Voltage-dependent K+ currents in smooth muscle cells from mouse gallbladder

Functional ion channels in human pulmonary artery smooth muscle cells: Voltage-dependent cation channels

The activity of voltage-gated ion channels is critical for the maintenance of cellular membrane potential and generation of action potentials. In turn, membrane potential regulates cellular ion homeostasis, triggering the opening and closing of ion channels in the plasma membrane and, thus, enabling ion transport across the membrane. Such transmembrane ion fluxes are important for excitation–contraction coupling in pulmonary artery smooth muscle cells (PASMC). Families of voltage-dependent cation channels known to be present in PASMC include voltage-gated K⁺ (Kv) channels, voltage-dependent Ca²⁺-activated K⁺ (Kca) channels, L- and T- type voltage-dependent Ca²⁺ channels, voltage-gated Na⁺ channels and voltage-gated proton channels. When cells are dialyzed with Ca²⁺-free K⁺- solutions, depolarization elicits four components of 4-aminopyridine (4-AP)-sensitive Kvcurrents based on the kinetics of current activation and inactivation. In cell-attached membrane patches, depolarization elicits a wide range of single-channel K⁺ currents, with conductances ranging between 6 and 290 pS. Macroscopic 4-AP-sensitive Kv currents and iberiotoxin-sensitive Kca currents are also observed. Transcripts of (a) two Na⁺ channel α-subunit genes (SCN5A and SCN6A), (b) six Ca²⁺ channel α–subunit genes (α_1A, α_1B, α_1X, α_1D, α_1Eand α_1G) and many regulatory subunits (α₂δ₁, β_1-4, and γ₆), (c) 22 Kv channel α–subunit genes (Kv1.1 - Kv1.7, Kv1.10, Kv2.1, Kv3.1, Kv3.3, Kv3.4, Kv4.1, Kv4.2, Kv5.1, Kv 6.1-Kv6.3, Kv9.1, Kv9.3, Kv10.1 and Kv11.1) and three Kv channel β-subunit genes (Kvβ1-3) and (d) four Kca channel α–subunit genes (Sloα1 and SK2-SK4) and four Kca channel β-subunit genes (Kcaβ1-4) have been detected in PASMC. Tetrodotoxin-sensitive and rapidly inactivating Na⁺ currents have been recorded with properties similar to those in cardiac myocytes. In the presence of 20 mM external Ca²⁺, membrane depolarization from a holding potential of -100 mV elicits a rapidly inactivating T-type Ca²⁺ current, while depolarization from a holding potential of -70 mV elicits a slowly inactivating dihydropyridine-sensitive L-type Ca²⁺ current. This review will focus on describing the electrophysiological properties and molecular identities of these voltage-dependent cation channels in PASMC and their contribution to the regulation of pulmonary vascular function and its potential role in the pathogenesis of pulmonary vascular disease.

Role of mitochondria in spontaneous rhythmic activity and intracellular calcium waves in the guinea pig gallbladder smooth muscle

Mitochondrial Ca(2+) handling has been implicated in spontaneous rhythmic activity in smooth muscle and interstitial cells of Cajal. In this investigation we evaluated the effect of mitochondrial inhibitors on spontaneous action potentials (APs), Ca(2+) flashes, and Ca(2+) waves in gallbladder smooth muscle (GBSM). Disruption of the mitochondrial membrane potential with carbonyl cyanide 3-chlorophenylhydrazone, carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone, rotenone, and antimycin A significantly reduced or eliminated APs, Ca(2+) flashes, and Ca(2+) waves in GBSM. Blockade of ATP production with oligomycin did not alter APs or Ca(2+) flashes but significantly reduced Ca(2+) wave frequency. Inhibition of mitochondrial Ca(2+) uptake and Ca(2+) release with Ru360 and CGP-37157, respectively, reduced the frequency of Ca(2+) flashes and Ca(2+) waves in GBSM. Similar to oligomycin, cyclosporin A did not alter AP and Ca(2+) flash frequency but significantly reduced Ca(2+) wave activity. These data suggest that mitochondrial Ca(2+) handling is necessary for the generation of spontaneous electrical activity and may therefore play an important role in gallbladder tone and motility.

