Guanine nucleotide effects on agonist binding at D2 dopamine receptors in bovine anterior pituitary.

[3H]Spiperone binding to D2 dopamine receptors on bovine anterior pituitary membranes is of high affinity, saturable and unaffected by guanine nucleotides. Antagonist displacement of [3H]spiperone binding is characterised by pseudo Hill coefficients close to one and is unaffected by guanine nucleotides. Agonist displacements show pseudo Hill coefficients less than one and agonist affinities are reduced by guanine nucleotides, although pseudo Hill coefficients are essentially unchanged. Agonist displacement curves may be treated in terms of two agonist binding sites with different agonist affinities, and in the presence of guanine nucleotides the two-binding site model remains the best description of the data.