Norepinephrine at 5 μM induces a rapid (60 s) and specific loss of phosphatidylinositol (PtdIns) when added to isolated rat liver plasma membranes. The hormone action is inhibited by the α-adrenergic antagonist phentolamine (20 μM). Depletion of Mg2+ and Ca2+ singly or in combination from the incubation buffer mimicks the hormone effect on PtdIns breakdown. No further effect on PtdIns degradation could be measured when norepinephrine was added to the cation-depleted buffers. Addition of the Ca2+ ionophore A23187 to the isolated membranes has no effect. It is concluded that PtdIns degradation can be provoked in isolated rat liver plasma membrane through α-adrenergic receptor activation and that this effect is dependent on divalent cations in the sense that loss of cations from the membrane allows degradation to commence.
