A dopaminergic binding site in the high speed supernatant of steer anterior pituitary homogenates

The high speed supernatant fraction from homogenates of the steer anterior pituitary was shown to contain a high affinity (Kd = 0.10-0.20 nM), saturable (Bmax - 1.2-3.5 fmol/mg protein), stereoselective binding site for 3H-spiperone (3H-SPIP). The Kd (0.10 nM) calculated from the experimentally determined rate constants k1 and k2 was in excellent agreement with that derived from equilibrium measurements. The rank order of potencies of dopaminergic agents to compete for binding was consistent with known dopamine (DA) receptors. Soluble binding sites represented 3% of the total specific binding found in the anterior pituitary, and several lines of evidence suggested they were not due to enzymatic, mechanical or osmotic displacement of membrane receptors. Further studies are necessary to elucidate the possible significance of these soluble binding sites; however, this observation is intriguing since both DA and membrane bound DA receptors enter the cell.