Resting potentials and potassium currents during development of pulmonary artery smooth muscle cells

Role of ryanodine receptor 2 and FK506-binding protein 12.6 dissociation in pulmonary hypertension

Pulmonary hypertension (PH) is a devastating disease characterized by a progressive increase in pulmonary arterial pressure leading to right ventricular failure and death. A major cellular response in this disease is the contraction of smooth muscle cells (SMCs) of the pulmonary vasculature. Cell contraction is determined by the increase in intracellular Ca2+ concentration ([Ca2+]i), which is generated and regulated by various ion channels. Several studies by us and others have shown that ryanodine receptor 2 (RyR2), a Ca2+-releasing channel in the sarcoplasmic reticulum (SR), is an essential ion channel for the control of [Ca2+]i in pulmonary artery SMCs (PASMCs), thereby mediating the sustained vasoconstriction seen in PH. FK506-binding protein 12.6 (FKBP12.6) strongly associates with RyR2 to stabilize its functional activity. FKBP12.6 can be dissociated from RyR2 by a hypoxic stimulus to increase channel function and Ca2+ release, leading to pulmonary vasoconstriction and PH. More specifically, dissociation of the RyR2-FKBP12.6 complex is a consequence of increased mitochondrial ROS generation mediated by the Rieske iron-sulfur protein (RISP) at the mitochondrial complex III after hypoxia. Overall, RyR2/FKBP12.6 dissociation and the corresponding signaling pathway may be an important factor in the development of PH. Novel drugs and biologics targeting RyR2, FKBP12.6, and related molecules may become unique effective therapeutics for PH.

AMPK deficiency in smooth muscles causes persistent pulmonary hypertension of the new-born and premature death

AMPK has been reported to facilitate hypoxic pulmonary vasoconstriction but, paradoxically, its deficiency precipitates pulmonary hypertension. Here we show that AMPK-α1/α2 deficiency in smooth muscles promotes persistent pulmonary hypertension of the new-born. Accordingly, dual AMPK-α1/α2 deletion in smooth muscles causes premature death of mice after birth, associated with increased muscularisation and remodeling throughout the pulmonary arterial tree, reduced alveolar numbers and alveolar membrane thickening, but with no oedema. Spectral Doppler ultrasound indicates pulmonary hypertension and attenuated hypoxic pulmonary vasoconstriction. Age-dependent right ventricular pressure elevation, dilation and reduced cardiac output was also evident. KV1.5 potassium currents of pulmonary arterial myocytes were markedly smaller under normoxia, which is known to facilitate pulmonary hypertension. Mitochondrial fragmentation and reactive oxygen species accumulation was also evident. Importantly, there was no evidence of systemic vasculopathy or hypertension in these mice. Moreover, hypoxic pulmonary vasoconstriction was attenuated by AMPK-α1 or AMPK-α2 deletion without triggering pulmonary hypertension.

On a Magical Mystery Tour with 8-Bromo-Cyclic ADP-Ribose: From All-or-None Block to Nanojunctions and the Cell-Wide Web

A plethora of cellular functions are controlled by calcium signals, that are greatly coordinated by calcium release from intracellular stores, the principal component of which is the sarco/endooplasmic reticulum (S/ER). In 1997 it was generally accepted that activation of various G protein-coupled receptors facilitated inositol-1,4,5-trisphosphate (IP3) production, activation of IP3 receptors and thus calcium release from S/ER. Adding to this, it was evident that S/ER resident ryanodine receptors (RyRs) could support two opposing cellular functions by delivering either highly localised calcium signals, such as calcium sparks, or by carrying propagating, global calcium waves. Coincidentally, it was reported that RyRs in mammalian cardiac myocytes might be regulated by a novel calcium mobilising messenger, cyclic adenosine diphosphate-ribose (cADPR), that had recently been discovered by HC Lee in sea urchin eggs. A reputedly selective and competitive cADPR antagonist, 8-bromo-cADPR, had been developed and was made available to us. We used 8-bromo-cADPR to further explore our observation that S/ER calcium release via RyRs could mediate two opposing functions, namely pulmonary artery dilation and constriction, in a manner seemingly independent of IP3Rs or calcium influx pathways. Importantly, the work of others had shown that, unlike skeletal and cardiac muscles, smooth muscles might express all three RyR subtypes. If this were the case in our experimental system and cADPR played a role, then 8-bromo-cADPR would surely block one of the opposing RyR-dependent functions identified, or the other, but certainly not both. The latter seemingly implausible scenario was confirmed. How could this be, do cells hold multiple, segregated SR stores that incorporate different RyR subtypes in receipt of spatially segregated signals carried by cADPR? The pharmacological profile of 8-bromo-cADPR action supported not only this, but also indicated that intracellular calcium signals were delivered across intracellular junctions formed by the S/ER. Not just one, at least two. This article retraces the steps along this journey, from the curious pharmacological profile of 8-bromo-cADPR to the discovery of the cell-wide web, a diverse network of cytoplasmic nanocourses demarcated by S/ER nanojunctions, which direct site-specific calcium flux and may thus coordinate the full panoply of cellular processes.

