Muscarinic receptors controlling the carbachol-activated nonselective cationic current in guinea pig gastric smooth muscle cells

Muscarinic receptor subtypes controlling the nonselective cationic current in response to carbachol (I-CCh) were studied in circular smooth muscle cel ls of the guinea pig gastric antrum using putative muscarinic agonists and antagonists. Both oxotremorine-M (an M-2-selective agonist) and CCh dose-de pendently activated the cationic current with EC50 values of 0.21 +/- 0.01 mu m and 0.97 +/- 0.06 mu M, respectively. In contrast, pilocarpine and McN -A 343 (an M-1-selective and a putative M-4 agonist) were weak partial agon ists. In response to 10 mu M CCh, 4-DAMP, methoctramine and pirenzepine dos e-dependently inhibited I-CCh and had IC50 values of 1.91 +/- 0.2 nM, 0.46 +/- 0.07 mu M and 8.33 +/- 0.4 mu M, respectively. 4-DAMP, methoctramine an d pirenzepine shifted the concentration-response curves of I-CCh to the rig ht without significantly reducing the maximal current. Values of the appare nt dissociation constant pA(2) obtained from Schild plot analysis were 9.24 , 7.72 and 6.62 for 4-DAMP, methoctramine and pirenzepine, respectively. Al so, pertussis toxin completely blocked I-CCh generation. These results sugg est that the M-2-subtype plays a crucial role in the activation of the I-CC h, and a block of the M-3-subtype reduces the sensitivity of the M-2-mediat ed response with no significant reduction of maximum response.