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

Primary root growth in the absence or presence of exogenous NO(3)(-) was studied by a quantitative genetic approach in a recombinant inbred line (RIL) population of Medicago truncatula. A quantitative trait locus (QTL) on chromosome 5 appeared to be particularly relevant because it was seen in both N-free medium (LOD score 5.7; R(2)=13.7) and medium supplied with NO(3)(-) (LOD score, 9.5; R(2)=21.1) which indicates that it would be independent of the general nutritional status. Due to its localization exactly at the peak of this QTL, the putative NRT1-NO(3)(-) transporter (Medtr5g093170.1), closely related to Arabidopsis AtNRT1.3, a putative low-affinity nitrate transporter, appeared to be a significant candidate involved in the control of primary root growth and NO(3)(-) sensing. Functional characterization in Xenopus oocytes using both electrophysiological and (15)NO(3)(-) uptake approaches showed that Medtr5g093170.1, named MtNRT1.3, encodes a dual-affinity NO(3)(-) transporter similar to the AtNRT1.1 'transceptor' in Arabidopsis. MtNRT1.3 expression is developmentally regulated in roots, with increasing expression after completion of germination in N-free medium. In contrast to members of the NRT1 superfamily characterized so far, MtNRT1.3 is environmentally up-regulated by the absence of NO(3)(-) and down-regulated by the addition of the ion to the roots. Split-root experiments showed that the increased expression stimulated by the absence of NO(3)(-) was not the result of a systemic signalling of plant N status. The results suggest that MtNRT1.3 is involved in the response to N limitation, which increases the ability of the plant to acquire NO(3)(-) under N-limiting conditions.

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cDNAs encoding novel long-chain scorpion toxins (64 amino acid residues, including only six cysteines) were isolated from cDNA libraries produced from the venom glands of the scorpions Androctonus australis from Old World and Tityus serrulatus from New World. The encoded peptides were very similar to a recently identified toxin from T. serrulatus, which is active against the voltage-sensitive`delayed-rectifier' potassium channel, but they were completely different from the long-chain and short-chain scorpion toxins already characterised. However, there was some sequence similarity (42%) between these new toxins, Aa TX KL L and Ts TX KL L, and scorpion defensins purified from the hemolymph of Buthidae scorpions Leiurus quinquestriatus and A. australis. Thus, according to a multiple sequence alignment using CLUSTAL, these new toxins seem to be related to the scorpion defensins.z 1998 Federation of European Biochemical Societies.