Whole-cell Patch-clamp Recordings of Isolated Primary Epithelial Cells from the Epididymis

The epididymis is an essential organ for sperm maturation and reproductive health. The epididymal epithelium consists of intricately connected cell types that are distinct not only in molecular and morphological features but also in physiological properties. These differences reflect their diverse functions, which together establish the necessary microenvironment for the post-testicular sperm development in the epididymal lumen. The understanding of the biophysical properties of the epididymal epithelial cells is critical for revealing their functions in sperm and reproductive health, under both physiological and pathophysiological conditions. While their functional properties have yet to be fully elucidated, the epididymal epithelial cells can be studied using the patch-clamp technique, a tool for measuring the cellular events and the membrane properties of single cells. Here, we describe the methods of cell isolation and whole-cell patch-clamp recording to measure the electrical properties of primary dissociated epithelial cells from the rat cauda epididymides.

Hypoxic pulmonary hypertension of the newborn

Hypoxic pulmonary hypertension of the newborn is characterized by elevated pulmonary vascular resistance and pressure due to vascular remodeling and increased vessel tension secondary to chronic hypoxia during the fetal and newborn period. In comparison to the adult, the pulmonary vasculature of the fetus and the newborn undergoes tremendous developmental changes that increase susceptibility to a hypoxic insult. Substantial evidence indicates that chronic hypoxia alters the production and responsiveness of various vasoactive agents such as endothelium-derived nitric oxide, endothelin-1, prostanoids, platelet-activating factor, and reactive oxygen species, resulting in sustained vasoconstriction and vascular remodeling. These changes occur in most cell types within the vascular wall, particularly endothelial and smooth muscle cells. At the cellular level, suppressed nitric oxide-cGMP signaling and augmented RhoA-Rho kinase signaling appear to be critical to the development of hypoxic pulmonary hypertension of the newborn.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Contractile and electrophysiological properties of pulmonary artery smooth muscle are not altered in TASK-1 knockout mice

The acid-sensitive, two-pore domain K+ channel, TASK-1, contributes to the background K+ conductance and membrane potential (Em) of rat and human pulmonary artery smooth muscle cells (PASMCs), but its role in regulating tone remains elusive. This study aimed to clarify the role of TASK-1 by determining the functional properties of pulmonary artery (PA) from mice in which the TASK-1 gene was deleted (TASK-1/3 KO), in comparison with wild-type (WT) C57BL/6 controls. Small vessel wire myography was used to measure isometric tension developed by intact PA. Em and currents were recorded from freshly isolated PASMCs using the perforated patch-clamp technique. Reverse transcription-polymerase chain reaction (RT-PCR) was used to estimate K+ channel expression. We could find no difference between PA from WT and TASK-1/3 KO mice. They showed similar constrictor responses to a range of agonists and K+ concentrations, the K+ channel blockers 4-aminopyridine, tetraethylammonium ions and XE991. Treprostinil, proposed to dilate by activating TASK-1, was just as effective in TASK-1/3 KO arteries. Blocking Ca2+ influx with nifedipine (1 μM) or levcromakalim (10 μM) had no effect on resting tone in either strain. The resting Em of PASMCs and its responses to K+ channel blockers were unchanged in TASK-1/3 KO mice as were voltage-activated K+ currents, including the non-inactivating K+ current (IKN) measured at 0 mV. The Em was, however, depolarised in comparison with other species.Mouse IKN was much smaller than in rat and showed no sensitivity to pH. The results imply that TASK-1 does not form a functional channel in mouse PASMCs.

