Electrophysiological effects of SN-6, a novel Na+/Ca2+ exchange inhibitor on membrane currents in guinea pig ventricular myocytes

T-type voltage-gated channels, Na+/Ca2+-exchanger, and calpain-2 promote photoreceptor cell death in inherited retinal degeneration

Inherited retinal degenerations (IRDs) are a group of untreatable and commonly blinding diseases characterized by progressive photoreceptor loss. IRD pathology has been linked to an excessive activation of cyclic nucleotide-gated channels (CNGC) leading to Na+- and Ca2+-influx, subsequent activation of voltage-gated Ca2+-channels (VGCC), and further Ca2+ influx. However, a connection between excessive Ca2+ influx and photoreceptor loss has yet to be proven.Here, we used whole-retina and single-cell RNA-sequencing to compare gene expression between the rd1 mouse model for IRD and wild-type (wt) mice. Differentially expressed genes indicated links to several Ca2+-signalling related pathways. To explore these, rd1 and wt organotypic retinal explant cultures were treated with the intracellular Ca2+-chelator BAPTA-AM or inhibitors of different Ca2+-permeable channels, including CNGC, L-type VGCC, T-type VGCC, Ca2+-release-activated channel (CRAC), and Na+/Ca2+ exchanger (NCX). Moreover, we employed the novel compound NA-184 to selectively inhibit the Ca2+-dependent protease calpain-2. Effects on the retinal activity of poly(ADP-ribose) polymerase (PARP), sirtuin-type histone-deacetylase, calpains, as well as on activation of calpain-1, and - 2 were monitored, cell death was assessed via the TUNEL assay.While rd1 photoreceptor cell death was reduced by BAPTA-AM, Ca2+-channel blockers had divergent effects: While inhibition of T-type VGCC and NCX promoted survival, blocking CNGCs and CRACs did not. The treatment-related activity patterns of calpains and PARPs corresponded to the extent of cell death. Remarkably, sirtuin activity and calpain-1 activation were linked to photoreceptor protection, while calpain-2 activity was related to degeneration. In support of this finding, the calpain-2 inhibitor NA-184 protected rd1 photoreceptors.These results suggest that Ca2+ overload in rd1 photoreceptors may be triggered by T-type VGCCs and NCX. High Ca2+-levels likely suppress protective activity of calpain-1 and promote retinal degeneration via activation of calpain-2. Overall, our study details the complexity of Ca2+-signalling in photoreceptors and emphasizes the importance of targeting degenerative processes specifically to achieve a therapeutic benefit for IRDs. Video Abstract.

The properties, distribution and function of Na(+)-Ca(2+) exchanger isoforms in rat cutaneous sensory neurons

The Na(+)-Ca(2+) exchanger (NCX) appears to play an important role in the regulation of the high K(+)-evoked Ca(2+) transient in putative nociceptive dorsal root ganglion (DRG) neurons. The purpose of the present study was to (1) characterize the properties of NCX activity in subpopulations of DRG neurons, (2) identify the isoform(s) underlying NCX activity, and (3) begin to assess the function of the isoform(s) in vivo. In retrogradely labelled neurons from the glabrous skin of adult male Sprague-Dawley rats, NCX activity, as assessed with fura-2-based microfluorimetry, was only detected in putative nociceptive IB4+ neurons. There were two modes of NCX activity: one was evoked in response to relatively large and long lasting (∼325 nm for >12 s) increases in the concentration of intracellular Ca(2+) ([Ca(2+)]i), and a second was active at resting [Ca(2+)]i > ∼150 nm. There also were two modes of evoked activity: one that decayed relatively rapidly (<5 min) and a second that persisted (>10 min). Whereas mRNA encoding all three NCX isoforms (NCX1-3) was detected in putative nociceptive cutaneous neurons with single cell PCR, pharmacological analysis and small interfering RNA (siRNA) knockdown of each isoform in vivo suggested that NCX2 and 3 were responsible for NCX activity. Western blot analyses suggested that NCX isoforms were differentially distributed within sensory neurons. Functional assays of excitability, action potential propagation, and nociceptive behaviour suggest NCX activity has little influence on excitability per se, but instead influences axonal conduction velocity, resting membrane potential, and nociceptive threshold. Together these results indicate that the function of NCX in the regulation of [Ca(2+)]i in putative nociceptive neurons may be unique relative to other cells in which these exchanger isoforms have been characterized and it has the potential to influence sensory neuron properties at multiple levels.

Effect of chronic perinatal hypoxia on the role of rho-kinase in pulmonary artery contraction in newborn lambs

Exposure to chronic hypoxia during gestation predisposes infants to neonatal pulmonary hypertension, but the underlying mechanisms remain unclear. Here, we test the hypothesis that moderate continuous hypoxia during gestation causes changes in the rho-kinase pathway that persist in the newborn period, altering vessel tone and responsiveness. Lambs kept at 3,801 m above sea level during gestation and the first 2 wk of life were compared with those with gestation at low altitude. In vitro studies of isolated pulmonary arterial rings found a more forceful contraction in response to KCl and 5-HT in high-altitude compared with low-altitude lambs. There was no difference between the effects of blockers of various pathways of extracellular Ca(2+) entry in low- and high-altitude arteries. In contrast, inhibition of rho-kinase resulted in significantly greater attenuation of 5-HT constriction in high-altitude compared with low-altitude arteries. High-altitude lambs had higher baseline pulmonary artery pressures and greater elevations in pulmonary artery pressure during 15 min of acute hypoxia compared with low-altitude lambs. Despite evidence for an increased role for rho-kinase in high-altitude arteries, in vivo studies found no significant difference between the effects of rho-kinase inhibition on hypoxic pulmonary vasoconstriction in intact high-altitude and low-altitude lambs. We conclude that chronic hypoxia in utero results in increased vasopressor response to both acute hypoxia and serotonin, but that rho-kinase is involved only in the increased response to serotonin.

