A novel pattern of fast calcium oscillations points to calcium and electrical activity cross-talk in rat chromaffin cells

Cav1.3 Channels as Key Regulators of Neuron-Like Firings and Catecholamine Release in Chromaffin Cells

Neuronal and neuroendocrine L-type calcium channels (Cav1.2, Cav1.3) open readily at relatively low membrane potentials and allow Ca(2+) to enter the cells near resting potentials. In this way, Cav1.2 and Cav1.3 shape the action potential waveform, contribute to gene expression, synaptic plasticity, neuronal differentiation, hormone secretion and pacemaker activity. In the chromaffin cells (CCs) of the adrenal medulla, Cav1.3 is highly expressed and is shown to support most of the pacemaking current that sustains action potential (AP) firings and part of the catecholamine secretion. Cav1.3 forms Ca(2+)-nanodomains with the fast inactivating BK channels and drives the resting SK currents. These latter set the inter-spike interval duration between consecutive spikes during spontaneous firing and the rate of spike adaptation during sustained depolarizations. Cav1.3 plays also a primary role in the switch from "tonic" to "burst" firing that occurs in mouse CCs when either the availability of voltage-gated Na channels (Nav) is reduced or the β2 subunit featuring the fast inactivating BK channels is deleted. Here, we discuss the functional role of these "neuron-like" firing modes in CCs and how Cav1.3 contributes to them. The open issue is to understand how these novel firing patterns are adapted to regulate the quantity of circulating catecholamines during resting condition or in response to acute and chronic stress.

Electrophysiological and morphological features underlying neurotransmission efficacy at the splanchnic nerve-chromaffin cell synapse of bovine adrenal medulla

The ability of adrenal chromaffin cells to fast-release catecholamines relies on their capacity to fire action potentials (APs). However, little attention has been paid to the requirements needed to evoke the controlled firing of APs. Few data are available in rodents and none on the bovine chromaffin cell, a model extensively used by researchers. The aim of this work was to clarify this issue. Short puffs of acetylcholine (ACh) were fast perifused to current-clamped chromaffin cells and produced the firing of single APs. Based on the currents generated by such ACh applications and previous literature, current waveforms that efficiently elicited APs at frequencies up to 20 Hz were generated. Complex waveforms were also generated by adding simple waveforms with different delays; these waveforms aimed at modeling the stimulation patterns that a chromaffin cell would conceivably undergo upon strong synaptic stimulation. Cholinergic innervation was assessed using the acetylcholinesterase staining technique on the supposition that the innervation pattern is a determinant of the kind of stimuli chromaffin cells can receive. It is concluded that 1) a reliable method to produce frequency-controlled APs by applying defined current injection waveforms is achieved; 2) the APs thus generated have essentially the same features as those spontaneously emitted by the cell and those elicited by fast-ACh perifusion; 3) the higher frequencies attainable peak at around 30 Hz; and 4) the bovine adrenal medulla shows abundant cholinergic innervation, and chromaffin cells show strong acetylcholinesterase staining, consistent with a tight cholinergic presynaptic control of firing frequency.

Unifying electrostatic mechanism for metal cations in receptors and cell signaling

Previously, an electrostatic mechanism was proposed for receptor-ligand action and for cell signaling by phosphate and sulfate. The hypothesis is further elaborated by application to metal ions, mainly calcium, magnesium, zinc, iron, and copper, in receptors and cell signaling. Evidence is provided for involvement of electrostatics in various reaction modes in biosystems. Calcium plays an important role electrochemically in neurotransmission. In some cases, electron transfer and redox processes are also involved. Electrostatics are known to participate in plant biochemistry. Mechanistically, the electrostatic field may act as a conduit for electrons and radicals and in involvement with energetics.

