Protein kinase C mediates the desensitization of CCh-activated nonselective cationic current in guinea-pig gastric myocytes

The possibility of the protein kinase C (PKC) pathway being a mechanism underlying the desensitization of carbachol- (CCh-)activated nonselective cationic current (I(CCh)) was investigated in a study of guinea-pig gastric myocytes. Using the conventional whole-cell patch-clamp technique with symmetrical CsCl-rich solution in pipette and bath, I(CCh) was induced by bath application of 50 μM CCh. With 0.5 mM EGTA [ethyleneglycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid] in the pipette solution (0.5 mM [EGTA](i)), I(CCh) decayed spontaneously (desensitization of I(CCh)) to around 20% within 10 min. Desensitization of I(CCh) was significantly attenuated with 2 mM [EGTA](i). At a concentration of 20 μM OAG (1-oleoyl-2-acetyl-sn-glycerol), a PKC activator, inhibited I(CCh) at 0.5 mM [EGTA](i) but far less at 2 mM [EGTA](i) (18% and 81% of control, respectively). The same cationic current induced by intracellular guanosine-5'-O-(3-thiotriphosphate) (GTP[γ-S]) was not inhibited by OAG with 0.5 mM [EGTA](i). The pretreatment of gastric myocytes with PKC inhibitors, either 1 μM chelerythrine or 1 μM peptide inhibitor, attenuated the desensitization of I(CCh). [Ca²⁺](i) was also measured by single cell microfluorometry using fura-2. Under CCh stimulation with 2 mM [EGTA](i), [Ca²⁺](i) did not increase above 100 nM while it increased to around 260 nM with 0.5 mM [EGTA](i). These results suggest that the desensitization of I(CCh) is partly due to the Ca²⁺-dependent PKC pathway in guinea-pig gastric myocytes.