Effects of myosin light chain kinase inhibitors on caubachol-activated nonselective cationic current in guinea-pig gastric myocytes

The effects of myosin light chain kinase inhibitors on muscarinic stimulation-activated nonselective cationic current (I(CCh)) in guinea-pig gastric antral myocytes were studied using the whole-cell patch-clamp technique. I(CCh) was induced by carbachol (CCh, 50 μM) at a holding potential of -30 mV or -60 mV. ML-7, a chemical inhibitor of myosin light chain kinase (MLCK), inhibited I(CCh) concentration dependently in a reversible manner (53 ± 8.6% at 1 μM, mean ± SE, n = 11). In addition, amplitudes of I(CCh) were only 37 ± 2.7% of the daily control values following the addition of a peptide inhibitor of MLCK to the pipette solution. On the other hand, ML-7 had an inhibitory effect on voltage-operated Ca ²⁺ channel current. The peak value of Ba ²⁺ current at 0 mV was reduced to 35 ± 7.4% (n = 9) by 3 μM of ML-7. As I(CCh) is known to have an intracellular Ca ²⁺ dependence, we tried to exclude the possibility that ML-7 inhibited I(CCh) indirectly via suppression of Ca ²⁺ current and the similar inhibitory effects of ML7 on I(CCh) were confirmed under the following conditions: (1) clamp of membrane potential at -60 mV; (2) clamp of intracellular [Ca ²⁺] to 1 μM by 10 mM BAPTA; (3) pre-inhibition of Ca ²⁺ channel by verapamil. Different from the effects on I(CCh), ML-7 barely inhibited the same cationic current induced by guanosine 5'-O-(3-thiotriphosphate) (GTP[γS], 0.2 mM) in the pipette solution. These results suggest that a Ca ²⁺-calmodulin-MLCK-dependent pathway can modulate the activation of I(CCh) in guinea-pig gastric antral myocytes.