Different calcium requirements during incorporation of myo-inositol into membrane lipids for phosphatidylinositol hydrolysis stimulated by carbachol and noradrenaline.

Stimulation of phosphatidylinositol hydrolysis by noradrenaline requires the presence of calcium in the medium during the whole incubation and preincubation procedure, while carbachol-induced inositol phosphate formation is equally efficient in calcium-free and calcium-containing medium. This finding confirms the notion that phosphatidylinositols mobilized after stimulation of various receptors are present in different metabolic pools: the pool mobilized by alpha 1 adrenoceptors is apparently formed from myo-inositol incorporated into membrane phospholipids by a calcium-dependent process.