ROLE OF CALMODULIN IN THE ACTIVATION OF CARBACHOL-ACTIVATED CATIONIC CURRENT IN GUINEA-PIG GASTRIC ANTRAL MYOCYTES

In mammalian gastrointestinal myocytes, it is known that muscarinic st imulation activates nonselective cation channels through a G-protein a nd a Ca2+-dependent pathway. We recorded inward cationic currents foll owing application of carbachol (I-CCh) to guinea-pig gastric myocytes, which were held at -20 mV using the whole-cell patch-clamp method. I- CCh was suppressed by nicardipine or removal of Ca2+ from the bath sol ution. The peak value of inward current induced by repetitive applicat ions of carbachol (CCh) decreased progressively (run-down phenomenon). This run-down was significantly alleviated by the addition of calmodu lin to the pipette solution (0.15 mg/ml) or by using the perforated-pa tch whole-cell voltage-clamp technique. Moreover, W-7 6-aminohexyl)-5- chloro-1-naphthalenesulphonamide], a calmodulin antagonist, was a reve rsible inhibitor of I-CCh. However, W-7 had only a weak inhibitory eff ect on the same cationic current which was induced by guanosine 5'-O-( 3-thio-triphosphate) (GTP[gamma S] 0.2 mM) in the pipette solution. Th is GTP[gamma S]-induced cationic current was still markedly suppressed by the Ca2+-free bath solution. W-7 itself had a weak inhibitory effe ct on voltage-operated Ca2+ channels as well as the effects on I-CCh. These data suggest that multiple Ca2+-dependent pathways are involved in the activation of CCh-gated cation channels in guinea-pig antral my ocytes and a Ca2+/calmodufin-dependent pathway would be one of them.