PROTEIN-KINASE-C MEDIATES SPONTANEOUS TONE IN THE CAT LOWER ESOPHAGEAL SPHINCTER

The intracellular pathways responsible for maintenance of tone in the lower esophageal sphincter (LES) are not well understood. We show that the protein kinase C (PKC) antagonists (1-(5-isoquinolinesulphonyl)-2 -methylpiperazine dihydrochloride) and calphostin C reduce spontaneous resting tone in LES muscle strips, whereas the calmodulin antagonist N-(6-aminohexyl-5-chloro-1-naphthalenesulfonamide hydrochloride) has n o effect, which suggests that LES tone is maintained by a PKC-mediated mechanism. In addition, U73122, an inhibitor of phosphatidylinositol- 4,5-bisphosphate (PIP2)-specific phospholipase C, and D609, an inhibit or of phosphatidycholine-specific phospholipase C, reduced diacylglyce rol formation and LES tone in a concentration-dependent manner. Finall y, diacylglycerol levels and PKC activity were reduced during relaxati on of the LES induced by the inhibitory neurotransmitter vasoactive in testinal peptide. These data suggest that resting LES tone is associat ed with elevated diacylglycerol levels and PKC activity, which are red uced during relaxation. Diacylglycerol is derived from at least two di fferent sources. Hydrolysis of PIP2 by PIP2-specific phospholipase C p roduces equimolar amounts of inositol 1,4,5-triphosphate and diacylgly cerol, which may interact synergistically to activate PKC and develop tone. Furthermore, PKC-mediated contraction may be augmented by additi onal diacylglycerol production arising from the hydrolysis of phosphat idylcholine by phosphatidylcholine-specific phospholipase C.