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 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 μM and 0.97 ± 0.06 μM, respectively. In contrast, pilocarpine and McN-A 343 (an M₁-selective and a putative M₄ agonist) were weak partial agonists. In response to 10 μM CCh, 4-DAMP, methoctramine and pirenzepine dose-dependently inhibited I(CCh) and had IC₅₀ values of 1.91 ± 0.2 nM, 0.46 ± 0.07 μM and 8.33 ± 0.4 μM, respectively. 4-DAMP, methoctramine and pirenzepine shifted the concentration-response curves of I(CCh) 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 I(CCh) generation. These results suggest that the M₂-subtype plays a crucial role in the activation of the I(CCh), and a block of the M₃-subtype reduces the sensitivity of the M₂-mediated response with no significant reduction of maximum response.