Regulation of the Pulmonary Circulation in the Fetus and Newborn

During the development of the pulmonary vasculature in the fetus, many structural and functional changes occur to prepare the lung for the transition to air breathing. The development of the pulmonary circulation is genetically controlled by an array of mitogenic factors in a temporo-spatial order. With advancing gestation, pulmonary vessels acquire increased vasoreactivity. The fetal pulmonary vasculature is exposed to a low oxygen tension environment that promotes high intrinsic myogenic tone and high vasocontractility. At birth, a dramatic reduction in pulmonary arterial pressure and resistance occurs with an increase in oxygen tension and blood flow. The striking hemodynamic differences in the pulmonary circulation of the fetus and newborn are regulated by various factors and vasoactive agents. Among them, nitric oxide, endothelin-1, and prostaglandin I2 are mainly derived from endothelial cells and exert their effects via cGMP, cAMP, and Rho kinase signaling pathways. Alterations in these signaling pathways may lead to vascular remodeling, high vasocontractility, and persistent pulmonary hypertension of the newborn.

Dynamic characterization and hemodynamic effects of pulmonary waves in fetal lambs using cardiac extrasystoles and beat-by-beat wave intensity analysis

Steady-state wave intensity ( WI) analysis indicates that characteristic midsystolic falls in fetal pulmonary trunk (PT) and artery (PA) blood flow are due to an extremely large backward-running compression wave (BCWms) that 1) originates from the pulmonary microvasculature by a combination of cyclical pulmonary vasoconstriction and vascular reflection of the forward-running compression wave (FCWis) associated with impulsive right ventricular ejection, and 2) is transmitted into the PT. However, no information is available about the dynamic properties of PA BCWms and its contribution to beat-to-beat regulation of pulmonary hemodynamics. Accordingly, beat-by-beat WI analysis was performed during brief increases in ventricular contractility accompanying an extrasystole (ES) in nine anesthetized late-gestation fetal sheep instrumented with PT and left PA micromanometer catheters to measure pressure (P) and transit-time flow probes to obtain blood velocity ( U). WI was calculated as the product of P and U rates of change. At steady state, the magnitude of PA BCWms, and its associated P and U changes (ΔP and Δ U, respectively), were similar to those of FCWis. The PA FCWis and BCWms, and their accompanying ΔP and Δ U, were all transiently potentiated after an ES. Beat-by-beat PA FCWis-BCWms wave area, ΔP and Δ U relationships were highly linear ( R2 ≥ 0.91) with slopes of 1.36–1.47 ( P < 0.001), consistent with the presence of a vasoconstrictor component in PA BCWms. PA-PT BCWms area and ΔP and Δ U relationships were also linear ( R2 ≥ 0.77) with slopes of 0.23–0.64 ( P < 0.001). These results indicate that the fetal PA BCWms contributes to beat-to-beat regulation of not only PA but also PT hemodynamics.

Voltage-gated K+ channels at an early stage of chronic hypoxia-induced pulmonary hypertension in newborn piglets

Our purpose was to determine whether smooth muscle cell membrane properties are altered in small pulmonary arteries (SPA) of piglets at an early stage of pulmonary hypertension. Piglets were raised in either room air (control) or hypoxia for 3 days. A microelectrode technique was used to measure smooth muscle cell membrane potential ( Em) in cannulated, pressurized SPA (100- to 300-μm diameter). SPA responses to the voltage-gated K+ (KV) channel antagonist 4-aminopyridine (4-AP) and the KV1 family channel antagonist correolide were measured. Other SPA were used to assess amounts of KV1.2, KV1.5, and KV2.1 (immunoblot technique). Em was more positive in SPA of chronically hypoxic piglets than in SPA of comparable-age control piglets. The magnitude of constriction elicited by either 4-AP or correolide was diminished in SPA from hypoxic piglets. Abundances of KV1.2 were reduced, whereas abundances of both KV1.5 and KV2.1 were unaltered, in SPA from hypoxic piglets. At least partly because of reduced amounts of KV1.2, smooth muscle cell membrane properties are altered such that Em is depolarized and KV channel family function is impaired in SPA of piglets at an early stage of chronic hypoxia-induced pulmonary hypertension.

