Rapid Decreases in Phosphatidylinositol in Isolated Luteal Plasma Membranes after Stimulation by Luteinizing Hormone1

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Abstract
Phospholipid concentrations were determined in plasma membrane preparations from porcine corpora lutea after incubation for 15 to 120 s without or with 0.5 .mu.g/ml luteinizing hormone (LH) or 2 .mu.M dibutyrl cyclic adenosine 3'',5''-monophosphate (dbcAMP). Treatment with LH caused a dramatic loss of 9 nmol in plasma membrane phosphatidylinositol (PI)/mg protein after 15 s incubation, but no significant changes in other measurable phospholipids. Also, phospholipid concentrations were unchanged in untreated and dbcAMP-treated plasma membranes. The nature of the LH-induced decrease in PI was studied by incubating plasma membrane preparations for 15 s with [.gamma.32P] adenosine 3'',5''-triphosphate (ATP). 32P was incorporated only into three phospholipids: phosphatidic acid, phosphatidylinositol 4''-phosphate (PIP), and phosphatidylinositol 4'',5''-bisphosphate (PIP2). Although LH generated small but significant increases in labeling of PIP and PIP2, less than 0.5 nmol of total phospholipids/mg protein were radiolabeled in 15 s. Phosphatidylinositol kinase activity, the enzyme that converts PI into PIP, was not affected by LH or dbcAMP treatment. However, incubation of luteal plasma membranes for 15 s with LH resulted in an increase of approximately 2 nmol 1,2-diacylglycerol/mg protein more than that observed in untreated or dbcAMP-treated plasma membranes. In summary, these experiments suggest that LH may stimulated hydrolysis of PI (and possibly PIP and PIP2) in isolated luteal plasma membranes.