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

Abstract

Key pointsr Mouse chromaffin cells (MCCs) of the adrenal medulla possess fast-inactivating Nav channels whose availability alters spontaneous action potential firing patterns and the Ca 2+ -dependent secretion of catecholamines.r Here, we report MCCs expressing large densities of neuronal fast-inactivating Nav1.3 and Nav1.7 channels that carry little or no subthreshold pacemaker currents and can be slowly inactivated by 50% upon slight membrane depolarization.r Reducing Nav1.3/Nav1.7 availability by tetrodotoxin or by sustained depolarization near rest leads to a switch from tonic to burst-firing patterns that give rise to elevated Ca 2+ -influx and increased catecholamine release.r Spontaneous burst firing is also evident in a small percentage of control MCCs. r Our results establish that burst firing comprises an intrinsic firing mode of MCCs that boosts their output. This occurs particularly when Nav channel availability is reduced by sustained splanchnic nerve stimulation or prolonged cell depolarizations induced by acidosis, hyperkalaemia and increased muscarine levels.Abstract 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

Regulation of Ca2+ channels by SNAP-25 via recruitment of syntaxin-1 from plasma membrane clusters

“…Owing to the very high input resistance of adrenal chromaffin cells, a few picoamperes of current is sufficient to make a difference for burst firing, which effectively increases catecholamine secretion (Vandael et al. , 2015a, b). The consequences of shifting from SNAP-25a to SNAP-25b expression will be a compromise between the larger primed vesicle pool supported by the b isoform (Sørensen et al.…”

Microelectrode Arrays of Diamond-Insulated Graphitic Channels for Real-Time Detection of Exocytotic Events from Cultured Chromaffin Cells and Slices of Adrenal Glands

“…In particular, the relevance of spontaneous and evoked secretion is related to the in vivo supply of catecholamines from the adrenal gland that has to be continuously tuned on the basis of the hormonal need, thus ranging from a basal resting release to a more sustained secretion triggered by continuous cell stimulation mimicking stressful conditions. Cultured mouse chromaffin cells (MCCs) exhibit spontaneous firing activity 36 finely modulated by the coupling of calcium and calcium-dependent potassium channels 37,38 and the availability of sodium channels 39 . Taken together, these mechanisms contribute to regulate catecholamines release 40 that occurs spontaneously both in isolated MCCs and mouse adrenal slices.…”

The expression of thioredoxin-1 in acute epinephrine stressed mice

“…This so-called stress response, an integral part of any adaptive biological system, is conserved throughout evolution. Many factors can initiate a stress response (Lucassen et al 2014), enhancing the secretion of catecholamines epinephrine and norepinephrine (Faraut et al 2015;Vandael et al 2015). These hormones can promote the intracellular expression of oxidative stress genes (Graziano et al 2014) and result in proto-oncogene activation, tumor suppressor gene inactivation, DNA damage, and genomic instability (Calvani et al 2015;Pacak 2011).…”