Effect of High Ba2+ on Norepinephrine-Induced Inhibition of N-Type Calcium Current in Bullfrog Sympathetic Neurons

Abstract

The voltage-dependent inhibition of N-type calcium current by neurotransmitters is the best-understood example of neuronal calcium channel inhibition. One of the mechanisms by which this pathway is thought to inhibit the calcium current is by reducing the permeation of divalent cations through the channel. In this study one prediction of this hypothesis was examined, that high concentrations of divalent cations reduce the maximum neurotransmitter-induced inhibition. Norepinephrine (NE)-induced inhibition was compared in external solutions containing either 2 or 100 mM Ba2+. Initially, NE dose-response curves were generated by averaging data from many neurons, and it was found that the relationship was right shifted in the high-Ba2+ external solution without an effect on maximum inhibition. The IC50 was 0.6 and 3 μM in 2 and 100 mM Ba2+, respectively. This shift was verified by comparing the effect of NE on single neurons exposed to both 2 and 100 mM Ba2+. The inhibition induced by 1 μM NE was reduced in 100 mM Ba2+ compared with that in 2 mM Ba2+. However, the response to 100 μM NE was identical between high and low Ba2+. Thus, divalent cations appear to act as a competitive inhibitor of NE binding, which likely results from these ions' interacting with negatively charged amino acids that are important for catecholamine binding to adrenergic receptors. Because the maximum inhibition induced by NE was similar in low and high Ba2+, the effect of inhibition on single N-type calcium channels was not altered by the divalent cation concentration.