In-utero exposure to nicotine alters the development of the rabbit cardiac conduction system and provides a potential mechanism for sudden infant death syndrome.

In-utero exposure to tobacco smoke remains the highest risk factor for sudden infant death syndrome (SIDS). To alleviate the risks, nicotine replacement therapies are often prescribed to women who wish to quit smoking during their pregnancy. Cardiac arrhythmias is considered the final outcome leading to sudden death. Our goal in this study was to determine if exposing rabbit fetus to nicotine altered the cardiac conduction system of newborn kittens in a manner susceptible to cause SIDS. Using neuronal markers and a series of immunohistological and electrophysiological techniques we found that nicotine delayed the development of the cardiac pacemaker center (sinoatrial node) and decreased its innervation. At the molecular level, nicotine favored the expression of cardiac sodium channels with biophysical properties that will tend to slow heart rate and diminish electrical conduction. Our results show that alterations of the cardiac sodium current may contribute to the bradycardia, conduction disturbances and other cardiac arrhythmias often associated to SIDS and raise awareness on the use of replacement therapy during pregnancy.

STIM1 participates in the contractile rhythmicity of HL-1 cells by moderating T-type Ca(2+) channel activity.

STIM1 plays a crucial role in Ca(2+) homeostasis, particularly in replenishing the intracellular Ca(2+) store following its depletion. In cardiomyocytes, the Ca(2+) content of the sarcoplasmic reticulum must be tightly controlled to sustain contractile activity. The presence of STIM1 in cardiomyocytes suggests that it may play a role in regulating the contraction of cardiomyocytes. The aim of the present study was to determine how STIM1 participates in the regulation of cardiac contractility. Atomic force microscopy revealed that knocking down STIM1 disrupts the contractility of cardiomyocyte-derived HL-1 cells. Ca(2+) imaging also revealed that knocking down STIM1 causes irregular spontaneous Ca(2+) oscillations in HL-1 cells. Action potential recordings further showed that knocking down STIM1 induces early and delayed afterdepolarizations. Knocking down STIM1 increased the peak amplitude and current density of T-type voltage-dependent Ca(2+) channels (T-VDCC) and shifted the activation curve toward more negative membrane potentials in HL-1 cells. Biotinylation assays revealed that knocking down STIM1 increased T-VDCC surface expression and co-immunoprecipitation assays suggested that STIM1 directly regulates T-VDCC activity. Thus, STIM1 is a negative regulator of T-VDCC activity and maintains a constant cardiac rhythm by preventing a Ca(2+) overload that elicits arrhythmogenic events.

Functional up-regulation of Nav1.8 sodium channel in Aß afferent fibers subjected to chronic peripheral inflammation.

BACKGROUND: Functional alterations in the properties of Aß afferent fibers may account for the increased pain sensitivity observed under peripheral chronic inflammation. Among the voltage-gated sodium channels involved in the pathophysiology of pain, Na(v)1.8 has been shown to participate in the peripheral sensitization of nociceptors. However, to date, there is no evidence for a role of Na(v)1.8 in controlling Aß-fiber excitability following persistent inflammation. METHODS: Distribution and expression of Na(v)1.8 in dorsal root ganglia and sciatic nerves were qualitatively or quantitatively assessed by immunohistochemical staining and by real time-polymerase chain reaction at different time points following complete Freund's adjuvant (CFA) administration. Using a whole-cell patch-clamp configuration, we further determined both total INa and TTX-R Na(v)1.8 currents in large-soma dorsal root ganglia (DRG) neurons isolated from sham or CFA-treated rats. Finally, we analyzed the effects of ambroxol, a Na(v)1.8-preferring blocker on the electrophysiological properties of Nav1.8 currents and on the mechanical sensitivity and inflammation of the hind paw in CFA-treated rats. RESULTS: Our findings revealed that Na(v)1.8 is up-regulated in NF200-positive large sensory neurons and is subsequently anterogradely transported from the DRG cell bodies along the axons toward the periphery after CFA-induced inflammation. We also demonstrated that both total INa and Na(v)1.8 peak current densities are enhanced in inflamed large myelinated Aß-fiber neurons. Persistent inflammation leading to nociception also induced time-dependent changes in Aß-fiber neuron excitability by shifting the voltage-dependent activation of Na(v)1.8 in the hyperpolarizing direction, thus decreasing the current threshold for triggering action potentials. Finally, we found that ambroxol significantly reduces the potentiation of Na(v)1.8 currents in Aß-fiber neurons observed following intraplantar CFA injection and concomitantly blocks CFA-induced mechanical allodynia, suggesting that Na(v)1.8 regulation in Aß-fibers contributes to inflammatory pain. CONCLUSIONS: Collectively, these findings support a key role for Na(v)1.8 in controlling the excitability of Aß-fibers and its potential contribution to the development of mechanical allodynia under persistent inflammation.

About half of the late sodium current in cardiac myocytes from dog ventricle is due to non-cardiac-type Na(+) channels.

