Spiroperidol binding sites

For each response examined, inhibition of isoproterenol-stimulated cAMP accumulation by intact cells ( ) inhibition of basal release of IR-aMSH by intact cells ( J occupancy of specific [3H]-spiroperidol binding sites in a cell-free homogenate (Q) and inhibition of isoproterenol-stimulated adenylate cyclase activity in a cell-free homogenate (M), the effect achieved with the indicated concentration of apomorphine is expressed as a percentage of the maximal effect of apomorphine (33). 

Key A, total ( ) and nonspecific binding (i.e., binding in the presence of 2 nM fluphenazine) (O) was determined in the presence of the indicated concentrations of [3HJ-spiroperidol and B, specifically bound [3H]-spiroperidol (i.e., the difference between total and nonspecific binding) is shown as a function of the concentration of [3H] spiroperidol. Left inset a plot of the data in B according to Rosenthal s method yields an apparent Kd of 0.2 nM and a maximal concentration of specific binding sites of 94.1 fmol/mg protein (this is equivalent to 20.2... 

Figure 7. Competition between [3H]-spiroperidol and dopamine for occupancy of binding sites in the NIL of the rat pituitary gland. The amount of [3H]-spiro-peridol bound to NIL tissue was determined in the presence of 2 fluphenazine (0), 2 (iM fluphenazine and 100 GTP (4) or dopamine (at the indicated concentrations) in the absence (O) or presence (9) of 100 GTP (33).

Treatment of membrane preparations with digitonin (1-2%) in 100 mM NaCl, 10 mM Tris-HCl pH 7.4 releases 20-30% of the membrane-binding sites in a soluble form that cannot be sedimented at 100,000 xg for 60 min. Saturation isotherms of [3H]spiroperidol as measured by a Sephadex G-50 assay (32) for separation of bound from free ligand reveal a dissociation constant (1C) of 0.5 nM for spiroperidol (not shown). 

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