ACTH receptors in nervous tissue. High affinity binding-sequestration of [125I]Phe2,Nle4]ACTH 1-24 in homogenates and slices from rat brain.

We have demonstrated specific, high affinity binding of a biologically active Tyr23-monoiodinated derivative of ACTH, [125I][Phe2,Nle4]ACTH 1-24, in rat brain homogenates. Similarly, in metabolically inhibited and noninhibited rat whole brain slices there is a specific "binding-sequestration" process that is dependent on time, protein concentration, and pH. In homogenates, binding curves were best described by a two-site model and provided the following parameters: Kd1 = 0.65 +/- 0.47 nM, Bmax1 = 21 +/- 41 fmol/mg protein; Kd2 = 97 +/- 48 nM, Bmax2 = 3.5 +/- 1.8 pmol/mg protein. In metabolically viable brain slices, concentration-competition curves of [125I][Phe2,Nle4]ACTH 1-24 binding-sequestration can be described by three components (Kd1 = 14 +/- 24 nM, Bmax1 = 50 +/- 95 fmol/mg protein; Kd2 = 2.4 +/- 1.9 microM, Bmax2 = 44 +/- 49 pmol/mg protein; Kd3 = 0.16 +/- 1.0 mM, Bmax3 = 5.3 +/- 54 nmol/mg protein). Metabolic inhibition, by removal of glucose and addition of 100 microM ouabain, abolishes the lowest affinity, highest capacity binding-sequestrian component only (Kd1 = 7.1 +/- 14 nM, Bmax1 = 8.7 +/- 16 fmol/mg protein; Kd2 = 7.4 +/- 4.49 microM, Bmax2 = 37 +/- 27 pmol/mg protein). The two binding-sequestration parameter estimates obtained from metabolically inhibited tissue slices are not significantly different from those of the two higher affinity components obtained with noninhibited tissue. Thus, metabolic inhibition permits demonstration of ACTH receptor binding only, unconfounded by sequestration or internalization of ligand:receptor complexes.(ABSTRACT TRUNCATED AT 250 WORDS)