We performed experiments to investigate the actions of protein kinase C (PKC) on mechanical contraction during agonist stimulation in guinea-pig stomach. We used carbachol and high K condition to enhance mechanical contraction by mobilizing intracellular Ca
2+ and increasing Ca
2+ influx through voltage-dependent Ca
2+ channel, respectively. Phorbol 12, 13-dibutyrate (PDBu) increased spontaneous contractions sensitive to verapamil (438±82.2%, n=7) and potentiated high K-induced contraction (189±22.5%, n=5). However, carbachol (CCh)-induced contractions in PDBu-treated condition depended on extracellular Ca
2+. In the presence of extracellualr Ca
2+, CCh-induced contraction was potentiated, while it was suppressed in the absence of extracellular Ca
2+ in Ca
2+-preloaded muscle strips. To prove the hypothesis that such phenomena might be related with changes of myoplasmic Ca
2+ concentration, we investigated the effect of PDBu on voltage-dependent Ca
2+ current (I
Ca) and CCh-induced Ca
2+-activated K current (I
K (Ca) ) transient using whole-cell voltage clamp technique. For recording voltage-dependent Ca
2+ current (I
Ca), 10mM Ba
2+, instead of Ca
2+, was used to enhance the current size. Voltage-dependent Ba
2+ current (I
K Ba) was increased by PDBu (212±32.2% of steady state currents, n=5), while CCh-induced increase of I
K (Ca) transient was inhibited by PDBu (n=5), the changes of which were similar to those of muscle contractions. To analyze the steps involved in the inhibition of CCh-induced I
K (Ca) transient by PDBu, we investigated the effect of PDBu on I
K (Ca) in the cells perfused with Ins (1, 4, 5) P3. However, Ins (1, 4, 5) P3-induced I
K (Ca) was not inhibited by the treatment with phorbol ester. From these results, it is concluded that inhibition of phosphatidylinositol-phospholipase C (PI-PLC) system and potentiation of I
Ca by PKC are important regulatory mechanisms in agonist-induced muscle contraction.
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