Effects of Phospholipase C on Electrical and Mechanical Activity of Guinea Pig Stomach Muscle

Abstract

Effects of phospholipase C (PLC) on factors involved in smooth muscle contraction were investigated in longitudinal muscle of the guinea pig antrum. This agent irreversibly reduced spontaneous contraction and action potentials; partially after 30 min and, sometimes , completely after 60 min of exposure to PLC. The membrane depolarized about 8 mV from the control level in 30 min and 16 mV in 60 min. ACh-and KC1-induced contracture were inhibited in an external Ca2+ concentration range from zero to three times normal. The phasic component of KC1-induced contracture was abolished occasionally in normal Ca2+ concentration, and frequently in Ca2+ -free solution. Ca-uptake by antrum tissue, which was the same for control, ACh-, and KC1-induced contracture before PLC treatment, was not statistically different in PLCtreated control, but the value decreased during ACh-induced, and increased during KCl-induced contracture after PLC treatment. The contents of phosphatidylcholine and phosphatidylethanolamine were reduced slightly after PLC treatment, but this reduction was statistically significant. Substantial damage and modification of physical characteristics appeared in electron micrographs of PLC-treated preparations . From the results it was concluded that changes in Ca2+ transport and membrane structural damage both contribute to inhibition of smooth muscle activity by PLC.In addition to their function as the principal structural component of cell membranes (ROBERTSON, 1960;SINGER and NICOLSON, 1972), phospholipids also contribute to ion exchange (TomAs, 1964;GOLDMAN, 1965), and enzyme functions (MARToNosi et al., 1968;VESSEY and ZAKIM, 1971) in membranes.Phospholipase C (PLC) effectively inhibits the ionic transport responsible for the generation of action potentials in skeletal muscle and nerves (TomAs, 1958;ALBUQUERQUE and THESLEFF, 1967). It was suggested that the PLC effect on inhibition of the ionic transport is through its action on the hydrophilic heads of certain membrane phospholipids, but the actual mechanism is not yet clear .