The role of calcium-activated potassium channels in respiratory control

Large conductance voltage-and calcium-activated K+ (BK) channel in health and disease

Large Conductance Voltage- and Calcium-activated K+ (BK) channels are transmembrane pore-forming proteins that regulate cell excitability and are also expressed in non-excitable cells. They play a role in regulating vascular tone, neuronal excitability, neurotransmitter release, and muscle contraction. Dysfunction of the BK channel can lead to arterial hypertension, hearing disorders, epilepsy, and ataxia. Here, we provide an overview of BK channel functioning and the implications of its abnormal functioning in various diseases. Understanding the function of BK channels is crucial for comprehending the mechanisms involved in regulating vital physiological processes, both in normal and pathological conditions, controlled by BK. This understanding may lead to the development of therapeutic interventions to address BK channelopathies.

Potassium ion channels and allergic asthma

High-conductive calcium-sensitive potassium channels (BK+Ca) and ATP-sensitive potassium (K+ATP) channels play a significant role in the airway smooth muscle cell and goblet cell function, and cytokine production. The present study evaluated the therapeutic potential of BK+Ca and K+ATP openers, NS 1619 and pinacidil, respectively, in an experimental model of allergic inflammation. Airway allergic inflammation was induced with ovalbumine in guinea pigs during 21 days, which was followed by a 14-day treatment with BK+Ca and K+ATP openers. The outcome measures were airway smooth muscle cells reactivity in vivo and in vitro, cilia beating frequency and the level of exhaled NO (ENO), and the level of pro-inflammatory cytokines in the plasma and bronchoalveolar lavage fluid. The openers of both channels decreased airway smooth muscle cells reactivity, cilia beating frequency, and cytokine levels in the serum. Furthermore, NS1619 reduced ENO and inflammatory cells infiltration. The findings confirmed the presence of beneficial effects of BK+Ca and K+ATP openers on airway defence mechanisms. Although both openers dampened pro-inflammatory cytokines and mast cells infiltration, an evident anti-inflammatory effect was provided only by NS1619. Therefore, we conclude that particularly BK+Ca channels represent a promising new drug target in treatment of airway's allergic inflammation.

Using a computational model to analyze the effects of firing frequency on synchrony of a network of gap junction-coupled hypoglossal motoneurons

Hypoglossal motoneurons (HMs) are located in the brainstem and play an important role in the maintenance of upper airway patency. HMs are known to be coupled to one another via gap junctions and exhibit synchronous firing behavior when driven by premotor inputs. In the current study, we used a computational model to analyze the influence of firing frequency on synchronous firing behavior of a network of gap junction-coupled HMs. As there are many factors that can influence excitability and firing frequency of HMs, our simulations were focused on the effects of modulation of SK channel conductance and the magnitude of the input currents as a means of modifying firing frequency. Our simulations revealed that regardless of the mechanism by which firing frequency was modulated, increasing the firing frequency disrupted the synchrony in gap junction-coupled HM networks with low, medium, and high levels of gap junction coupling.

Molecular cloning and tissue expression patterns of a small conductance calcium-activated potassium channel gene in turbot (Scophthalmus maximus L.)

Calcium-activated potassium channels on plasma membrane enable potassium influx into the cell with ensuing changes in plasma membrane potential and consequent effects on cellular metabolic functions. Recently, this potassium channel was reported to regulate the cellular responses of mammalian immune cells. We have postulated the presence of such a channel in fish immune cells and its potential role in immunoregulation in fish. Employing specific primers and RNA template, we cloned a segment of a novel gene from turbot blood sample and subsequently obtained a full cDNA sequence using RACE approaches. Bioinformatic analysis revealed structural and phylogenetic characteristics of a novel small conductance calcium-activated potassium channel gene, we called TSKCa, which exhibits homologous domains to other species particularly in the transmembrane regions. Full-length TSKCa cDNA is 1698 bp with a 1632 bp open reading frame encoding a protein of 544 amino acids. TSKCa gene is expressed in majority of the tested organs and tissues of turbot. To assess the postulated immune function of TSKCa, we infected turbot with the pathogen Vibrio anguillarum. Here, semi-quantitative RT-PCR analysis demonstrated increased mRNA expression of TSKCa in head kidney, spleen and blood, indicating an important role of TSKCa in these organ tissues that mediate the immune defense response of turbot. In contrast, there was less change in expression in the turbot intestines and liver which were less implicated in the immune response in present study.

Identification and characterization of Ca2+-activated K+ channels in granulosa cells of the human ovary

BACKGROUND

Granulosa cells (GCs) represent a major endocrine compartment of the ovary producing sex steroid hormones. Recently, we identified in human GCs a Ca2+-activated K+ channel (K(Ca)) of big conductance (BK(Ca)), which is involved in steroidogenesis. This channel is activated by intraovarian signalling molecules (e.g. acetylcholine) via raised intracellular Ca2+ levels. In this study, we aimed at characterizing 1. expression and functions of K(Ca) channels (including BK(Ca) beta-subunits), and 2. biophysical properties of BK(Ca) channels.

