Selective enrichment with alpha 1A- and alpha 1B-adrenoceptor subtypes in rat brain cortical membranes

Recent evidences from molecular biology, radioreceptor binding and functional studies indicate that the alpha 1-adrenoceptor population is heterogeneous and can at least be divided into two subclasses named alpha 1A and alpha 1B. The present study was designed to obtain, a selective enrichment of rat brain cortical membranes with each subtype of alpha 1-adrenoceptor using alkylating agents. [3H]prazosin binding to rat cortical membranes was saturable and of high affinity (KD = 0.11 +/- 0.02 nM; Bmax = 132.5 +/- 7.2 fmol/mg protein). All ligands competed for specific [3H]prazosin binding in a statistically significant biphasic manner (%Rhigh = 30-40%; %Rlow = 60-70%). These sites meet generally accepted and recently described pharmacologic criteria for their identification as the alpha 1A- and alpha 1B-adrenoceptors. After pretreatment of membranes with benextramine (1 microM) in the presence of clonidine (1 microM), the antagonists, WB4101, (+)-niguldipine and phentolamine, displaced the radioligand with an inhibition curve steeper than in control membranes and with Ki values that agree with those obtained for the low affinity site present in control membranes. On the other hand, after pretreatment with chloroethylclonidine (10 microM) in the presence of WB4101 (1 nM), Hill coefficients for the displacement of the radioligand by WB4101, (+)-niguldipine, and phentolamine, were also increased, but in contrast to the situation described above, the Ki values agree with those obtained for the high affinity site present in control membranes. In conclusion, this method of partial alkylation of receptors could be a valuable tool for separately studying the pharmacological characteristics of the alpha 1-adrenoceptor subtypes in native membranes of cerebral tissue.