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 stimulation activates nonselective cation channels through a G-protein and a Ca2+-dependent pathway. We recorded inward cationic currents following 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 solution. The peak value of inward current induced by repetitive applications of carbachol (CCh) decreased progressively (run-down phenomenon). This run-down was significantly alleviated by the addition of calmodulin to the pipette solution (0.15 mg/ml) or by using the perforated-patch whole-cell voltage-clamp technique. Moreover, W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide], a calmodulin antagonist, was a reversible inhibitor of I-CCh. However, W-7 had only a weak inhibitory effect 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. This GTP[gamma S]-induced cationic current was still markedly suppressed by the Ca2+-free bath solution. W-7 itself had a weak inhibitory effect 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 myocytes and a Ca2+/calmodufin-dependent pathway would be one of them.