Pharmacological and molecular evidence for the involvement of Kv4.3 in ultra-fast activating K+ currents in murine portal vein myocytes

Castration Induces Down-Regulation of A-Type K+ Channel in Rat Vas Deferens Smooth Muscle

A-type K⁺ channels contribute to regulating the propagation and frequency of action potentials in smooth muscle cells (SMCs). The present study (i) identified the molecular components of A-type K⁺ channels in rat vas deferens SMs (VDSMs) and (ii) showed the long-term, genomic effects of testosterone on their expression in VDSMs. Transcripts of the A-type K⁺ channel α subunit, Kv4.3L and its regulatory β subunits, KChIP3, NCS1, and DPP6-S were predominantly expressed in rat VDSMs over the other related subtypes (Kv4.2, KChIP1, KChIP2, KChIP4, and DPP10). A-type K⁺ current (I_A) density in VDSM cells (VDSMCs) was decreased by castration without changes in I_A kinetics, and decreased I_A density was compensated for by an oral treatment with 17α-methyltestosterone (MET). Correspondingly, in the VDSMs of castrated rats, Kv4.3L and KChIP3 were down-regulated at both the transcript and protein expression levels. Changes in Kv4.3L and KChIP3 expression levels were compensated for by the treatment with MET. These results suggest that testosterone level changes in testosterone disorders and growth processes control the functional expression of A-type K⁺ channels in VDSMCs.

VNUT and VMAT2 segregate within sympathetic varicosities and localize near preferred Cav2 isoforms in the rat tail artery

ATP and norepinephrine (NE) are coreleased from peripheral sympathetic nerve terminals. Whether they are stored in the same vesicles has been debated for decades. Preferential dependence of NE or ATP release on Ca²⁺ influx through specific voltage-gated Ca²⁺ channel (Cav2) isoforms suggests that NE and ATP are stored in separate vesicle pools, but simultaneous imaging of NE and ATP containing vesicles within single varicosities has not been reported. We conducted an immunohistochemical study of vesicular monoamine transporter 2 (VMAT2/SLC18A2) and vesicular nucleotide translocase (VNUT/SLC17A9) as markers of vesicles containing NE and ATP in sympathetic nerves of the rat tail artery. A large fraction of varicosities exhibited neighboring, rather than overlapping, VNUT and VMAT2 fluorescent puncta. VMAT2, but not VNUT, colocalized with synaptotagmin 1. Cav2.1, Cav2.2, and Cav2.3 are expressed in nerves in the tunica adventitia. VMAT2 preferentially localized adjacent to Cav2.2 and Cav2.3 rather than Cav2.1. VNUT preferentially localized adjacent to Cav2.3 > Cav2.2 >> Cav2.1. With the use of wire myography, inhibition of field-stimulated vasoconstriction with the Cav2.3 blocker SNX-482 (0.25 µM) mimicked the effects of the P2X inhibitor suramin (100 µM) rather than the α-adrenergic inhibitor phentolamine (10 µM). Variable sensitivity to SNX-482 and suramin between animals closely correlated with Cav2.3 staining. We concluded that a majority of ATP and NE stores localize to separate vesicle pools that use different synaptotagmin isoforms and that localize near different Cav2 isoforms to mediate vesicle release. Cav2.3 appears to play a previously unrecognized role in mediating ATP release in the rat tail artery. NEW & NOTEWORTHY Immunofluorescence imaging of vesicular nucleotide translocase and vesicular monoamine transporter 2 in rat tail arteries revealed that ATP and norepinephrine, classical cotransmitters, localize to well-segregated vesicle pools. Furthermore, vesicular nucleotide translocase and vesicular monoamine transporter 2 exhibit preferential localization with specific Cav2 isoforms. These novel observations address long-standing debates regarding the mechanism(s) of sympathetic neurotransmitter corelease.

Unravelling the complexities of vascular smooth muscle ion channels: Fine tuning of activity by ancillary subunits

Which ion channel is the most important for regulating vascular tone? Which one is responsible for controlling the resting membrane potential or repolarization? Which channels are recruited by different intracellular signalling pathways or change in certain vascular diseases? Many different ion channels have been identified in the vasculature over the years and claimed as future therapeutic targets. Unfortunately, several of these ion channels are not just found in the vasculature, with many of them also found to have prominent functional roles in different organs of the body, which then leads to off-target effects. As cardiovascular diseases are expected to increase worldwide to epidemic proportions, ion channel research and the hunt for the next major therapeutic target to treat different vascular diseases has never been more important. However, I believe that the question we should now be asking is: which ancillary subunits are involved in regulating specific ion channels in the vasculature and do they have the potential to be new therapeutic targets?

