• Archives of Pharmacal Research
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• v.24 no.6
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• pp.546-551
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• 2001

We have previously shown that, in circular muscle cells of the lower esophageal sphincter (LES) isolated by enzymatic digestion, contraction in response to maximally effective doses of acetylcholine (ACh) or Inositol Triphosphate ($IP_3$) depends on the release of $Ca^{2+}$ from intracellular stores and activation of a $Ca6{2+}$-calmodulin (CaM)-dependent pathway. On the contrary, maintenance of LES tone, and response to low doses of ACh or $IP_3$ depend on a protein kinase C (PKC) mediated pathway. In the present investigation, we have examined requirements for $Ca6{2+}$ regulation of the interaction between CaM- and PKC-dependent pathways in LES contraction. Thapsigargin (TG) treatment for 30 min dose dependently reduced ACh-induced contraction of permeable LES cells in free $Ca6{2+}$ medium. ACh-induced contraction following the low level of reduction of $Ca6{2+}$ stores by a low dose of TG ($10^{-9}{\;}M$) was blocked by the CaM antagonist, CCS9343B but not by the PKC antagonists chelerythrine or H7, indicating that the contraction is CaM-dependent. After maximal reduction in intracellular $Ca{2+}$ from $Ca6{2+}$stores by TG ($10^{-6}{\;}M$), ACh-induced contraction was blocked by chelerythrine or H7, but not by CCS9343B, indicating that it is PKC-dependent. In normal $Ca^{2+}$medium, the contraction by ACh after TG ($10^{-9}{\;}M$) treatment was also CaM-dependent, whereas the contraction by ACh after TG ($10^{-9}{\;}M$) treatment was PKC-dependent. We examined whether PKC activation was inhibited by activated CaM. CCS 7343B Inhibited the CaM-induced contraction, but did not inhibit the DAC-induced contraction. CaM inhibited the DAC-induced contraction in the presence of CCS 9343B. This inhibition by CaM was $Ca{2+}$dependent. These data are consistent with the view that the switch from a PKC-dependent pathway to a CaM dependent pathway can occur and can be regulated by cytosolic $Ca{2+}$ in the LES.