Protection of presynaptic alpha-adrenoceptors against irreversible blockade by phenoxybenzamine: preservation of the modulatory effects of exogenous noradrenaline and yohimbine.

1. Receptor protection experiments were carried out in order to study the site of action of alpha-adrenoceptor agonists and antagonists on the release of noradrenaline. Cerebrocortical slices from rabbits were preincubated with 3H-noradrenaline. They were then superfused with medium containing cocaine 30 mumol/l and stimulated electrically (3 Hz) three times, after 60, 250 and 295 min of superfusion (S1, S2, S3). Phenoxybenzamine 10 mumol/l, when used, was added between S1 and S2 for 30 min; putative protecting drugs (clonidine 100 mumol/l or yohimbine 10 mumol/l) were present 5 min before and during the exposure to phenoxybenzamine and then washed out together with the latter. Either the voltage drop between the electrodes at S2 and S3 or the Ca2+ -concentration of the superfusion medium at S2 and S3 was diminished, if necessary, in order to bring the overflow evoked by S2 close to the overflow at S1. Blockade by phenoxybenzamine, or protection against the blockade, was examined by addition of the test compounds noradrenaline 0.1 mumol/l or yohimbine 1 mumol/l before S3. 2. In slices not exposed previously to alpha-adrenoceptor ligands, noradrenaline 0.1 mumol/l greatly reduced, whereas yohimbine 1 mumol/l greatly increased the evoked overflow of tritium. Both effects were abolished in slices treated with phenoxybenzamine 10 mumol/l alone between S1 and S2. 3. In contrast to phenoxybenzamine alone, exposure to phenoxybenzamine 10 mumol/l in the presence of either clonidine 100 mumol/l or yohimbine 10 mumol/l failed to abolish the effects of the test compounds noradrenaline 0.1 mumol/l and yohimbine 1 mumol/l, although the effects were reduced.(ABSTRACT TRUNCATED AT 250 WORDS)