Delayed rectifier K+ current of dog bronchial myocytes: effect of pollen sensitization and PKC activation

KCNQ (Kv7) potassium channel activators as bronchodilators: combination with a β2-adrenergic agonist enhances relaxation of rat airways

KCNQ (Kv7 family) potassium (K(+)) channels were recently found in airway smooth muscle cells (ASMCs) from rodent and human bronchioles. In the present study, we evaluated expression of KCNQ channels and their role in constriction/relaxation of rat airways. Real-time RT-PCR analysis revealed expression of KCNQ4 > KCNQ5 > KCNQ1 > KCNQ2 > KCNQ3, and patch-clamp electrophysiology detected KCNQ currents in rat ASMCs. In precision-cut lung slices, the KCNQ channel activator retigabine induced a concentration-dependent relaxation of small bronchioles preconstricted with methacholine (MeCh; EC50 = 3.6 ± 0.3 μM). Bronchoconstriction was also attenuated in the presence of two other structurally unrelated KCNQ channel activators: zinc pyrithione (ZnPyr; 1 μM; 22 ± 7%) and 2,5-dimethylcelecoxib (10 μM; 24 ± 8%). The same three KCNQ channel activators increased KCNQ currents in ASMCs by two- to threefold. The bronchorelaxant effects of retigabine and ZnPyr were prevented by inclusion of the KCNQ channel blocker XE991. A long-acting β2-adrenergic receptor agonist, formoterol (10 nM), did not increase KCNQ current amplitude in ASMCs, but formoterol (1-1,000 nM) did induce a time- and concentration-dependent relaxation of rat airways, with a notable desensitization during a 30-min treatment or with repetitive treatments. Coadministration of retigabine (10 μM) with formoterol produced a greater peak and sustained reduction of MeCh-induced bronchoconstriction and reduced the apparent desensitization observed with formoterol alone. Our findings support a role for KCNQ K(+) channels in the regulation of airway diameter. A combination of a β2-adrenergic receptor agonist with a KCNQ channel activator may improve bronchodilator therapy.

The effects of protein kinase C (PKC) on the tension of normal and passively sensitized human airway smooth muscle and the activity of voltage-dependent delayed rectifier potassium channel (Kv)

The effects of protein kinase C (PKC) on the tension and the activity of voltage-dependent delayed rectifier potassium channel (K(y)) were examined in normal and passively sensitized human airway smooth muscle (HASM), by measuring tones and whole-cell patch clamp techniques, and the K(v) activities and membrane potential (E (m)) were also detected. The results showed that phorbol 12-myristate 13-acetate (PMA), a PKC activator, caused a concentration-dependent constriction in normal HASM rings. The constriction of the passively sensitized muscle in asthma serum group was significantly higher than that of the normal group (P<0.05), and the constrictions of both groups were completely abolished by PKC inhibitor Ro31-8220 and calcium channel inhibitor nifedipine. K(v) activities of HASM cells were significantly inhibited by PMA, and the E (m) became more positive, as compared with the DMSO (a PMA menstruum)-treated group (P<0.01). This effect could be blocked by Ro31-8220 (P<0.01). It was concluded that activation of PKC could increase the tones of HASM, which might be related to the reduced K(v) activity. In passively sensitized HASM rings, this effect was more notable.

Bradykinin activates calcium-dependent potassium channels in cultured human airway smooth muscle cells

Bradykinin (BK) is an inflammatory mediator that can cause bronchoconstriction. In this study, we investigated the membrane currents induced by BK in cultured human airway smooth muscle (ASM) cells. Depolarization of the cells induced outward currents, which were inhibited by tetraethylammonium (TEA) in a concentration-dependent manner with an IC50 of 0.33 microM. The currents were increased by elevating intracellular free Ca2+ concentration, suggesting they are calcium-activated potassium channels [I(K(Ca))]. Preexposure to inhibitor of I(K(Ca)) of large conductance (BKCa), iberiotoxin, and small conductance (SKCa), apamin, inhibited the increase of outward current induced by BK. The relative contribution of BKCa was greatest in early passage cells. Both nickel and SKF-96365 (10 microM) inhibited the increase of the I(K(Ca)) induced by BK; however, the l-type Ca2+ channel blocker, nifedipine, had no effect. Activation of the BK-induced current was inhibited by heparin, indicating dependence on intact inositol 1,4,5-triphosphate (IP3)-sensitive intracellular Ca2+ stores. BK also increased inositol phosphate accumulation and induced a transient Ca2+-activated chloride current (CACC) and a sustained nonselective cation current (I(CAT)). In summary, BK activates BKCa, SKCa, CACC, and I(CAT) via IP3-sensitive stores in human ASM.

