Phosphatidylinositol Metabolism and Myoinositol 1,2-Cyclic Phosphate Action in Smooth Muscle

Abstract

Enhanced labelling of phosphatidylinositol occurs in different type of cells during stiinulation (1,2). This effect includes phosphatidylinositol breakdown and resynthesis (see cycle of reaction in Hawthorne et al., this Synposium) and has recently been related in various tissues to α -adrenergic and muscarinic cholinergic agonists (2). According to the available information it seems most probably that the site of control of this effect is confined to phosphatidylinositol breakdown to 1,2-diacylglycerol. Pancreas, parotid and longitudinal ileum smooth muscle show a fall in the concentration of phosphatidylinositol during stimulation (3,4,5). It is then possible that studies of phosphatidylinositol labelling are only a reflection of the primary event which is phosphatidylinositol breakdown. A specific phospholipase-C type of activity against phosphatidylinositol has been found in particulate fractions of brain and vas deferens smooth muscle (6-8). This activity produces breakdown of phosphatidylinositol with production of 1,2- diacylglycerol, myoinositol 1,2-cyclic phosphate and myoinositol 1-phosphate (8,9)