Modulation of BK channel calcium affinity by differential phosphorylation in developing ovine basilar artery myocytes

Large-conductance Ca2+-sensitive K+ (BK) channel activity is greater in basilar artery smooth muscle cells (SMCs) of the fetus than the adult, and this increased activity is associated with a lower BK channel Ca2+ set point (Ca0). Associated PKG activity is three times greater in BK channels from fetal than adult myocytes, whereas associated PKA activity is three times greater in channels from adult than fetal myocytes. We hypothesized that the change in Ca0 during development results from different levels of channel phosphorylation. In inside-out membrane patch preparations of basilar artery SMCs from adult and fetal sheep, we measured BK channel activity in four states of phosphorylation: native, dephosphorylated, PKA phosphorylated, and PKG phosphorylated. BK channels from adult and fetus exhibited similar voltage-activation curves, Ca0 values, and Ca2+ dissociation constants (Kd) for the dephosphorylated, PKA phosphorylated, and PKG phosphorylated states. However, voltage-activation curves of native fetal BK channels shifted significantly to the left of those of the adult, with Ca0 and Kd values half those of the adult. For the two age groups at each of the phosphorylation states, Ca0 and Kd produced linear relations when plotted against voltage at half-maximal channel activation. We conclude that the Ca0 and Kd values of the BK channel can be modulated by differential channel phosphorylation. Lower Ca0 and Kd values in BK channels of fetal myocytes can be explained by a greater extent of channel phosphorylation of fetal than adult myocytes.

Effects of aging on Ca2+ signaling in murine mesenteric arterial smooth muscle cells

Pathophysiological changes in arterial smooth muscle structure and function occur with aging and there are a number of reports illustrating reductions in vascular responsiveness with aging. While much is known about arterial remodeling and functional adaptations with aging, very little is known about the biophysical adaptations in individual arterial myocytes. Cytosolic Ca2+ signaling, involving activation of L-type Ca2+ channels on the plasma membrane as well as InsP3 and ryanodine receptors on the sarcoplasmic reticulum, is integral to vascular tone and reactivity. Thus, we tested the hypothesis that aging results in reductions in the functional expression of L-type channels and temporal aspects of ryanodine receptor and InsP3 receptor Ca2+ signaling, in mesenteric arterial smooth muscle cells isolated from 6 and 30 months old C57Bl/6 mice. Comparisons of L-type current activity were made using dialyzed, whole-cell voltage-clamp techniques and Ba2+ as charge carrier. Ca2+ signaling was measured using fura-2 fluorescence microscopy techniques. Cell morphological changes were also investigated using electrophysiological and immunocytochemical approaches. The amplitudes of L-type Ca2+ currents were increased in older mice, but this was associated with membrane surface area increases of approximately 50%, due to increases in cell length not cell width. Consequently, L-type Ca2+ current densities were preserved with age, indicating functional channel expression was unchanged. In contrast, aging was associated with decrements in Ca2+ signaling in response to either ryanodine receptor stimulation by caffeine or InsP3 receptor activation with phenylephrine. These changes with aging may be related to the previously reported depression in myogenic reactivity.

