An activity-dependent increased role for L-type calcium channels in exocytosis is regulated by adrenergic signaling in chromaffin cells

Cav1.3 and Cav1.2 channels of adrenal chromaffin cells: emerging views on cAMP/cGMP-mediated phosphorylation and role in pacemaking

Voltage-gated Ca²⁺ channels (VGCCs) are voltage sensors that convert membrane depolarizations into Ca²⁺ signals. In the chromaffin cells of the adrenal medulla, the Ca²⁺ signals driven by VGCCs regulate catecholamine secretion, vesicle retrievals, action potential shape and firing frequency. Among the VGCC-types expressed in these cells (N-, L-, P/Q-, R- and T-types), the two L-type isoforms, Ca(v)1.2 and Ca(v)1.3, control key activities due to their particular activation-inactivation gating and high-density of expression in rodents and humans. The two isoforms are also effectively modulated by G protein-coupled receptor pathways delimited in membrane micro-domains and by the cAMP/PKA and NO/cGMP/PKG phosphorylation pathways which induce prominent Ca²⁺ current changes if opposingly regulated. The two L-type isoforms shape the action potential and directly participate to vesicle exocytosis and endocytosis. The low-threshold of activation and slow rate of inactivation of Ca(v)1.3 confer to this channel the unique property of carrying sufficient inward current at subthreshold potentials able to activate BK and SK channels which set the resting potential, the action potential shape, the cell firing mode and the degree of spike frequency adaptation during spontaneous firing or sustained depolarizations. These properties help chromaffin cells to optimally adapt when switching from normal to stress-mimicking conditions. Here, we will review past and recent findings on cAMP- and cGMP-mediated modulations of Ca(v)1.2 and Ca(v)1.3 and the role that these channels play in the control of chromaffin cell firing. This article is part of a Special Issue entitled: Calcium channels.

Prolactin regulates tuberoinfundibular dopamine neuron discharge pattern: novel feedback control mechanisms in the lactotrophic axis

Balance in the body's hormonal axes depends on feedback onto neuroendocrine hypothalamic neurons. This phenomenon involves transcriptional and biosynthetic effects, yet less is known about the potential rapid modulation of electrical properties. Here, we investigated this issue in the lactotrophic axis, in which the pituitary hormone prolactin is tonically inhibited by tuberoinfundibular dopamine (TIDA) neurons located in the hypothalamic arcuate nucleus. Whole-cell recordings were performed on slices of the rat hypothalamus. In the presence of prolactin, spontaneously oscillating TIDA cells depolarized, switched from phasic to tonic discharge, and exhibited broadened action potentials. The underlying prolactin-induced current is composed of separate low- and high-voltage components that include the activation of a transient receptor potential-like current and the inhibition of a Ca(2+)-dependent BK-type K(+) current, respectively, as revealed by ion substitution experiments and pharmacological manipulation. The two components of the prolactin-induced current appear to be mediated through distinct signaling pathways as the high-voltage component is abolished by the phosphoinositide 3-kinase blocker wortmannin, whereas the low-voltage component is not. This first description of the central electrophysiological actions of prolactin suggests a novel feedback mechanism. By simultaneously enhancing the discharge and spike duration of TIDA cells, increased serum prolactin can promote dopamine release to limit its own secretion with implications for the control of lactation, sexual libido, fertility, and body weight.

Inhibition of Ca2+ channels and adrenal catecholamine release by G protein coupled receptors

Catecholamines and other transmitters released from adrenal chromaffin cells play central roles in the "fight-or-flight" response and exert profound effects on cardiovascular, endocrine, immune, and nervous system function. As such, precise regulation of chromaffin cell exocytosis is key to maintaining normal physiological function and appropriate responsiveness to acute stress. Chromaffin cells express a number of different G protein coupled receptors (GPCRs) that sense the local environment and orchestrate this precise control of transmitter release. The primary trigger for catecholamine release is Ca2+ entry through voltage-gated Ca2+ channels, so it makes sense that these channels are subject to complex regulation by GPCRs. In particular G protein βγ heterodimers (Gbc) bind to and inhibit Ca2+ channels. Here I review the mechanisms by which GPCRs inhibit Ca2+ channels in chromaffin cells and how this might be altered by cellular context. This is related to the potent autocrine inhibition of Ca2+ entry and transmitter release seen in chromaffin cells. Recent data that implicate an additional inhibitory target of Gβγ on the exocytotic machinery and how this might fine tune neuroendocrine secretion are also discussed.