Functional characterization of voltage-gated K+ channels in mouse pulmonary artery smooth muscle cells

Mice are useful animal models to study pathogenic mechanisms involved in pulmonary vascular disease. Altered expression and function of voltage-gated K(+) (K(V)) channels in pulmonary artery smooth muscle cells (PASMCs) have been implicated in the development of pulmonary arterial hypertension. K(V) currents (I(K(V))) in mouse PASMCs have not been comprehensively characterized. The main focus of this study was to determine the biophysical and pharmacological properties of I(K(V)) in freshly dissociated mouse PASMCs with the patch-clamp technique. Three distinct whole cell I(K(V)) were identified based on the kinetics of activation and inactivation: rapidly activating and noninactivating currents (in 58% of the cells tested), rapidly activating and slowly inactivating currents (23%), and slowly activating and noninactivating currents (17%). Of the cells that demonstrated the rapidly activating noninactivating current, 69% showed I(K(V)) inhibition with 4-aminopyridine (4-AP), while 31% were unaffected. Whole cell I(K(V)) were very sensitive to tetraethylammonium (TEA), as 1 mM TEA decreased the current amplitude by 32% while it took 10 mM 4-AP to decrease I(K(V)) by a similar amount (37%). Contribution of Ca(2+)-activated K(+) (K(Ca)) channels to whole cell I(K(V)) was minimal, as neither pharmacological inhibition with charybdotoxin or iberiotoxin nor perfusion with Ca(2+)-free solution had an effect on the whole cell I(K(V)). Steady-state activation and inactivation curves revealed a window K(+) current between -40 and -10 mV with a peak at -31.5 mV. Single-channel recordings revealed large-, intermediate-, and small-amplitude currents, with an averaged slope conductance of 119.4 +/- 2.7, 79.8 +/- 2.8, 46.0 +/- 2.2, and 23.6 +/- 0.6 pS, respectively. These studies provide detailed electrophysiological and pharmacological profiles of the native K(V) currents in mouse PASMCs.

Identification of a spontaneously active, Na+-permeable channel in guinea pig gallbladder smooth muscle

The action potential in gallbladder smooth muscle (GBSM) is caused by Ca2+ entry through voltage-dependent Ca2+ channels (VDCC), which contributes to the GBSM contractions. Action potential generation in GBSM is critically dependent on the resting membrane potential (about -50 mV), which is approximately 35 mV more positive of the K+ equilibrium potential. We hypothesized that a tonic, depolarizing conductance is present in GBSM and contributes to the regulation of the resting membrane potential and action potential frequency. GBSM cells were isolated from guinea pig gallbladders, and the whole cell patch-camp technique was used to record membrane currents. After eliminating the contribution of VDCC and K+ channels, we identified a novel spontaneously active cation conductance (I(cat)) in GBSM. This I(cat) was mediated predominantly by influx of Na+. Na+ substitution with N-methyl-D-glucamine (NMDG), a large relatively impermeant cation, caused a negative shift in the reversal potential of the ramp current and reduced the amplitude of the inward current at -50 mV by 65%. Membrane potential recordings with intracellular microelectrodes or in current-clamp mode of the patch-clamp technique indicated that the inhibition of I(cat) conductance by NMDG is associated with membrane hyperpolarization and inhibition of action potentials. Extracellular Ca2+, Mg2+, and Gd3+ attenuated the I(cat) in GBSM. Muscarinic stimulation did not activate the I(cat). Our results indicate that, in GBSM, an Na+-permeable channel contributes to the maintenance of the resting membrane potential and action potential generation and therefore plays a critical role in the regulation of GBSM excitability and contractility.

Diversity of voltage-dependent K+ channels in human pulmonary artery smooth muscle cells

Electrical excitability, which plays an important role in excitation-contraction coupling in the pulmonary vasculature, is regulated by transmembrane ion flux in pulmonary artery smooth muscle cells (PASMC). This study examined the heterogeneous nature of native voltage-dependent K(+) channels in human PASMC. Both voltage-gated K(+) (K(V)) currents and Ca(2+)-activated K(+) (K(Ca)) currents were observed and characterized. In cell-attached patches of PASMC bathed in Ca(2+)-containing solutions, depolarization elicited a wide range of K(+) unitary conductances (6-290 pS). When cells were dialyzed with Ca(2+)-free and K(+)-containing solutions, depolarization elicited four components of K(V) currents in PASMC based on the kinetics of current activation and inactivation. Using RT-PCR, we detected transcripts of 1) 22 K(V) channel alpha-subunits (K(V)1.1-1.7, K(V)1.10, K(V)2.1, K(V)3.1, K(V)3.3-3.4, K(V)4.1-4.2, K(V)5.1, K(V) 6.1-6.3, K(V)9.1, K(V)9.3, K(V)10.1, and K(V)11.1), 2) three K(V) channel beta-subunits (K(V)beta 1-3), 3) four K(Ca) channel alpha-subunits (Slo-alpha 1 and SK2-SK4), and 4) four K(Ca) channel beta-subunits (K(Ca)beta 1-4). Our results show that human PASMC exhibit a variety of voltage-dependent K(+) currents with variable kinetics and conductances, which may result from various unique combinations of alpha- and beta-subunits forming the native channels. Functional expression of these channels plays a critical role in the regulation of membrane potential, cytoplasmic Ca(2+), and pulmonary vasomotor tone.