Properties and molecular basis of the mouse urinary bladder voltage-gated K+ current

Potassium channels play an important role in controlling the excitability of urinary bladder smooth muscle (UBSM). Here we describe the biophysical, pharmacological and molecular properties of the mouse UBSM voltage-gated K+ current (IK(V)). The IK(V) activated, deactivated and inactivated slowly with time constants of 29.9 ms at +30 mV, 131 ms at -40 mV and 3.4 s at +20 mV. The midpoints of steady-state activation and inactivation curves were 1.1 mV and -61.4 mV, respectively. These properties suggest that IK(V) plays a role in regulating the resting membrane potential and contributes to the repolarization and after-hyperpolarization phases of action potentials. The IK(V) was blocked by tetraethylammonium ions with an IC50 of 5.2 mM and was unaffected by 1 mM 4-aminopyridine. RT-PCR for voltage-gated K+ channel (KV) subunits revealed the expression of Kv2.1, Kv5.1, Kv6.1, Kv6.2 and Kv6.3 in isolated UBSM myocytes. A comparison of the biophysical properties of UBSM IK(V) with those reported for Kv2.1 and Kv5.1 and/or Kv6 heteromultimeric channels demonstrated a marked similarity. We propose that heteromultimeric channel complexes composed of Kv2.1 and Kv5.1 and/or Kv6 subunits form the molecular basis of the mouse UBSM IK(V).

Endogenous Kv channels in human embryonic kidney (HEK-293) cells

The human embryonic kidney cells (HEK-293) have been widely used as one mammalian expression system in the study of voltage-gated K+ (Kv) channels. Understanding the endogenous Kv channels in these cells is the prerequisite for the characterization of the heterogeneously expressed Kv channels in these cells. In the present study we screened the transcriptional expression of different Kv genes in HEK-293 cells using reverse transcribed DNApolymerase chain reaction (RT-PCR) method. Among 16 Kv genes examined in native HEK-293 cells 10 Kv genes were reproducibly amplified, including those Kv a subunits encoding for the delayed rectifier (IK) [Kv1.1, Kv1.2, Kv1.3, Kv1.6, and Kv3.1], and for the transient outward Kv channels (IA) [Kv1.4, Kv3.3, Kv3.4, and Kv4.1] as well as a Kvbeta2 subunit. The whole-cell outward rectifier IK currents in the native HEK-293 cells were recorded (203 +/- 13 pA at +30 mV, n = 82) with the patch-clamp technique. In about 42% of the examined cells, IA coexisted with IK currents. IK currents were inhibited by tetraethylammonium chloride (TEA) at 1 and 10 mM by 39.5 and 48.4%, respectively. A 39.6% inhibition of IK currents was also observed in the presence of4-aminopyridine (4-AP, 5 mM). Interestingly, both TEAand 4-AP also inhibited IA currents. 4-acelamido-4'-isothiocyanalostilbene-2, 2'-disulfonic acid (1 mM), a Cl- channel blocker, had no effect on the endogenous outward currents. We concluded that multiple endogenous Kv genes were expressed in native HEK-293 cells, which possessed significant endogenous IK and IA currents with unique pharmacological properties.

Ionic mechanisms and Ca(2+) regulation in airway smooth muscle contraction: do the data contradict dogma?

In general, excitation-contraction coupling in muscle is dependent on membrane depolarization and hyperpolarization to regulate the opening of voltage-dependent Ca(2+) channels and, thereby, influence intracellular Ca(2+) concentration ([Ca(2+)](i)). Thus Ca(2+) channel blockers and K(+) channel openers are important tools in the arsenals against hypertension, stroke, and myocardial infarction, etc. Airway smooth muscle (ASM) also exhibits robust Ca(2+), K(+), and Cl(-) currents, and there are elaborate signaling pathways that regulate them. It is easy, then, to presume that these also play a central role in contraction/relaxation of ASM. However, several lines of evidence speak to the contrary. Also, too many researchers in the ASM field view the sarcoplasmic reticulum as being centrally located and displacing its contents uniformly throughout the cell, and they have focused almost exclusively on the initial single [Ca(2+)] spike evoked by excitatory agonists. Several recent studies have revealed complex spatial and temporal heterogeneity in [Ca(2+)](i), the significance of which is only just beginning to be appreciated. In this review, we will compare what is known about ion channels in ASM with what is believed to be their roles in ASM physiology. Also, we will examine some novel ionic mechanisms in the context of Ca(2+) handling and excitation-contraction coupling in ASM.