Dopaminergic and adrenergic binding affinities in rabbit retinal synaptosomes

1. Dopamine binding to amacrine membrane vesicles isolated as synaptosomal fractions P1 and P2 from rabbit retinas showed saturation within less than a minute. 2. Dopamine binding to retinal synaptosomal membranes (RSM) in P1 and P2 is a two-component system: the first saturated at 1.00 microM 14C-dopamine in P1 and 1.25 microM in P2, and the second saturated at 2.00 microM in both pellets. 3. The affinity of RSM receptors to dopamine in P1 was equal to that in P2 (Km = 2.00 microM), whereas the calculated Vmax of dopamine binding was increased in P2 (1.25 pmol/micrograms protein) as compared to P1 (0.625 pmol/micrograms protein). 4. Dopamine binding to the beta-adrenergic sites showed a lower affinity (Km = 10 microM) in P2 relative to P1 (Km = 4.0 microM), whereas Vmax in P2 (5.0 pmol/micrograms protein) was 4-fold higher than P1 (1.25 pmol/micrograms protein). 5. The P1 and P2 fractions of rabbit RSM contain dopaminergic and beta-adrenergic binding sites with higher concentration of dopaminergic receptors and lower concentration of beta-adrenergic receptors in P2 relative to P1.