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).

Differential regulation of SK and BK channels by Ca(2+) signals from Ca(2+) channels and ryanodine receptors in guinea-pig urinary bladder myocytes

Small-conductance (SK) and large-conductance (BK) Ca(2+)-activated K(+) channels are key regulators of excitability in urinary bladder smooth muscle (UBSM) of guinea-pig. The overall goal of this study was to define how SK and BK channels respond to Ca(2+) signals from voltage-dependent Ca(2+) channels (VDCCs) in the surface membrane and from ryanodine-sensitive Ca(2+) release channels or ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR) membrane. To characterize the role of SK channels in UBSM, the effects of the SK channel blocker apamin on phasic contractions were examined. Apamin caused a dose-dependent increase in the amplitude of phasic contractions over a broad concentration range (10(-10) to 10(-6) M). To determine the effects of Ca(2+) signals from VDCCs and RyRs to SK and BK channels, whole cell membrane current was measured in isolated myocytes bathed in physiological solutions. Depolarization (-70 to +10 mV for 100 ms) of isolated myocytes caused an inward Ca(2+) current (I(Ca)), followed by an outward current. The outward current was reduced in a dose-dependent manner by apamin (10(-10) to 10(-6) M), and designated I(SK). I(SK) had a mean amplitude of 53.8 +/- 6.1 pA or approximately 1.4 pA pF(-1) at +10 mV. The amplitude of I(SK) correlated with the peak I(Ca). Blocking I(Ca) abolished I(SK). In contrast, I(SK) was insensitive to the RyR blocker ryanodine (10 microM). These data indicate that Ca(2+) signals from VDCCs, but not from RyRs, activate SK channels. BK channel currents (I(BK)) were isolated from other currents by using the BK channel blockers tetraethylammonium ions (TEA(+); 1 mM) or iberiotoxin (200 nM). Voltage steps evoked transient and steady-state I(BK) components. Transient BK currents have previously been shown to result from BK channel activation by local Ca(2+) release through RyRs ('Ca(2+) sparks'). Transient BK currents were inhibited by ryanodine (10 microM), as expected, and had a mean amplitude of 152.6 pA at +10 mV. The mean number of transient BK currents during a voltage step (range 0 to 3) correlated with I(Ca). There was a long delay (52.4 +/- 2.7 ms) between activation of I(Ca) and the first transient BK current. In contrast, ryanodine did not affect the steady-state BK current (mean amplitude 135.4 pA) during the voltage step. The steady-state BK current was reduced 95 % by inhibition of VDCCs, suggesting that this process depends largely on Ca(2+) entry through VDCCs and not Ca(2+) release through RyRs. These results indicate that Ca(2+) entry through VDCCs activates both BK and SK channels, but Ca(2+) release (Ca(2+) sparks) through RyRs activates only BK channels.

Characterization of outward K(+) currents in isolated smooth muscle cells from sheep urethra

The perforated-patch technique was used to measure membrane currents in smooth muscle cells from sheep urethra. Depolarizing pulses evoked large transient outward currents and several components of sustained current. The transient current and a component of sustained current were blocked by iberiotoxin, penitrem A, and nifedipine but were unaffected by apamin or 4-aminopyridine, suggesting that they were mediated by large-conductance Ca(2+)-activated K(+) (BK) channels. When the BK current was blocked by exposure to penitrem A (100 nM) and Ca(2+)-free bath solution, there remained a voltage-sensitive K(+) current that was moderately sensitive to blockade with tetraethylammonium (TEA; half-maximal effective dose = 3.0 +/- 0.8 mM) but not 4-aminopyridine. Penitrem A (100 nM) increased the spike amplitude and plateau potential in slow waves evoked in single cells, whereas addition of TEA (10 mM) further increased the plateau potential and duration. In conclusion, both Ca(2+)-activated and voltage-dependent K(+) currents were found in urethral myocytes. Both of these currents are capable of contributing to the slow wave in these cells, suggesting that they are likely to influence urethral tone under certain conditions.