Role of Kv7 channels in responses of the pulmonary circulation to hypoxia.

Hypoxic pulmonary vasoconstriction (HPV) is a beneficial mechanism that diverts blood from hypoxic alveoli to better ventilated areas of the lung, but breathing hypoxic air causes the pulmonary circulation to become hypertensive. Responses to airway hypoxia are associated with depolarization of smooth muscle cells in the pulmonary arteries and reduced activity of K(+) channels. As Kv7 channels have been proposed to play a key role in regulating the smooth muscle membrane potential, we investigated their involvement in the development of HPV and hypoxia-induced pulmonary hypertension. Vascular effects of the selective Kv7 blocker, linopirdine, and Kv7 activator, flupirtine, were investigated in isolated, saline-perfused lungs from rats maintained for 3-5 days in an isobaric hypoxic chamber (FiO2 = 0.1) or room air. Linopirdine increased vascular resistance in lungs from normoxic, but not hypoxic rats. This effect was associated with reduced mRNA expression of the Kv7.4 channel α-subunit in hypoxic arteries, whereas Kv7.1 and Kv7.5 were unaffected. Flupirtine had no effect in normoxic lungs but reduced vascular resistance in hypoxic lungs. Moreover, oral dosing with flupirtine (30 mg/kg/day) prevented short-term in vivo hypoxia from increasing pulmonary vascular resistance and sensitizing the arteries to acute hypoxia. These findings suggest a protective role for Kv7.4 channels in the pulmonary circulation, limiting its reactivity to pressor agents and preventing hypoxia-induced pulmonary hypertension. They also provide further support for the therapeutic potential of Kv7 activators in pulmonary vascular disease.

Amplified NO/cGMP-mediated relaxation and ryanodine receptor-to-BKCa channel signalling in corpus cavernosum smooth muscle from phospholamban knockout mice.

BACKGROUND AND PURPOSE: Relaxation of corpus cavernosum smooth muscle (CCSM) is induced by NO. NO promotes the formation of cGMP, which activates cGMP-dependent protein kinase I (PKGI). The large conductance calcium-activated potassium (BK(Ca) ) channel is regarded as a major target of NO/cGMP signalling; however, the mechanism of BK(Ca) activation remains unclear. The aim of the present study was to determine whether sarcoplasmic reticulum (SR) Ca(2+) load and Ca(2+) release from the SR via ryanodine receptors (RyRs) is important for BK(Ca) channel activation in response to NO/cGMP. EXPERIMENTAL APPROACH: In vitro myography was performed on CCSM strips from wild-type and PLB knockout (PLB(-/-)) mice to evaluate contraction and relaxation in response to pharmacological agents and electrical field stimulation (EFS). KEY RESULTS: In CCSM strips from PLB(-/-) mice, a model of increased SR Ca(2+) load, contractile force in response to EFS or phenylephrine (PE) was increased by nearly 100%. EFS of strips precontracted with PE induced transient relaxation in CCSM, an effect that was significantly larger in PLB(-/-) strips. Likewise, the relaxation of PE-induced contraction in response to SNP and cGMP was greater in PLB(-/-) , as demonstrated by a shift in the concentration-response curve towards lower concentrations. Blocking RyRs and BK(Ca) channels diminished the induced relaxations and eliminated the difference between wild-type and PLB(-/-). CONCLUSIONS AND IMPLICATIONS: NO/cGMP activates BK(Ca) channels through RyR-mediated Ca(2+) release. This signalling pathway is responsible for approximately 40% of the NO/cGMP effects and is amplified by increased SR Ca(2+) concentrations.

KCNQ potassium channels: new targets for pulmonary vasodilator drugs?

Smooth muscle cells regulate the diameter of pulmonary arteries and the resistance to blood flow in the pulmonary circulation. These cells are normally relaxed to maintain low intrinsic vessel tone, but are contracted in pulmonary arterial hypertension (PAH). Potassium channels in the smooth muscle cell help to maintain low tone by polarising the membrane and preventing Ca(2+) influx through voltage-operated Ca(2+) channels. There is a loss of K(+) channel activity in PAH, so drugs that open K(+) channels are predicted to have a beneficial effect, provided their action can be restricted to the pulmonary circulation. Here we review the myriad of K(+) channels that are expressed in pulmonary arteries and suggest the roles that each might play in regulating pulmonary artery tone. We conclude that members of the KCNQ family of K(+) channels, the most recent K(+) channels to be discovered in pulmonary artery, may be a useful therapeutic target for the treatment of PAH. KCNQ channels appear to be preferentially expressed in pulmonary arteries and drugs that modulate their activity have potent effects on pulmonary artery tone.

KCNQ modulators reveal a key role for KCNQ potassium channels in regulating the tone of rat pulmonary artery smooth muscle.

