muscle