NAVigating a transition from single action potential firing to bursting in chromaffin cells

Chromaffin cells (CCs) of the adrenal medulla act as depots of stored catech-olamines (CAs), adrenaline (epinephrine) and noradrenaline (norepinephrine), which are released into the general circulation as part of the classic response to stress (de Diego et al. 2008). The primary physiological trigger of CA secretion from CCs is stress-induced elevation of splanchnic nerve activity. Release of acetylcholine from sympathetic nerve terminals stimulates nicotinic (and muscarinic) receptors on CCs leading to an increase an action potential (AP) firing, an elevation of cytosolic Ca²⁺ ([Ca²⁺]_i) resulting from Ca²⁺ influx thorugh voltage-dependent Ca²⁺ (Cav) channels, and the Ca²⁺-dependent release of CAs resulting in concomitant changes in blood pressure and metabolism. Yet, exocytosis of CA-containing secretory granules from CCs is only weakly coupled to AP-induced elevation of [Ca²⁺]_i (Duan et al. 2003). Isolated CCs typically exhibit spontaneous AP firing, but the basal occurrence of exocytotic events exhibits little correlation with spontaneous APs. In contrast, with brief higher frequency stimulation of APs, secretion is increased, reflecting a higher average [Ca²⁺]_i resulting from temporally summed contributions of Ca²⁺ influx from closely associated APs. Quite naturally, most attention on the physiological role of the adrenal medulla therefore focuses on splanchnic nerve-evoked stimulation of the adrenals (de Diego et al. 2008). Now, in this issue of The Journal of Physiology, Vandael et al. (2015) show that mouse CCs can undergo a change from a spontaneous repetitive AP firing mode to a spontaneous bursting activity with an associated increase in CA secretion. This therefore raises the possibility that mechanisms may exist that enhance non-neurogenic secretion of CAs from CCs.

How does this bursting arise? Vandael et al. reveal that bursting activity is unmasked via two distinct manipulations, both of which alter voltage-dependent Na⁺ (Nav) current availability. In one, Nav current is reduced by tetrodotoxin (TTX) and, in the other, small depolarizations are used to favour Nav inactivation. The authors show that all Nav current in mouse CCs is TTX sensitive and inactivating, thereby allowing TTX to be used as a tool to manipulate Nav availability. However, most importantly, the steady-state inactivation properties of the endogenous Nav current and its slow time-course of recovery from inactivation appear to be ideally suited to allow dynamic modulation of Nav availability over membrane potentials from −40 to −55 mV, the precise membrane potential range over which CCs normally reside. Intriguingly, spontaneous bursting behaviour in CCs has now also been unmasked by an entirely different sort of manipulation. Specifically, genetic deletion of the auxiliary β2 subunit of the Ca²⁺- and voltage-activated BK-type K⁺ channel results in a qualitatively similar spontaneous bursting in mouse CCs (Martinez-Espinosa et al. 2014). Together, these papers raise the possibility that modulation of intrinsic conductances may permit mouse CCs to transition from a spontaneous firing behaviour (∼1 Hz APs) to a bursting mode, with slow wave bursts also occurring at ∼1 Hz. Both papers also observe that a certain fraction (∼10–15%) of control cells exhibit spontaneous bursting, indicative that the capacity to burst occurs normally. This raises the possibility that endogenous modulatory influences might alter membrane conductances in a fashion that would favour bursting behaviour. Vandael et al. suggest that physiological conditions such as plasma hyperkalaemia, acidosis, or increased histamine levels might be pathways through which a sustained depolarization could create conditions leading to sufficient Nav inactivation to promote bursting.

To illuminate the specific ion mechanisms underlying the bursting behaviour, the authors utilize an elegant approach typical of other contributions from the Carbone group. Specifically, AP and burst waveforms are employed as voltage-clamp commands to identify those current components active during the burst behaviour and the specific changes that occur with changes in Nav availability. Earlier work had established that repetitive pacemaking activity in mouse CCs arises from the coupled action of the Cav1.3 Ca²⁺ channel with BK channels (Marcantoni et al. 2010). Here, this same combination presumably underlies the timing of slow-wave bursts, but also appears to define the plateau level of depolarization during the slow wave. The consequence of reduced Nav availability is that the upswing of the initial AP is reduced, with an associated decrease in AP peak. This in turn reduces voltage-dependent K⁺ channel (Kv) activation during the AP, thereby allowing more persistent activation of Cav and BK that then defines the depolarized membrane potential. Although the ionic participants in this bursting mechanism might be considered atypical, these recent results are consistent with the view that bursting pacemaker activity or other patterns of seemingly similar electrical activity may arise via a variety of distinct conductance mechanisms (Marder & Taylor, 2011).

The existence of endogenous bursting behaviour in CCs will require some fresh consideration of the possibility that non-neurogenic release of CAs from CCs may have potential physiological implications. The grouping of APs in endogenous bursts in mouse CCs would elevate average [Ca²⁺]_i to levels sufficient to promote endogenous CA secretion, without involvement of splanchnic nerve activity. In support of this idea, Vandael et al., using amperometric measurements of single-cell CA secretion, observe that the increased APs occurring during bursting activity in the presence of TTX result in enhanced CA secretion over that evoked by simple spontaneous AP firing. If spontaneous bursting was elicited by naturally occurring physiological conditions independent of splanchnic nerve activity, this would require modification of the prevailing view that elevation of circulating CAs and consequent changes in blood pressure arise almost exclusively from splanchnic nerve-evoked release of CAs from the adrenal medulla.

Additional information

Competing interests

None declared.

Funding

The author's research is funded by NIH grant R01 GM081748.