EFFECT OF CALCIUM-CHANNEL BLOCKERS ON NOREPINEPHRINE RELEASE AND MODULATION BY PREJUNCTIONAL D-2 DOPAMINE-RECEPTORS

Influx of calcium through voltage-dependent N-type calcium channels pr omotes the release of norepinephrine from nerve terminals. Contributio ns by L- and N-type calcium channels at different levels of nerve stim ulation were examined in perfused rat tail arteries loaded with [H-3]n orepinephrine. Nifedipine had no effect while omega-conotoxin reduced tritium efflux by 69 to 82% depending on the stimulation intensity. Th us, N-type calcium channels predominated in the control of norepinephr ine release, and the relative contribution of L- and N-type channels d id not change when stimulation intensity was altered. We also explored the effect of calcium channel blockers on modulation of norepinephrin e release by D-2 dopamine receptors. Inhibition of stimulation-evoked tritium efflux by the D-2 agonist N-0923 was similar in the absence an d presence of nifedipine and/or omega-conotoxin. We conclude that D-2 dopamine receptors are not coupled to L-type calcium channels; however , the role of N-type calcium channels requires further investigation.