Arachidonic acid, bradykinin and phospholipase A2 modify both prolactin binding capacity and fluidity of mouse hepatic membranes

Abstract The objective of this study was to determine if arachidonic acid, a precursor of prostaglandin synthesis, bradykinin, a decapeptide known to stimulate membrane phospholipid methylation, arachidonic acid release and prostacyclin synthesis, and enzyme phospholipase A 2 , capable of liberating arachidonic acid, alter the fluidity of hepatic membranes which could in turn modify the functionality of prolactin receptors. Liver homogenates of adult C 3 H female mice incubated at 28°C for various times with 1–20 μg/ml arachidonic acid, 1–100 μg/ml bradykinin or 0.26–0.00026 U/ml phospholipase A 2 provided the 100,000 × g membrane pellets for subsequent ovine prolactin binding and membrane fluidity studies. Membrane microviscosity was determined by fluorescence polarization techniques using the lipid probe 1,6 diphenylhexatriene. Arachidonic acid, bradykinin and phospholipase A 2 stimulated specific oPRL binding, in a dose-related fashion, with maximum increases of 73%, 21% and 46%, at 4 μg/ml arachidonic acid, 5 μg/ml bradykinin and 0.026 U/ml PLA 2 , respectively. This induction, occurring within 30 min of incubation, was found to be due to an increase in the number of receptor sites. Under the same conditions, arachidonic acid, bradykinin and PLA 2 induced 22%, 16%, and 18% decreases in membrane microviscosity, respectively. These data suggest that prostaglandin synthesis modifying agents may modulate the number of prolactin receptors in vivo by changing the lipid fluidity of the target cell membranes by either of their known effects: arachidonic acid release from the phospholipid matrix, synthesizing appropriate prostaglandins at correct concentration or methylation of membrane phospholipids.