METHODS

GCs were obtained from in vitro-fertilization patients and cultured. Expression of mRNA was determined by standard RT-PCR and protein expression in human ovarian slices was detected by immunohistochemistry. Progesterone production was measured in cell culture supernatants using ELISAs. Single channels were recorded in the inside-out configuration of the patch-clamp technique.

RESULTS

We identified two K(Ca) types in human GCs, the intermediate- (IK) and the small-conductance K(Ca) (SK). Their functionality was concluded from attenuation of human chorionic gonadotropin-stimulated progesterone production by K(Ca) blockers (TRAM-34, apamin). Functional IK channels were also demonstrated by electrophysiological recording of single K(Ca) channels with distinctive features. Both, IK and BK(Ca) channels were found to be simultaneously active in individual GCs. In agreement with functional data, we identified mRNAs encoding IK, SK1, SK2 and SK3 in human GCs and proteins of IK and SK2 in corresponding human ovarian cells. Molecular characterization of the BK(Ca) channel revealed the presence of mRNAs encoding several BK(Ca) beta-subunits (beta2, beta3, beta4) in human GCs. The multitude of beta-subunits detected might contribute to variations in Ca2+ dependence of individual BK(Ca) channels which we observed in electrophysiological recordings.

CONCLUSION

Functional and molecular studies indicate the presence of active IK and SK channels in human GCs. Considering the already described BK(Ca), they express all three K(Ca) types known. We suggest that the plurality and co-expression of different K(Ca) channels and BK(Ca) beta-subunits might allow differentiated responses to Ca2+ signals over a wide range caused by various intraovarian signalling molecules (e.g. acetylcholine, ATP, dopamine). The knowledge of ovarian K(Ca) channel properties and functions should help to understand the link between endocrine and paracrine/autocrine control in the human ovary.

Somatic Ca2+ transients do not contribute to inspiratory drive in preBötzinger Complex neurons

PreBötzinger Complex (preBötC) neurons are postulated to underlie respiratory rhythm generation. The inspiratory phase of the respiratory cycle in vitro results from preBötC neurons firing synchronous bursts of action potentials (APs) on top of 10-20 mV, 0.3-0.8 s inspiratory drive potentials. Is the inspiratory drive in individual neurons simply the result of the passive integration of inspiratory-modulated synaptic currents or do active processes modulate these currents? As somatic Ca(2+) is known to increase during inspiration, we hypothesized that it affects inspiratory drive. We combined whole cell recording in an in vitro slice preparation with Ca(2+) microfluorometry to detect single inspiratory neuron somatic Ca(2+) transients with high temporal resolution ( approximately mus). In neurons loaded with either Fluo-4 or Oregon Green BAPTA 5 N, we observed Ca(2+) transients associated with each AP. During inspiration, significant somatic Ca(2+) influx was a direct consequence of activation of voltage-gated Ca(2+) channels by APs. However, when we isolated the inspiratory drive potential in active preBötC neurons (by blocking APs with intracellular QX-314 or by hyperpolarization), we did not detect somatic Ca(2+) transients; yet, the parameters of inspiratory drive were the same with or without APs. We conclude that, in the absence of APs, somatic Ca(2+) transients do not shape the somatic inspiratory drive potential. This suggests that in preBötC neurons, substantial and widespread somatic Ca(2+) influx is a consequence of APs during the inspiratory phase and does not contribute substantively to the inspiratory drive potential. Given evidence that the Ca(2+) buffer BAPTA can significantly reduce inspiratory drive, we hypothesize that dendritic Ca(2+) transients amplify inspiratory-modulated synaptic currents.

Calcium-activated potassium currents differentially modulate respiratory rhythm generation

The pre-Bötzinger complex (PBC) generates eupnea and sighs in normoxia and gasping during hypoxia through particular mixtures of intrinsic and synaptic properties. Among intrinsic properties, little is known about the role of Ca(2+)-activated potassium channels in respiratory rhythms generation. To examine this role, we tested the effects of openers and blockers of the large-conductance (BK) and small-conductance (SK) Ca(2+)-activated potassium channels on the respiratory rhythms recorded both in vitro and in vivo, as well as on the discharge pattern of respiratory neurons in the PBC. Activation of SK channels with 1-ethyl-2-benzimidazolinone (1-EBIO) abolished sigh-like activity and inhibited eupneic-like activity, whereas blockade of SK channels with apamine (APA) increased frequency in both rhythms. In hypoxia, APA did not affect the transition to gasping-like activity. At the cellular level, activation of SK channels abolished pacemaker activity and decreased non-pacemaker neurons discharge; opposite effects were observed with SK blockade. In contrast to SK channel modulation, either activation or blockade of BK channels with NS 1619 or iberiotoxin and paxilline, respectively, produced mild effects on eupneic-like and sigh-like bursts during normoxia in vitro. However, BK blockers prevented the changes associated with the transition to gasping-like activity in vitro and perturbed gasping generation and autoresuscitation in vivo. At the cellular level BK channel modulation did not affect respiratory neurons discharge. We conclude that K(Ca) participate in rhythm generation in a state-dependent manner; SK channels are preferentially involved in rhythm generation in normoxia whereas BK channels participate in the transition to gasping generation in hypoxia.