Ca2+-activated K+ channel-associated phosphatase and kinase activities during development

In ovine basilar arterial smooth muscle cells (SMCs), the fetal "big" Ca2+-activated K+ (BK) channel activity is significantly greater and has a lower Ca2+ setpoint than BK channels from adult cells. In the present study, we tested the hypothesis that these differences result from developmentally regulated phosphorylation of these channels. Using the patch-clamp technique and a novel in situ enzymological approach, we measured the rates and extents of changes in BK channel voltage activation from SMC inside-out patch preparations in response to selective activation and inhibition of channel-associated protein phosphatases and kinases (CAPAKs). We show that BK channel activity is modulated during development by differential phosphorylation and that the activities of CAPAKs change substantially during development. In particular, excised membrane patches from adult SMCs exhibited greater protein kinase A activity than those from a fetus. In contrast, fetal SMCs exhibited greater protein kinase G activity and phosphatase activity than adult SMCs. These findings extend our previous observation that the BK channel Ca2+ setpoint differs significantly in adult and fetal cerebrovascular myocytes and suggest a biochemical mechanism for this difference. In addition, these findings suggest that the functional stoichiometry of CAPAKs varies significantly during development and that such variation may be a hitherto unrecognized mechanism of ion channel regulation.

Ionic mechanisms mediating the myogenic response in newborn porcine cerebral arteries

Mechanisms that underlie autoregulation in the newborn vasculature are unclear. Here we tested the hypothesis that in newborn porcine cerebral arteries intravascular pressure elevates wall tension, leading to an increase in intracellular calcium concentration ([Ca2+]i) and a constriction that is opposed by pressure-induced K+channel activation. Incremental step (20 mmHg) elevations in intravascular pressure between 10 and 90 mmHg induced an immediate transient elevation in arterial wall [Ca2+]iand a short-lived constriction that was followed by a smaller steady-state [Ca2+]ielevation and sustained constriction. Pressures between 10 and 90 mmHg increased steady-state arterial wall [Ca2+]ibetween ∼142 and 299 nM and myogenic (defined as passive-active) tension between 25 and 437 dyn/cm. The relationship between pressure and myogenic tension was strongly Ca2+dependent until forced dilation. At low pressure, 60 mM K+induced a steady-state elevation in arterial wall [Ca2+]iand a constriction. Nimodipine, a voltage-dependent Ca2+channel blocker, and removal of extracellular Ca2+similarly dilated arteries at low or high pressures. 4-Aminopyridine, a voltage-dependent K+(Kv) channel blocker, induced significantly larger constrictions at high pressure, when compared with those at low pressure. Although selective Ca2+-activated K+(KCa) channel blockers and intracellular Ca2+release inhibitors induced only small constrictions at low and high pressures, a low concentration of caffeine (1 μM), a ryanodine-sensitive Ca2+release (RyR) channel activator, increased KCachannel activity and induced dilation. These data suggest that in newborn cerebral arteries, intravascular pressure elevates wall tension, leading to voltage-dependent Ca2+channel activation, an increase in wall [Ca2+]iand Ca2+-dependent constriction. In addition, pressure strongly activates Kvchannels that opposes constriction but only weakly activates KCachannels.

Functional evidence of a role for two-pore domain potassium channels in rat mesenteric and pulmonary arteries

1. Experiments were performed to elucidate the mechanism by which alterations of extracellular pH (pH(o)) change membrane potential (E(M)) in rat mesenteric and pulmonary arteries. 2. Changing pH(o) from 7.4 to 6.4 or 8.4 produced a depolarisation or hyperpolarisation, respectively, in mesenteric and pulmonary arteries. Anandamide (10 microm) or bupivacaine (100 microm) reversed the hyperpolarisation associated with alkaline pH(o), shifting the E(M) of both vessels to levels comparable to that at pH 6.4. In pulmonary arteries, clofilium (100 microm) caused a significant reversal of hyperpolarisation seen at pH 8.4 but was without effect at pH 7.4. 3. K(+) channel blockade by 4-aminopyridine (4-AP) (5 mm), tetraethylammonium (TEA) (10 mm), Ba(2+) (30 microm) and glibenclamide (10 microm) depolarised the pulmonary artery. However, shifts in E(M) with changes in pH(o) remained and were sensitive to anandamide (10 microm), bupivacaine (100 microm) or Zn(2+) (200 microm). 4. Anandamide (0.3-60 microm) or bupivacaine (0.3-300 microm) caused a concentration-dependent increase in basal tone in pulmonary arteries. 5. RT-PCR demonstrated the expression of TASK-1, TASK-2, THIK-1, TRAAK, TREK-1, TWIK-1 and TWIK-2 in mesenteric arteries and TASK-1, TASK-2, THIK-1, TREK-2 and TWIK-2 in pulmonary arteries. TASK-1, TASK-2, TREK-1 and TWIK-2 protein was demonstrated in both arteries by immunostaining. 6. These experiments provide evidence for the presence of two-pore domain K(+) channels in rat mesenteric and pulmonary arteries. Collectively, they strongly suggest that modulation of TASK-1 channels is most likely to have mediated the pH-induced changes in membrane potential observed in these vessels, and that blockade of these channels by anandamide or bupivacaine generates a small increase in pulmonary artery tone.

