Evidence that phentolamine is not an inhibitor of extraneuronal uptake

1. Tracheal segments from guinea-pigs pretreated with 6-hydroxydopamine were incubated in isoprenaline at 37°C for 5 min in the absence or presence of phentolamine. Catechol-O-methyl transferase was inhibited by 100 μmol l-1 U-0521. Tissues were prepared for fluorescence histochemistry and accumulated isoprenaline in trachealis smooth muscle cells (fluorescence) was measured by microphotometry. 2. Phentolamine, in concentrations up to 100 μmol l-1, had no effect on isoprenaline fluorescence. 3. It is concluded that phentolamine does not inhibit extraneuronal uptake in concentrations used to block α-adrenoceptors in isolated tissue experiments. Thus, it can be present in experiments designed to examine the effects of extraneuronal uptake inhibitor drugs on β-adrenoceptormediated responses