A physiological view of the central and peripheral mechanisms that regulate the release of catecholamines at the adrenal medulla

Here we review the tight neural control of the differential secretion into the circulation, of the adrenal medullary hormones adrenaline and noradrenaline. One or the other catecholamines are differentially released on various stress conditions. This is specifically controlled by central nervous system nuclei at the cortex, hypothalamus and spinal cord. Different firing patterns of splanchnic nerves and nicotinic or muscarinic receptors cause the selective release of noradrenaline or adrenaline, to adapt the body to the 'fight or flight' reaction, or during severe hypoglycaemia, haemorrhage, cold, acute myocardial infarction or other severe stressful conflicts. Endogenously acetylcholine (ACh) released at the splanchnic nerve-chromaffin cell synapse, acting on muscarinic and nicotinic receptors, causes membrane depolarization and action potentials (AP) in chromaffin cells. These changes vary with the animal species, the cell preparation (intact bisected adrenal, adrenal slices, or isolated fresh or cultured cells) or the recording technique (intracellular microelectrodes, patch-clamp, perforated-patch, cell-attached). Conflicting results leave many open questions concerning the actions of ACh on chromaffin cell excitability. The use of adrenal slices and field electrical stimulation will surely provide new insights into these mechanisms. Chromaffin cells have been thoroughly used as models to study the relationship between Ca2+ entry, cytosolic Ca2+ signals, exocytosis and endocytosis, using patch-clamp and amperometric techniques. Cells have been stimulated with single depolarizing pulses (DPs), DP trains and with simulated AP waveforms. These approaches have provided useful information but we have no data on APs generated by pulsatile secretory quanta of ACh, trying to mimic the intermittent and repetitive splanchnic nerve discharge of the neurotransmitter. We present some recent experiments using ultrashort ACh pulses (25 ms), that cause non-desensitizing repetitive APs with each ACh pulse, at low ACh concentrations (30 microM). Ultrashort pulses of a high ACh concentration (1000 microM) causes a single AP followed by a prolonged depolarization. It could be interesting trying to correlate these 'patterns of splanchnic nerve discharge' with Ca2+ signals and exocytosis. This, together with the use of adrenal slices and transmural electrical stimulation of splanchnic nerves will provide new physiologically sound data on the regulation of adrenal medullary secretion.

Fast exocytosis mediated by T- and L-type channels in chromaffin cells: distinct voltage-dependence but similar Ca2+ -dependence

Expression, spatial distribution and specific roles of different Ca(2+) channels in stimulus-secretion coupling of chromaffin cells are intriguing issues still open to discussion. Most of the evidence supports a role of high-voltage activated (HVA) Ca(2+) channels (L-, N-, P/Q- and R-types) in the control of exocytosis: some suggesting a preferential coupling of specific Ca(2+) channel subunits with the secretory apparatus, others favoring the idea of a contribution to secretion proportional to the expression density and gating properties of Ca(2+) channels. In this work we review recent findings and bring new evidence in favor of the hypothesis that also the LVA (low-voltage-activated, T-type) Ca(2+) channels effectively control fast exocytosis near resting potential in adrenal chromaffin cells of adult rats. T-type channels recruited after long-term treatments with pCPT-cAMP (or chronic hypoxia) are shown to control exocytosis with the same efficacy of L-type channels, which are the dominant Ca(2+) channel types expressed in rodent chromaffin cells. A rigorous comparison of T- and L-type channel properties shows that, although operating at different potentials and with different voltage-sensitivity, the two channels possess otherwise similar Ca(2+)-dependence of exocytosis, size and kinetics of depletion of the immediately releasable pool and mobilize vesicles of the same quantal size. Thus, T- and L-type channels are coupled with the same Ca(2+)-efficiency to the secretory apparatus and deplete the same number of vesicles ready for release. The major difference of the secretory signals controlled by the two channels appear to be the voltage range of operation, suggesting the idea that stressful conditions (hypoxia and persistent beta-adrenergic stimulation) can lower the threshold of cell excitability by recruiting new Ca(2+) channels and activate an additional source of catecholamine secretion.