Five subtypes of muscarinic receptors are expressed in gastric smooth muscles of guinea pig.

Muscarinic receptors play key roles in the control of gastrointestinal smooth muscle activity. However, specific physiological functions of each subtype remain to be determined. In this study, the nonselective cation channel activated by carbachol (I(CCh)) was examined in circular smooth muscle cells of the guinea pig gastric antrum using patch-clamp technique. 4-DAMP inhibited I(CCh) dose-dependently with IC(50) of 1.1 0.1 nM (n = 6). GTPgS-induced current, however, was not inhibited by 10 nM 4-DAMP. I(CCh) was not recorded in pertussis-toxin (PTX)-pretreated smooth muscle cells of gastric antrum. I(CCh) values in response to 10 mM CCh at a holding potential of 60 mV were -330 32 pA (n=4) and -15 3 pA (n = 6) in the control and PTX-treated cells, respectively (P 0.01). Sensitivities to nanomolar 4-DAMP and PTX suggest the possible involvement of m4 subtype. Using sequence information obtained from cloned guinea pig muscarinic receptor genes, it is possible to amplify the cDNAs encoding m1-m5 from guinea pig brain tissue. Single cell RT-PCR experiments showed that all five subtypes of muscarinic receptor were present in circular smooth muscle cells of the guinea pig gastric antrum. Together with our previous results showing that G(o) protein is important for activation of ACh-activated NSC channels, our results suggest that I(CCh) might be activated by acetylcholine through m4 subtype as well as m2 and m3 subtypes in guinea-pig stomach.