N-Ethylmaleimide Diminishes α2-Adrenoceptor-Mediated Effects on Norepinephrine Release in Rat Tail Arteries

SummaryIsolated tail arteries from Wistar rats, prelabeled with [H]norepinephrine (NE) were subjected to electrical field stimulation (24 pulses at 0.4 Hz and 200 mA). Both NE release and vasoconstriction were measured in parallel. The selective α2-adrenoceptor agonist B-HT 933 diminished the evoked NE release in a concentration-dependent manner. This effect of B-HT 933 was counteracted by the selective α2-adrenoceptor antagonist rauwolscine, which given alone enhanced evoked transmitter release, indicating the presence of autoinhibition. N-Ethylmaleimide (NEM) (3 μ M), which also in itself increased transmitter release, virtually abolished facilitation of release by 0.1 μM rauwolscine and diminished its inhibition by 10 μM B-HT 933. The diminution of the inhibitory effect of B-HT 933 was even more pronounced when the current strength was decreased from 200 mA to 90 mA to compensate for the NEM-induced increase in transmitter release. Treatment of the arteries with NEM did not affect the perfusion pressure. In contrast, however, the B-HT 933-induced increase in basal perfusion pressure was significantly diminished by NEM. Although 10 μM B-HT 933 given alone did not affect stimulation-evoked vasoconstriction, it caused a significant increase in arteries treated with NEM. In conclusion, the observed NEM-sensitivity of the presynaptic and vascular α2-adrenoceptor mechanisms is compatible with the idea that both pre- and postsynaptic α2-adrenoceptors couple to Pertussis toxin (PTX)-sensitive G proteins.