Phase 2 Re-Entry Without Ito: Role of Sodium Channel Kinetics in Brugada Syndrome Arrhythmias.

BACKGROUND: In Brugada syndrome (BrS), phase 2 re-excitation/re-entry (P2R) induced by the transient outward potassium current (Ito) is a proposed arrhythmia mechanism; yet, the most common genetic defects are loss-of-function sodium channel mutations. OBJECTIVES: The authors used computer simulations to investigate how sodium channel dysfunction affects P2R-mediated arrhythmogenesis in the presence and absence of Ito. METHODS: Computer simulations were carried out in 1-dimensional cables and 2-dimensional tissue using guinea pig and human ventricular action potential models. RESULTS: In the presence of Ito sufficient to generate robust P2R, reducing sodium current (INa) peak amplitude alone only slightly potentiated P2R. When INa inactivation kinetics were also altered to simulate reported effects of BrS mutations and sodium channel blockers, however, P2R occurred even in the absence of Ito. These effects could be potentiated by delaying L-type calcium channel activation or increasing ATP-sensitive potassium current, consistent with experimental and clinical findings. INa-mediated P2R also accounted for sex-related, day and night-related, and fever-related differences in arrhythmia risk in BrS patients. CONCLUSIONS: Altered INa kinetics synergize powerfully with reduced INa amplitude to promote P2R-induced arrhythmias in BrS in the absence of Ito, establishing a robust mechanistic link between altered INa kinetics and the P2R-mediated arrhythmia mechanism.

Evidence of Permethrin Resistance and Fipronil Tolerance in Rhipicephalus sanguineus s.l. (Acari: Ixodidae) Populations From Florida and California.

Rhipicephalus sanguineus s.l. (Latreille), is a vector of multiple disease-causing pathogens to humans and dogs. Permethrin and fipronil are two acaricides used to manage R. sanguineus s.l. infestations. Homeowners have reported treatment failures in managing brown dog ticks using permethrin and fipronil based products. Previous studies demonstrated that high permethrin resistance in some R. sanguineus s.l. populations was due to metabolic detoxification and target site insensitivity. In this study, three R. sanguineus s.l. strains, one from a laboratory colony (NC) and two colonies originally collected from Florida (FL) and California (CA), were evaluated for resistance expression against permethrin and fipronil. Metabolic detoxification mechanisms were evaluated in the FL strain using three synergists, while a polymerase chain reaction assay was used to detect a resistance mutation in all strains. The NC strain was susceptible to both permethrin and fipronil, while both the FL and CA strains exhibited high resistance to permethrin and tolerance to fipronil. The synergist tests and PCR results indicated that the FL strain utilized both metabolic resistance and target site insensitivity against permethrin, while the CA strain was documented to have the target-site insensitivity resistant allele. This study confirmed permethrin resistance in both California and Florida populations and its persistence in Florida populations, although its susceptibility can potentially be increased by adding a synergist. Fipronil resistance was not detected suggesting this acaricide may provide suitable tick control.

Evaluation of acaricidal resistance status of Rhipicephalus microplus ticks from the hilly state (Uttarakhand) of India and evaluation of efficacy of a natural formulation for the management of resistant ticks.

The resistance status against deltamethrin, cypermethrin, coumaphos and ivermectin was assessed of Rhipicephalus microplus from five districts of Uttarakhand, through adult immersion test and larval packet test. The field isolates were highly resistant to deltamethrin (median resistance ratio [RR50] = 9.10-29.13-fold) followed by cypermethrin (2.23-3.55). Surprisingly the isolates were susceptible to coumaphos (0.34-3.17). Emerging resistance against ivermectin (1.55-3.27) was also observed in all the isolates. Elevated levels of esterases (enzyme ratio = 2.93-5.84-fold), glutathione S-transferases (5.10-10.06) and monooxygenases (1.68-4.02) in resistant fields isolates were highly correlated (47.4-86.0%) with the resistant factor (RR50) determined by bioassay. All the isolates except Uttarkashi possess mutation at the 190th position in domain II of the sodium channel gene. As a mitigation strategy an Ageratum conyzoides-based characterized natural formulation was tested against all the isolates and was found effective at the concentration of 10.1-11.5%. The possibility of using the natural formulation for the management of multi-acaricide resistant ticks is discussed.

The contribution of detoxification pathways to pyrethroid resistance in Hyalella azteca.