Hypoxia sensitivity of a voltage-gated potassium current in porcine intrapulmonary vein smooth muscle cells

Hypoxia contracts the pulmonary vein, but the underlying cellular effectors remain unclear. Utilizing contractile studies and whole cell patch-clamp electrophysiology, we report for the first time a hypoxia-sensitive K(+) current in porcine pulmonary vein smooth muscle cells (PVSMC). Hypoxia induced a transient contractile response that was 56 ± 7% of the control response (80 mM KCl). This contraction required extracellular Ca(2+) and was sensitive to Ca(2+) channel blockade. Blockade of K(+) channels by tetraethylammonium chloride (TEA) or 4-aminopyridine (4-AP) reversibly inhibited the hypoxia-mediated contraction. Single-isolated PVSMC (typically 159.1 ± 2.3 μm long) had mean resting membrane potentials (RMP) of -36 ± 4 mV with a mean membrane capacitance of 108 ± 3.5 pF. Whole cell patch-clamp recordings identified a rapidly activating, partially inactivating K(+) current (I(KH)) that was hypoxia, TEA, and 4-AP sensitive. I(KH) was insensitive to Penitrem A or glyburide in PVSMC and had a time to peak of 14.4 ± 3.3 ms and recovered in 67 ms following inactivation at +80 mV. Peak window current was -32 mV, suggesting that I(KH) may contribute to PVSMC RMP. The molecular identity of the potassium channel is not clear. However, RT-PCR, using porcine pulmonary artery and vein samples, identified Kv(1.5), Kv(2.1), and BK, with all three being more abundant in the PV. Both artery and vein expressed STREX, a highly conserved and hypoxia-sensitive BK channel variant. Taken together, our data support the hypothesis that hypoxic inhibition of I(KH) would contribute to hypoxic-induced contraction in PVSMC.

New tricks for old dogs: KCNQ expression and role in smooth muscle

Ion channels encoded by the KCNQ gene family (K(v)7.1-7.5) are major determinants of neuronal membrane potential and the cardiac action potential. This key physiological role is highlighted by the existence of a number of hereditary disorders caused by mutations to KCNQ genes. Recently, KCNQ gene expression has been identified in vascular and non-vascular smooth muscles. In addition, experiments with an array of pharmacological modulators of KCNQ channels have supported a crucial role for these channels in regulating smooth muscle contractility. This article will provide an overview of present understanding in this nascent area of KCNQ research and will offer guidance as to future directions.

Loss of functional K+ channels encoded by ether-à-go-go-related genes in mouse myometrium prior to labour onset

There is a growing appreciation that ion channels encoded by the ether-à-go-go-related gene family have a functional impact in smooth muscle in addition to their accepted role in cardiac myocytes and neurones. This study aimed to assess the expression of ERG1-3 (KCNH1-3) genes in the murine myometrium (smooth muscle layer of the uterus) and determine the functional impact of the ion channels encoded by these genes in pregnant and non-pregnant animals. Quantitative RT-PCR did not detect message for ERG2 and 3 in whole myometrial tissue extracts. In contrast, message for two isoforms of mERG1 were readily detected with mERG1a more abundant than mERG1b. In isometric tension studies of non-pregnant myometrium, the ERG channel blockers dofetilide (1 microM), E4031 (1 microM) and Be-KM1 (100 nM) increased spontaneous contractility and ERG activators (PD118057 and NS1643) inhibited spontaneous contractility. In contrast, neither ERG blockade nor activation had any effect on the inherent contractility in myometrium from late pregnant (19 days gestation) animals. Moreover, dofetilide-sensitive K(+) currents with distinctive 'hooked' kinetics were considerably smaller in uterine myocytes from late pregnant compared to non-pregnant animals. Expression of mERG1 isoforms did not alter throughout gestation or upon delivery, but the expression of genes encoding auxillary subunits (KCNE) were up-regulated considerably. This study provides the first evidence for a regulation of ERG-encoded K(+) channels as a precursor to late pregnancy physiological activity.