Receptor for thyrotropin-releasing hormone in plasma membranes of bovine anterior pituitary gland. Role of lipids.

The receptor for thyrotropin-releasing hormone has been located in the plasma membranes of bovine anterior pituitary gland. Treatment of membranes with phospholipase A, phospholipase C, or digitonin or addition of lysophosphatidylcholine diminishes or completely destroys the ability of membranes to bind 3H-labeled thyrotropin-releasing hormone. Phospholipase D was without effect. These reductions in thyrotropin-releasing hormone binding capacity stem from a decrease in the afhnity for thyrotropin-relasing hormone of the membrane-located receptor. The effects of phospholipase A were completely Ca2+-dependent. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, and phosphatidylserine are the four major phospholipids of adenohypophyseal cell plasma membranes. When added in excess of 1 mg per ml to intact plasma membranes, sonicated suspensions of these four phospholipids present either singly or in combination result in an inhibition of thyrotropin-releasing hormone binding. If plasma membranes are used with a concentration of phospholipase A or C to give a reduction in thyrotropin-releasing hormone binding of at least 50 %, partial recovery of binding activity is achieved on phospholipid addition. Inhibition of thyrotropin-releasing hormone binding by phospholipid addition to intact membranes is mediated by a decrease in the V,, of the reaction. These data show membrane phospholipids to be intimately involved in the receptor for thyrotropin-releasing hormone.