Voltage gated sodium channels (Na(V)s) are essential to propagate neuronal and cardiac electrical impulses. While the cardiac Na(+) current (I(Na)) is often all attributed to the cardiac isoform, Na(V)1.5, some evidence suggests that other Na(+) channel isoforms are also expressed in the heart ventricle. One way to distinguish Na(+) channels is by their sensitivity to tetrodotoxin (TTX); various "non-cardiac-type" Na(+) channels are relatively sensitive to TTX (denoted tNa(V) channels) compared to Na(V)1.5 channels. tNa(V) channels have been detected in hearts with various pathological conditions such as hypertrophy, infarction and ischemia, where they might enhance the late Na(+) current (I(NaL)) thereby prolonging the action potential under such conditions (resulting in a prolonged QT interval on the EKG). The principal aim of this article is to evaluate the extent to which non-cardiac isotypes contribute to I(NaL) under normal physiological conditions. I(NaL) was measured in acutely dissociated dog cardiomyocytes using the patch-clamp technique. Our results indicate that 44% on average of the late I(Na) current is due to non-cardiac Na(V)s. Previous studies indicated that the overexpression of non-cardiac Na(V) channels is responsible for the prolonged duration of the cardiac action potential (and, thereby, a prolonged QT interval) under pathophysiological conditions associated with various heart diseases. Our finding indicates that non-cardiac Na(V) channels are strong contributors to I(NaL) under physiological conditions thereby suggesting that these channels are also major determinants of the duration of the cardiac action potential even in healthy hearts. Interestingly, these results may explain the observations of cardiac arrhythmias associated with prolonged QT intervals in people with inherited neuronal and musculoskeletal diseases involving mutations that enhance the current from non-cardiac-type Na(V)s, a connection which apparently was never made before.

The neuronal potassium current IA is a potential target for pain during chronic inflammation.

Voltage-gated ion channels play a key role in the action potential (AP) initiation and its propagation in sensory neurons. Modulation of their activity during chronic inflammation creates a persistent pain state. In this study, we sought to determine how peripheral inflammation caused by complete Freund's adjuvant (CFA) alters the fast sodium (INa ), L-type calcium (ICaL ), and potassium (IK ) currents in primary afferent fibers to increase nociception. In our model, intraplantar administration of CFA induced mechanical allodynia and thermal hyperalgesia at day 14 post-injection. Using whole-cell patch-clamp recording in dissociated small (C), medium (Aδ), and large-sized (Aß) rat dorsal root ganglion (DRG) neurons, we found that CFA prolonged the AP duration and increased the amplitude of the tetrodotoxin-resistant (TTX-r) INa in Aß fibers. In addition, CFA accelerated the recovery of INa from inactivation in C and Aδ nociceptive fibers but enhanced the late sodium current (INaL ) only in Aδ and Aß neurons. Inflammation similarly reduced the amplitude of ICaL in each neuronal cell type. Fourteen days after injection, CFA reduced both components of IK (IKdr and IA ) in Aδ fibers. We also found that IA was significantly larger in C and Aδ neurons in normal conditions and during chronic inflammation. Our data, therefore, suggest that targeting the transient potassium current IA represents an efficient way to shift the balance toward antinociception during inflammation, since its activation will selectively decrease the AP duration in nociceptive fibers. Altogether, our data indicate that complex interactions between IK , INa , and ICaL reduce pain threshold by concomitantly enhancing the activity of nociceptive neurons and reducing the inhibitory action of Aß fibers during chronic inflammation.

In utero exposure to nicotine abolishes the postnatal response of the cardiac sodium current to isoproterenol in newborn rabbit atrium.

BACKGROUND: In utero exposure to tobacco smoke is associated with sudden infant death syndrome (SIDS) and cardiac arrhythmias in newborns. The arrhythmogenic mechanisms seem linked to alterations of the cardiac sodium current (INa). We previously reported that in utero exposure to nicotine delays the postnatal development of the heart sinoatrial node in rabbits and altered expression of the sodium channels NaV1.5 and NaV1.1 in the atrium surrounding it. These channels react differently to sympathetic stimulation. OBJECTIVE: The purpose of this study was to test whether nicotine altered the response of INa to stimulation by the ß-adrenoreceptor agonist isoproterenol in atrial myocytes. Our hypothesis is that changes in the sympathetic response of sinoatrial node peripheral cells may create a substrate for arrhythmia. METHODS: Using the patch-clamp technique we measured the effect of nicotine on the response of INa to adrenergic stimulation in isolated cardiomyocytes. RESULTS: Isoproterenol increased INa by 50% in newborn sham rabbits but had no effect in newborn rabbits exposed to nicotine in utero. Our data also show that nicotine increases the late sodium current, an effect that may promote QT prolongation. CONCLUSION: We provide the first evidence linking fetal exposure to nicotine to long-term alterations of INa response to isoproterenol. These changes may impair INa adaptation to sympathetic tone and prevent awakening from sleep apnea, thus leading to arrhythmias that could potentially be involved in SIDS. Our data also raise concerns about the use of nicotine replacement therapies for pregnant women.

Prolongation of action potential duration and QT interval during epilepsy linked to increased contribution of neuronal sodium channels to cardiac late Na+ current: potential mechanism for sudden death in epilepsy.

BACKGROUND: Arrhythmias associated with QT prolongation on the ECG often lead to sudden unexpected death in epilepsy. The mechanism causing a prolongation of the QT interval during epilepsy remains unknown. Based on observations showing an upregulation of neuronal sodium channels in the brain during epilepsy, we tested the hypothesis that a similar phenomenon occurs in the heart and contributes to QT prolongation by altering cardiac sodium current properties (INa). METHODS AND RESULTS: We used the patch clamp technique to assess the effects of epilepsy on the cardiac action potential and INa in rat ventricular myocytes. Consistent with QT prolongation, epileptic rats had longer ventricular action potential durations attributable to a sustained component of INa (INaL). The increase in INaL was because of a larger contribution of neuronal Na channels characterized by their high sensitivity to tetrodotoxin. As in the brain, epilepsy was associated with an enhanced expression of the neuronal isoform NaV1.1 in cardiomyocyte. Epilepsy was also associated with a lower INa activation threshold resulting in increased cell excitability. CONCLUSIONS: This is the first study correlating increased expression of neuronal sodium channels within the heart to epilepsy-related cardiac arrhythmias. This represents a new paradigm in our understanding of cardiac complications related to epilepsy.