Altered expression of calcium-activated K and Cl channels in detrusor overactivity of rats with partial bladder outlet obstruction

OBJECTIVE

To evaluate the activity of large- and small-conductance calcium-activated potassium channels (BKCa, SKCa) and calcium-activated chloride channels (ClCa) in detrusor overactivity (DO) cells after partial bladder outlet obstruction (PBOO) in rats.

MATERIALS AND METHODS

Thirteen female Wistar rats with DO caused by PBOO were studied simultaneously with eight sham-operated rats. The expression of KCa and ClCa channels was assessed by reverse transcription-polymerase chain reaction, and the function of the two groups compared.

RESULTS

In the DO cells the expression of BKCa, SKCa2 and SKCa3 was lower, and that of ClCa channels higher, than in the control group cells. Using confocal laser scanning microscopic analysis, the function of BKCa and SKCa channels was suppressed, and that of ClCa channels was enhanced in DO group cells. KCa and ClCa effectors altered the cell membrane potentials more significantly in the DO cells than in the control cells, indicating a decrease in KCa and an increase in ClCa in DO group in either iso- or hypo-osmolar medium. Moreover, the change in BKCa, SKCa and ClCa channel activators in DO cells showed a more excitable state in hypo-osmolar medium than in iso-osmolar medium.

CONCLUSION

In DO myocytes after PBOO, the expression and function of KCa channels were decreased, and those of ClCa channels increased. These changes all provoke greater cell excitability, and could partly account for the DO.

The progressive effects of ageing on chemosensitivity in healthy subjects

The aim of this study was to compare the central inspiratory drive (P(0.1)) response to hypoxia and hypercapnia between different age groups of elderly, nonsmoker, healthy subjects and young healthy controls. A random sample, proportionally stratified by age (65-69, 70-74, 75-79 and 80-84 yrs) from a sample of nonsmoker elderly subjects representative of a general population and 47 healthy subjects aged 20-40 were selected. Arterial blood gas, lung volumes, diffusing capacity, maximal respiratory pressure and oxygen uptake measurements were performed. Breathing pattern and mouth occlusion pressure, as well as P(0.1) responses to hyperoxic progressive hypercapnia and isocapnic progressive hypoxia were evaluated. The elderly subjects had lower P0.1 responses to hypoxia (0.017+/-0.006 vs. 0.031+/-0.008 kPa/%, P<0.001) and hypercapnia (0.042+/-0.018 vs. 0.051+/-0.030 kPa/mmHg, P=0.047) than the young healthy controls. Hypoxic sensitivity gradually decreased as age increased to 70-74 and remained unchanged from 75 years of age onward. CO(2) threshold was lower in the elderly groups than in young healthy controls. Lung volumes, inspiratory muscle strength and baseline metabolic rate were the principal determinants of hypoxic sensitivity. In summary, during old age, a progressive decline in hypoxic sensitivity and a decrease in the CO(2) threshold are experienced. These alterations remain stable from the age of 75 onward.

Peripheral chemoreceptor control of ventilation following sustained hypoxia in young and older adult humans

The rate and duration of peripheral chemoreceptor resensitization following sustained hypoxia was characterized in young and older (74-year-old) adults. In addition, cerebral blood velocity (CBV) was measured in young subjects during and following the relief from sustained hypoxia. Following 20 min of sustained eucapnic hypoxia (50 mmHg), subjects were re-exposed to brief (1.5 min) hypoxic pulses (50 mmHg), and the magnitude of the ventilatory response was used to gauge peripheral chemosensitivity. Five minutes after the relief from sustained hypoxia, ventilation (V(E)) increased to 40.3 +/- 4.5% of the initial hypoxic ventilatory response, and by 36 min V(E) increased to 100%, indicating that peripheral chemosensitivity to hypoxia was restored. The V(E) response magnitude plotted versus time demonstrated that V(E), hence peripheral chemosensitivity, was restored at a rate of 1.9% per minute. Cerebral blood flow (CBF, inferred from CBV) remained constant during sustained hypoxia and increased by the same magnitude during the hypoxic pulses, suggesting that CBF has a small, if any, impact on the decline in V(E) during hypoxia and its subsequent recovery. To address the issue of whether hypoxic pulses affect subsequent challenges, series (continuous hypoxic pulses at various recovery intervals) and parallel (only 1 pulse per trial) methods were used. There were no differences in the ventilatory responses between the series and parallel methods. Older adults demonstrated a similar rate of recovery as in the young, suggesting that ageing in active older adults does not affect the peripheral chemoreceptor response.