Effects of Sodium Channel-Targeted Conotoxins on Catecholamine Release in Adrenal Chromaffin Cell
Elsie C. Jimenez1,2*, Nobuyuki Sasakawa1, and Konosuke Kumakura1
1Laboratory of Neurochemistry and Neuropharmacology, Life Science Institute
Sophia University 7-1 Kioi-cho, Chiyoda-ku, Tokyo, Japan
2Department of Physical Sciences, College of Science
University of the Philippines Baguio, Baguio City, Philippines
Veratridine, a sodium channel inactivation inhibitor causes Ca2+ influx and catecholamine (CA) release. Adrenal chromaffin cells were treated with sodium channel-targeted conotoxins, μO-MrVIA and μ-PIIIA, prior to the application of 20 μM veratridine that was completely inhibited by tetrodotoxin (TTX). At 2.5 μM, μO-MrVIA completely abolished the veratridine-induced TTX-sensitive CA release. On the other hand, μ-PIIIA (0.5 - 2.5 μM) is a less potent inhibitor; the inhibition leveled off at ~ 60%. The results indicate that in adrenal chromaffin cells, the TTX-sensitive sodium channels can be classified into two categories: (1) sensitive to both μO-MrVIA and μ-PIIIA (60%), and (2) sensitive to μO-MrVIA but resistant to μ-PIIIA (40%). Conotoxins μO-MrVIA and μ-PIIIA may be used in discriminating structurally different sodium channels. Specifically, PIIIA may be useful in determining the functional significance and regulation of co-expression of sodium channel subtypes during differentiation in various types of cells.
In adrenal chromaffin cells, stimulation of nicotinic receptors by acetylcholine causes influx of Na+, and to a lesser extent Ca2+, through the receptor-associated ion channel. This entry of Na+ results in a mild depolarization of the cell membrane, activation of voltage-dependent tetrodotoxin (TTX)-sensitive sodium channels, followed by activation of voltage-dependent calcium channels, and finally exocytotic release of catecholamine (CA) (Ceña et al. 1983; Garcia et al. 1984; Wada et al. 1985). Tetrodotoxin, a specific blocker of voltage-dependent sodium channels does not block CA release by nicotine at all (Ceña et al. 1983). The role of Na+ entry through voltage-dependent TTX-sensitive sodium channels in the secretory response upon a nicotinic stimulation is facilitatory but not obligatory. The entry of Na+ through voltage-dependent channels is observed only when stimulation is induced by low level of agonists (Kirshner 1987). Moreover, voltage-dependent sodium channels are not involved in the CA release elicited by high extracellular K+, that directly activates voltage-dependent calcium channels.
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