Revisiting the ionic mechanisms of early afterdepolarizations in cardiomyocytes: predominant by Ca waves or Ca currents?

Early afterdepolarizations (EADs) have been implicated in severe cardiac arrhythmias and sudden cardiac deaths. However, the mechanism(s) for EAD genesis, especially regarding the relative contribution of Ca(2+) wave (CaW) vs. L-type Ca current (I(Ca,L)), still remains controversial. In the present study, we simultaneously recorded action potentials (APs) and intracellular Ca(2+) images in isolated rabbit ventricular myocytes and systematically compared the properties of EADs in the following two pharmacological models: 1) hydrogen peroxide (H(2)O(2); 200 μM); and 2) isoproterenol (100 nM) and BayK 8644 (50 nM) (Iso + BayK). We assessed the rate dependency of EADs, the temporal relationship between EADs and corresponding CaWs, the distribution of EADs over voltage, and the effects of blockers of I(Ca,L), Na/Ca exchangers, and ryanodine receptors. The most convincing evidence came from the AP-clamp experiment, in which the cell membrane clamp was switched from current clamp to voltage clamp using a normal AP waveform without EAD; CaWs disappeared in the H(2)O(2) model, but persisted in the Iso + BayK model. We postulate that, although CaWs and reactivation of I(Ca,L) may act synergistically in either case, reactivation of I(Ca,L) plays a predominant role in EAD genesis under oxidative stress (H(2)O(2) model), while spontaneous CaWs are a predominant cause for EADs under Ca(2+) overload condition (Iso + BayK model).

Long-term maternal hypoxia: the role of extracellular Ca2+ entry during serotonin-mediated contractility in fetal ovine pulmonary arteries

Antenatal maternal long-term hypoxia (LTH) can alter serotonin (5-HT) and calcium (Ca(2+)) signaling in fetal pulmonary arteries (PAs) and is associated with persistent pulmonary hypertension of the newborn (PPHN). In humans, the antenatal maternal hypoxia can be secondary to smoking, anemia, and chronic obstructive pulmonary disorders. However, the mechanisms of antenatal maternal hypoxia-related PPHN are unresolved. Because both LTH and 5-HT are associated with PPHN, we tested the hypothesis that antenatal maternal LTH can increase 5-HT-mediated PA contraction and associated extracellular Ca(2+) influx through L-type Ca(2+) channels (Ca(L)), nonselective cation channels (NSCCs), and reverse-mode sodium-calcium exchanger (NCX) in the near-term fetus. We performed wire myography and confocal-Ca(2+) imaging approaches on fetal lamb PA (∼ 140 days of gestation) from normoxic ewes or those acclimatized to high-altitude LTH (3801 m) for ∼110 days. Long-term hypoxia reduced the potency but not the efficacy of 5-HT-induced PA contraction. Ketanserin (100 nmol/L), a 5-HT(2A) antagonist, shifted 5-HT potency irrespective of LTH, while GR-55562 (1 µmol/L), a 5-HT(1B/D) inhibitor, antagonized 5-HT-induced contraction in normoxic fetuses only. Various inhibitors for Ca(L), NSCC, and reverse-mode NCX were used in contraction studies. Contraction was reliant on extracellular Ca(2+) regardless of maternal hypoxia, NSCC was more important to contraction than Ca(L), and reverse-mode NCX had little or no role in contraction. Long-term hypoxia also attenuated the effects of 2-APB and flufenamic acid and reduced Ca(2+) responses observed by imaging studies. Overall, LTH reduced 5HT(1B/D) function and increased NSCC-related Ca(2+)-dependent contraction in ovine fetuses, which may compromise pulmonary vascular function in the newborn.

Sodium-calcium exchangers contribute to the regulation of cytosolic calcium levels in mouse taste cells

Taste cells use multiple signalling mechanisms to generate unique calcium responses to distinct taste stimuli. Some taste stimuli activate G-protein coupled receptors (GPCRs) that cause calcium release from intracellular stores while other stimuli depolarize taste cells to cause calcium influx through voltage-gated calcium channels (VGCCs). We recently demonstrated that a constitutive calcium influx exists in taste cells that is regulated by mitochondrial calcium transport and that the magnitude of this calcium influx correlates with the signalling mechanisms used by the taste cells. In this study, we used calcium imaging to determine that sodium-calcium exchangers (NCXs) also routinely contribute to the regulation of basal cytosolic calcium and that their relative role correlates with the signalling mechanisms used by the taste cells. RT-PCR analysis revealed that multiple NCXs and sodium-calcium-potassium exchangers (NCKXs) are expressed in taste cells. Thus, a dynamic relationship exists between calcium leak channels and calcium regulatory mechanisms in taste cells that functions to keep cytosolic calcium levels in the appropriate range for cell function.