Different roles attributed to Cav1 channel subtypes in spontaneous action potential firing and fine tuning of exocytosis in mouse chromaffin cells

J. Neurochem. (2011) 116, 105–121.

Abstract

This study examines the Cav1 isoforms expressed in mouse chromaffin cells and compares their biophysical properties and roles played in cell excitability and exocytosis. Using immunocytochemical and electrophysiological techniques in mice lacking the Cav1.3α1 subunit (Cav1.3^−/−) or the high sensitivity of Cav1.2α1 subunits to dihydropyridines, Cav1.2 and Cav1.3 channels were identified as the only Cav1 channel subtypes expressed in mouse chromaffin cells. Cav1.3 channels were activated at more negative membrane potentials and inactivated more slowly than Cav1.2 channels. Cav1 channels, mainly Cav1.2, control cell excitability by functional coupling to BK channels, revealed by nifedipine blockade of BK channels in wild type (WT) and Cav1.3^−/− cells (53% and 35%, respectively), and by the identical change in the shape of the spontaneous action potentials elicited by the dihydropyridine in both strains of mice. Cav1.2 channels also play a major role in spontaneous action potential firing, supported by the following evidence: (i) a similar percentage of WT and Cav1.3^−/− cells fired spontaneous action potentials; (ii) firing frequency did not vary between WT and Cav1.3^−/− cells; (iii) mostly Cav1.2 channels contributed to the inward current preceding the action potential threshold; and (iv) in the presence of tetrodotoxin, WT or Cav1.3^−/− cells exhibited spontaneous oscillatory activity, which was fully abolished by nifedipine perfusion. Finally, Cav1.2 and Cav1.3 channels were essential for controlling the exocytotic process at potentials above and below −10 mV, respectively. Our data reveal the key yet differential roles of Cav1.2 and Cav1.3 channels in mediating action potential firing and exocytotic events in the neuroendocrine chromaffin cell.

Interaction between the second messengers cAMP and Ca2+ in mouse presynaptic taste cells

The second messenger, 3',5'-cyclic adenosine monophosphate (cAMP), is known to be modulated in taste buds following exposure to gustatory and other stimuli. Which taste cell type(s) (Type I/glial-like cells, Type II/receptor cells, or Type III/presynaptic cells) undergo taste-evoked changes of cAMP and what the functional consequences of such changes are remain unknown. Using Fura-2 imaging of isolated mouse vallate taste cells, we explored how elevating cAMP alters Ca(2+) levels in identified taste cells. Stimulating taste buds with forskolin (Fsk; 1 microm) + isobutylmethylxanthine (IBMX; 100 microm), which elevates cellular cAMP, triggered Ca(2+) transients in 38% of presynaptic cells (n = 128). We used transgenic GAD-GFP mice to show that cAMP-triggered Ca(2+) responses occur only in the subset of presynaptic cells that lack glutamic acid decarboxylase 67 (GAD). We never observed cAMP-stimulated responses in receptor cells, glial-like cells or GAD-expressing presynaptic cells. The response to cAMP was blocked by the protein kinase A inhibitor H89 and by removing extracellular Ca(2+). Thus, the response to elevated cAMP is a PKA-dependent influx of Ca(2+). This Ca(2+) influx was blocked by nifedipine (an inhibitor of L-type voltage-gated Ca(2+) channels) but was unperturbed by omega-agatoxin IVA and omega-conotoxin GVIA (P/Q-type and N-type channel inhibitors, respectively). Single-cell RT-PCR on functionally identified presynaptic cells from GAD-GFP mice confirmed the pharmacological analyses: Ca(v)1.2 (an L-type subunit) is expressed in cells that display cAMP-triggered Ca(2+) influx, while Ca(v)2.1 (a P/Q subunit) is expressed in all presynaptic cells, and underlies depolarization-triggered Ca(2+) influx. Collectively, these data demonstrate cross-talk between cAMP and Ca(2+) signalling in a subclass of taste cells that form synapses with gustatory fibres and may integrate tastant-evoked signals.

Differential variations in Ca2+ entry, cytosolic Ca2+ and membrane capacitance upon steady or action potential depolarizing stimulation of bovine chromaffin cells

AIMS

This study looks into the physiology of the exocytosis of catecholamines released by adrenal medullary chromaffin cells. We have comparatively explored the exocytotic responses elicited by two different patterns of depolarizing stimulation: the widely employed square depolarizing pulses (DPs) and trains of acetylcholine-like action potentials (APs), likely the physiological mode of stimulation in the intact innervated adrenal medulla. APs were applied at 30 Hz, a frequency similar to that produced in a stressful situation.

METHODS

Patch-clamp, cell membrane capacitance, single cell amperometry and fluorescence were the techniques used. The variations of calcium entry measured as the integral of the calcium current, cytosolic calcium (measured with the calcium-sensitive fluorescent probe fluo-4) and exo-endocytosis (membrane capacitance variations) were the parameters measured.

RESULTS

Trains of AP depolarizations produced distinct responses compared to those of square depolarizations: (1) Calcium current amplitude decreased to a lesser extent along the AP train; (2) calcium entry and capacitance increments raised linearly with stimulation time whereas they deviated from linearity when square depolarizations were used; (3) slower activation and faster delayed decay phase of cytosolic calcium transients; (4) capacitance increments varied linearly with calcium entry with APs and deviated from linearity with longer depolarizations; (5) little endocytosis after stimulation with longer trains of APs and pronounced endocytosis with longer square depolarizations.

