Reduced availability of voltage-gated sodium channels by depolarization or blockade by tetrodotoxin boosts burst firing and catecholamine release in mouse chromaffin cells

David H F Vandael (1) , Matteo M Ottaviani (1) , Christian Legros (2) , Claudie Lefort (2) , Nathalie C. Guérineau (2) , Arianna Allio (1) , Valentina Carabelli (1) , Emilio Carbone (1)

1 UNITO - Università degli studi di Torino = University of Turin
2 BNMI - Biologie Neurovasculaire et Mitochondriale Intégrée

Action potential (AP) firing in mouse chromaffin cells (MCCs) is mainly sustained by Cav1.3 L-type channels that drive BK and SK currents and regulate the pacemaking cycle. As secretory units, CCs optimally recruit Ca(2+) channels when stimulated, a process potentially dependent on the modulation of the AP waveform. Our previous work has shown that a critical determinant of AP shape is voltage-gated sodium channel (Nav) channel availability. Here, we studied the contribution of Nav channels to firing patterns and AP shapes at rest (-50 mV) and upon stimulation (-40 mV). Using quantitative RT-PCR and immunoblotting, we show that MCCs mainly express tetrodotoxin (TTX)-sensitive, fast-inactivating Nav1.3 and Nav1.7 channels that carry little or no Na(+) current during slow ramp depolarizations. Time constants and the percentage of recovery from fast inactivation and slow entry into closed-state inactivation are similar to that of brain Nav1.3 and Nav1.7 channels. The fraction of available Nav channels is reduced by half after 10 mV depolarization from -50 to -40 mV. This leads to low amplitude spikes and a reduction in repolarizing K(+) currents inverting the net current from outward to inward during the after-hyperpolarization. When Nav channel availability is reduced by up to 20% of total, either by TTX block or steady depolarization, a switch from tonic to burst firing is observed. The spontaneous occurrence of high frequency bursts is rare under control conditions (14% of cells) but leads to major Ca(2+)-entry and increased catecholamine release. Thus, Nav1.3/Nav1.7 channel availability sets the AP shape, burst-firing initiation and regulates catecholamine secretion in MCCs. Nav channel inactivation becomes important during periods of high activity, mimicking stress responses.

Mots clés: ion channels; burst firing; chromaffin cells

Domaines: Neurosciences [q-bio.NC]

David H F Vandael, Matteo M Ottaviani, Christian Legros, Claudie Lefort, Nathalie C. Guérineau, et al.. Reduced availability of voltage-gated sodium channels by depolarization or blockade by tetrodotoxin boosts burst firing and catecholamine release in mouse chromaffin cells. The Journal of Physiology, 2015, 593 (4), pp.905-927. ⟨10.1113/jphysiol.2014.283374⟩. ⟨hal-02484488⟩