LH-RH binding to purified pituitary plasma membranes: Absence of adenylate cyclase activation

Abstract

Purified bovine pituitary plasma membranes possess two specific LH-RH binding sites. The high affinity site (2.5 × 10 9 l/mol) has low capacity (9 × 10 −1.5 mol/mg membrane protein) while the low affinity site (6.1 × 10 5l/mol) has a much higher capacity (1.1 × 10 −10 mol/mg). Specific LH-RH binding to plasma membranes is increased 8.5-fold during purification from homogenate whilst adenylate cyclase activity is enriched 7–8-fold. Distribution of specific LH-RH binding to sucrose density gradient interface fractions parallels that of adenylate cyclase activity. Mg 2+ and Ca 2+ inhibit specific [ 125I]LH-RH binding at micromolar concentrations. Synthetic LH-RH, up to 250 μg/ml, failed to stimulate adenylate cyclase activity of the purified bovine membranes. Using a crude 10,800 g rat pituitary membrane preparation, LH-RH similarly fails to activate adenylate cyclase even in the presence of guanyl nucleotides. These data confirm the presence of LH-RH receptor sites on pituitary plasma membranes and suggest that LH-RH-induced gonadotrophin release may be mediated by mechanisms other than activation of adenylate cyclase.