Phenoxybenzamine Selectively and Irreversibly Inactivates Dopaminergic D<sub>2</sub> Receptors on Primary Cultured Rat Lactotrophs
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abstract
Lactotrophs have several different kinds of receptors, such as dopaminergic D2, somatostatin, angiotensin II and thyrotropin-releasing hormone receptors, which stimulate or inhibit prolactin release. We have studied the specificity of phenoxybenzamine on receptors in lactotrophs. Phenoxybenzamine is a beta-haloalkylamine which alkylates chemically active radicals such as hydroxy, sulfhydryl, and amino groups. This alkylation is an irreversible chemical reaction in contrast to the receptor-secretagogue complex which is present in a state of dynamic equilibrium. Primary cultured rat adenohypophyseal cells were used in this study. A dose-response relationship was examined between concentrations of phenoxybenzamine pretreatment and prolactin release using a monolayer cell culture system. The inhibitory action of dopamine (10 mumol/l) on the control group (13.0 +/- 0.1 ng/ml or 86% inhibition relative to the control) was significantly higher than on the 0.1-mumol/l phenoxybenzamine-pretreated group (39.0 +/- 0.2 ng/ml or 58% inhibition relative to the control), but the stimulatory effect of thyrotropin-releasing hormone on prolactin release was not significantly affected up to a 10-mumol/l phenoxybenzamine pretreatment as compared with the control group. We thus selected a phenoxybenzamine concentration of 0.1 mumol/l for the next series of perifusion experiments in order to examine dynamic changes in prolactin release. The basal prolactin release was decreased to almost half by phenoxybenzamine pretreatment. The inhibitory action of dopamine (0.1 mumol/l containing 0.1 mmol/l ascorbic acid) was significantly less in the phenoxybenzamine-pretreated group (68% of the basal prolactin concentration) than in the control group (31% of the basal concentration).(ABSTRACT TRUNCATED AT 250 WORDS)