CONCLUSIONS

Stimulation of chromaffin cells with trains of APs produced patterns of cytosolic calcium transients, exocytotic and endocytotic responses quite different from those elicited by the widely employed DPs. Our study is relevant from the methodological and physiological points of view.

Pharmacological and biophysical properties of Ca2+ channels and subtype distributions in human adrenal chromaffin cells

In this study, we explored the pharmacological and biophysical properties of voltage-activated Ca2+ channels in human chromaffin cells using the perforated-patch configuration of the patch-clamp technique. According to their pharmacological sensitivity to Ca2+ channel blockers, cells could be sorted into two groups of similar size showing the predominance of either N- or P/Q-type Ca2+ channels. R-type Ca2+ channels, blocked by 77% with 20 muM Cd2+ and not affected by 50 muM Ni2+, were detected for the first time in human chromaffin cells. Immunocytochemical experiments revealed an even distribution of alpha (1E) Ca2+ channels in these cells. With regard to their biophysical properties, L- and R-type channels were activated at membrane potentials that were 15-20 mV more negative than P/Q- and N-type channels. Activation time constants showed no variation with voltage for the L-type channels, decreased with increasing potentials for the R- and P/Q-type channels, and displayed a bell shape with a maximum at 0 mV for the N-type channels. R-type channels were also the most inactivated channels. We thus show here that human chromaffin cells possess all the Ca2+ channel types described in neurons, L, N, P/Q, and R channels, but the relative contributions of N and P/Q channels differ among cells. Given that N- and P/Q-type Ca2+ channel types can be differentially modulated, these findings suggest the possibility of cell-specific regulation in human chromaffin cells.

Measuring secretion in chromaffin cells using electrophysiological and electrochemical methods

Our present understanding of exocytosis of catecholamines has benefited tremendously from the arrival of single-cell electrochemical methods (amperometry and voltammetry), electrophysiological techniques (whole-cell and patch capacitance) and from the combination of both techniques (patch amperometry). In this brief review, we will outline the strengths and limitations of amperometric and electrophysiological methods and highlight the major contribution obtained with the use of these techniques in chromaffin cells.

Calcium channel subtypes differentially regulate fusion pore stability and expansion

Various studies have focused in the relative contribution of different voltage-activated Ca(2+) channels (VACC) to total transmitter release. However, how Ca(2+) entry through a given VACC subtype defines the pattern of individual exocytotic events remains unknown. To address this question, we have used amperometry in bovine chromaffin cells. L, N, and P/Q channels were individually or jointly blocked with furnidipine, omega-conotoxin GVIA, omega-agatoxin IVA, or omega-conotoxin MVIIC. The three channel types contributed similarly to cytosolic Ca(2+) signals induced by 70 mmol/L K(+). However, they exhibited different contributions to the frequency of exocytotic events and they were shown to differently regulate the final steps of the exocytosis. When compared with the other VACC subtypes, Ca(2+) entry through P/Q channels effectively induced exocytosis, it decreased fusion pore stability and accelerated its expansion. Conversely, Ca(2+) entry through N channels was less efficient in inducing exocytotic events, also slowing fusion pore expansion. Finally, Ca(2+) entry through L channels inefficiently induced exocytosis, and the individual blockade of this channel significantly modified fusion pore dynamics. The distance between a given VACC subtype and the release sites could account for the differential effects of the distinct VACC on the fusion pore dynamics.

Differential regulation of endogenous N- and P/Q-type Ca2+ channel inactivation by Ca2+/calmodulin impacts on their ability to support exocytosis in chromaffin cells

P/Q-type (Ca(V)2.1) and N-type (Ca(V)2.2) Ca2+ channels are critical to stimulus-secretion coupling in the nervous system; feedback regulation of these channels by Ca2+ is therefore predicted to profoundly influence neurotransmission. Here we report divergent regulation of Ca2+-dependent inactivation (CDI) of native N- and P/Q-type Ca2+ channels by calmodulin (CaM) in adult chromaffin cells. Robust CDI of N-type channels was observed in response to prolonged step depolarizations, as well as repetitive stimulation with either brief step depolarizations or action potential-like voltage stimuli. Adenoviral expression of Ca2+-insensitive calmodulin mutants eliminated CDI of N-type channels. This is the first demonstration of CaM-dependent CDI of a native N-type channel. CDI of P/Q-type channels was by comparison modest and insensitive to expression of CaM mutants. Cloning of the C terminus of the Ca(V)2.1 alpha1 subunit from chromaffin cells revealed multiple splice variants lacking structural motifs required for CaM-dependent CDI. The physiological relevance of CDI on stimulus-coupled exocytosis was revealed by combining perforated-patch voltage-clamp recordings of pharmacologically isolated Ca2+ currents with membrane capacitance measurements of exocytosis. Increasing stimulus intensity to invoke CDI resulted in a significant decrease in the exocytotic efficiency of N-type channels compared with P/Q-type channels. Our results reveal unexpected diversity in CaM regulation of native Ca(V)2 channels and suggest that the ability of individual Ca2+ channel subtypes to undergo CDI may be tailored by alternative splicing to meet the specific requirements of a particular cellular function.

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.