Chronic exposure to pyrethroid insecticides can result in strong selective pressures on non-target species in aquatic systems and drive the evolution of resistance and population-level changes. Characterizing the underlying mechanisms of resistance is essential to better understanding the potential consequences of contaminant-driven microevolution. The current study found that multiple mechanisms enhance the overall tolerance of Hyalella azteca to the pyrethroid permethrin. In H. azteca containing mutations in the voltage-gated sodium channel (VGSC), both adaptation and acclimation played a role in mitigating the adverse effects of pyrethroid exposures. Pyrethroid resistance is primarily attributed to the heritable mutation at a single locus of the VGSC, resulting in reduced target-site sensitivity. However, additional pyrethroid tolerance was conferred through enhanced enzyme-mediated detoxification. Cytochrome P450 monooxygenases (CYP450) and general esterases (GE) significantly contributed to the detoxification of permethrin in H. azteca. Over time, VGSC mutated H. azteca retained most of their pyrethroid resistance, though there was some increased sensitivity from parent to offspring when reared in the absence of pyrethroid exposure. Permethrin median lethal concentrations (LC50s) declined from 1809 ng/L in parent (P0) individuals to 1123 ng/L in the first filial (F1) generation, and this reduction in tolerance was likely related to alterations in acclimation mechanisms, rather than changes to target-site sensitivity. Enzyme bioassays indicated decreased CYP450 and GE activity from P0 to F1, whereas the VGSC mutation was retained. The permethrin LC50s in resistant H. azteca were still two orders-of-magnitude higher than non-resistant populations indicating that the largest proportion of resistance was maintained through the inherited VGSC mutation. Thus, the noted variation in tolerance in H. azteca is likely associated with inducible traits controlling enzyme pathways. A better understanding of the mechanistic and genomic basis of acclimation is necessary to more accurately predict the ecological and evolutionary consequences of contaminant-driven change in H. azteca.

Brugada syndrome and sinus node dysfunction.

Brugada syndrome (BrS) is a well-known catastrophic disease first reported in 1992 by the Brugada brothers. Ventricular fibrillation (VF) is an essential arrhythmia in BrS. An association between BrS and atrial tachyarrhythmias is not uncommon. However, sinus node dysfunction (SND) associated with BrS has not been well discussed. In this review, we focus on the association between BrS and SND. Based on previous reports describing clinical, epidemiological, and genetic evidence, SND is not a rare concomitant disorder in BrS. BrS may be a multiple conduction or arrhythmogenic disorder including not only the His-Purkinje system and right ventricle, but also the sinus node and atrium, derived from ion channel mutations.

Identification of Novel Voltage-Gated Sodium Channel Mutations in Human Head and Body Lice (Phthiraptera: Pediculidae).

In recent years, the increase of head louse infestation in Iran (7.4%) and especially in West-Azerbaijan Province (248%) has raised the hypothesis of insecticide resistance development. There are different mechanisms of resistance to various groups of insecticides, and knockdown resistance (kdr) is a prominent mechanism of resistance to pyrethroids, an insecticide group which is used conventionally for pediculosis control. For detection of kdr-type well-known amino acid substitutions (M815I-T917I-L920F) and additional sodium channel mutations potentially associated with kdr resistance in head and body lice, louse populations were collected from West-Azerbaijan and Zanjan Provinces of Iran. Six novel mutations were found to be located in the IIS1-2 extracellular loop (H813P) and IIS5 (I927F, L928A, R929V, L930M, and L932M) of the α-subunit. Genotyping results showed that all specimens (100%) have at least one of these or the well-known mutations. Therefore, the presence of kdr-related and novel mutations in the sodium channel is likely to be the reason for the frequent use of pyrethroid insecticides due to treatment failure against lice. Further studies are now required to evaluate the prevalence of the kdr-like mutant allele for monitoring of insecticide resistance and the management of head and body lice in other provinces of the country.

Exonic mutations in SCN9A (NaV1.7) are found in a minority of patients with erythromelalgia.

Background and aim "Gain-of-function" mutations in voltage-gated sodium channel NaV1.7 have been linked to erythromelalgia (EM), characterized by painful hot and red hands and feet. We investigated the proportion of patients with EM that carry a mutation in NaV1.7 or in other pain-related genes and studied possible clinical differences. Methods In this study, 48 patients with EM were screened for mutations in a total of 29 candidate genes, including all sodium channel subunits, transient receptor potential channels (TRPA1, TRPV1, TRPM8), neurotrophic factors (NGF, NGFR, BDNF, GDNF, NTRK1 and WNK1) and other known pain-related genes (CACNG2, KCNS1, COMT, P2RX3, TAC1, TACR1), using a combination of next generation sequencing and classical Sanger sequencing. Results In 7/48 patients protein-modifying mutations of NaV1.7 (P187L, I228M, I848T (n = 4) and N1245S) were identified. Patients with the I848T mutation could be identified clinically based on early onset and severity of the disease. In contrast, there were no clinical characteristics that differentiated the other patients with NaV1.7 mutation from those patients without. We also found more than twenty rare protein-modifying genetic variants in the genes coding for sodium channels (NaV1.8, NaV1.9, NaV1.6, NaV1.5, NaV2.1, SCN1B, SCN3B), transient receptor potential channel (TRPA1, TRPV1), and other pain-related targets (WNK1 and NGFR). Conclusion We conclude that functionally characterized mutations of NaV1.7 (I848T) are present only in a minority of patient with EM. Albeit the majority of patients (27/48) carried rare protein-modifying mutations the vast majority of those will most probably not be causally linked to their disease. Implications The key question remaining to be solved is the possible role of rare variants of NaV1.8, NaV1.9, or beta-subunits in provoking chronic pain conditions or even EM.