Potassium channels are central to the regulation of pulmonary vascular tone. The smooth muscle cells of pulmonary artery display a background K(+) conductance with biophysical properties resembling those of KCNQ (K(V)7) potassium channels. Therefore, we investigated the expression and functional role of KCNQ channels in pulmonary artery. The effects of selective KCNQ channel modulators were investigated on K(+) current and membrane potential in isolated pulmonary artery smooth muscle cells (PASMCs), on the tension developed by intact pulmonary arteries, and on pulmonary arterial pressure in isolated perfused lungs and in vivo. The KCNQ channel blockers, linopirdine and XE991 [10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone], inhibited the noninactivating background K(+) conductance in PASMCs and caused depolarization, vasoconstriction, and raised pulmonary arterial pressure without constricting several systemic arteries or raising systemic pressure. The KCNQ channel openers, retigabine and flupirtine, had the opposite effects. PASMCs were found to express KCNQ4 mRNA, at higher levels than mesenteric artery, along with smaller amounts of KCNQ1 and 5. It is concluded that KCNQ channels, most probably KCNQ4, make an important contribution to the regulation of pulmonary vascular tone, with a greater contribution in pulmonary compared with systemic vessels. The pulmonary vasoconstrictor effect of KCNQ blockers is a potentially serious side effect, but the pulmonary vasodilator effect of the openers may be useful in the treatment of pulmonary hypertension.

The role of twin pore domain and other K+ channels in hypoxic pulmonary vasoconstriction.

Hypoxic pulmonary vasoconstriction (HPV) describes the vasoconstrictor response of pulmonary arteries to hypoxia, which directs blood flow towards better ventilated areas of the lung. Exactly how pulmonary arteries sense oxygen and mediate this response is widely debated and several hypotheses have emerged. One has smooth muscle K+ channels as the primary O2 sensor, hypoxia causing K+ channel inhibition, membrane depolarization and voltage-dependent Ca2+ influx. Even if this mechanism is not the primary response of pulmonary arteries to hypoxia, inhibition of K+ channel activity probably plays a role in HPV, due to enhanced membrane excitability and Ca2+ influx. Hypoxia inhibits several different K+ channels expressed in pulmonary artery smooth muscle, most from the K(v) class of voltage-gated K+ channels, but the properties of many K(v) channels are incompatible with a role in initiating HPV. Twin-pore domain K+ channels have emerged as prime candidates for controlling the resting membrane potential of cells. The identification of the twin-pore channel, TASK, in pulmonary artery smooth muscle, along with reports that it is inhibited by hypoxia, raises the possibility that a member of this family of channels acts as an O2 sensor in pulmonary artery. An unidentified low-threshold, voltage-dependent K+ channel might also contribute.

Pulmonary vasoconstrictor action of KCNQ potassium channel blockers.

BACKGROUND: KCNQ channels have been widely studied in the nervous system, heart and inner ear, where they have important physiological functions. Recent reports indicate that KCNQ channels may also be expressed in portal vein where they are suggested to influence spontaneous contractile activity. The biophysical properties of K+ currents mediated by KCNQ channels resemble a current underlying the resting K+ conductance and resting potential of pulmonary artery smooth muscle cells. We therefore investigated a possible role of KCNQ channels in regulating the function of pulmonary arteries by determining the ability of the selective KCNQ channel blockers, linopirdine and XE991, to promote pulmonary vasoconstriction. METHODS: The tension developed by rat and mouse intrapulmonary or mesenteric arteries was measured using small vessel myography. Contractile responses to linopirdine and XE991 were measured in intact and endothelium denuded vessels. Experiments were also carried out under conditions that prevent the contractile effects of nerve released noradrenaline or ATP, or block various Ca2+ influx pathways, in order to investigate the mechanisms underlying contraction. RESULTS: Linopirdine and XE991 both contracted rat and mouse pulmonary arteries but had little effect on mesenteric arteries. In each case the maximum contraction was almost as large as the response to 50 mM K+. Linopirdine had an EC50 of around 1 microM and XE991 was almost 10-fold more potent. Neither removal of the endothelium nor exposure to phentolamine or alpha,beta-methylene ATP, to block alpha1-adrenoceptors or P2X receptors, respectively, affected the contraction. Contraction was abolished in Ca2+-free solution and in the presence of 1 microM nifedipine or 10 microM levcromakalim. CONCLUSION: The KCNQ channel blockers are potent and powerful constrictors of pulmonary arteries. This action may be selective for the pulmonary circulation as mesenteric arteries showed little response. The results imply that the drugs act directly on smooth muscle cells and contraction requires voltage-dependent Ca2+ influx. It is concluded that the drugs probably act by blocking KCNQ channels in pulmonary artery myocytes, leading to membrane depolarization and Ca2+ influx through L-type Ca2+ channels. This implies a functional role for KCNQ channels in regulating the resting membrane potential of pulmonary artery myocytes.