Carbon monoxide activates KCa channels in newborn arteriole smooth muscle cells by increasing apparent Ca2+ sensitivity of alpha-subunits

Carbon monoxide (CO) is a gaseous vasodilator produced by many cell types, including endothelial and smooth muscle cells. The goal of the present study was to investigate signaling mechanisms responsible for CO activation of large-conductance Ca(2+)-activated K(+) (K(Ca)) channels in newborn porcine cerebral arteriole smooth muscle cells. In intact cells at 0 mV, CO (3 microM) or CO released from dimanganese decacarbonyl (10 microM), a novel light-activated CO donor, increased K(Ca) channel activity 4.9- or 3.5-fold, respectively. K(Ca) channel activation by CO was not blocked by 1-H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (25 microM), a soluble guanylyl cyclase inhibitor. In inside-out patches at 0 mV, CO shifted the Ca(2+) concentration-response curve for K(Ca) channels leftward and decreased the apparent dissociation constant for Ca(2+) from 31 to 24 microM. Western blotting data suggested that the low Ca(2+) sensitivity of newborn K(Ca) channels may be due to a reduced beta-subunit-to-alpha-subunit ratio. CO activation of K(Ca) channels was Ca(2+) dependent. CO increased open probability 3.7-fold with 10 microM free Ca(2+) at the cytosolic membrane surface but only 1.1-fold with 300 nM Ca(2+). CO left shifted the current-voltage relationship of cslo-alpha currents expressed in HEK-293 cells, increasing currents 2.2-fold at +50 mV. In summary, data suggest that in newborn arteriole smooth muscle cells, CO activates low-affinity K(Ca) channels via a direct effect on the alpha-subunit that increases apparent Ca(2+) sensitivity. The optimal tuning by CO of the micromolar Ca(2+) sensitivity of K(Ca) channels will lead to preferential activation by signaling modalities, such as Ca(2+) sparks, which elevate the subsarcolemmal Ca(2+) concentration within this range.

Developmental differences in Ca2+-activated K+ channel activity in ovine basilar artery

A primary determinant of vascular smooth muscle (VSM) tone and contractility is the resting membrane potential, which, in turn, is influenced heavily by K+ channel activity. Previous studies from our laboratory and others have demonstrated differences in the contractility of cerebral arteries from near-term fetal and adult animals. To test the hypothesis that these contractility differences result from maturational changes in voltage-gated K+ channel function, we compared this function in VSM myocytes from adult and fetal sheep cerebral arteries. The primary current-carrying, voltage-gated K+ channels in VSM myocytes are the large conductance Ca2+-activated K+ channels (BKCa) and voltage-activated K+ (KV) channels. We observed that at voltage-clamped membrane potentials of +60 mV in perforated whole cell studies, the normalized outward current densities in fetal myocytes were >30% higher than in those of the adult (P < 0.05) and that these were predominantly due to iberiotoxin-sensitive currents from BKCa channels. Excised, insideout membrane patches revealed nearly identical unitary conductances and Hill coefficients for BKCa channels. The plot of log intracellular [Ca2+] ([Ca2+]i) versus voltage for half-maximal activation (V(1/2)) yielded linear and parallel relationships, and the change in V(1/2) for a 10-fold change in [Ca2+] was also similar. Channel activity increased e-fold for a 19 +/- 2-mV depolarization for adult myocytes and for an 18 +/- 1-mV depolarization for fetal myocytes (P > 0.05). However, the relationship between BKCa open probability and membrane potential had a relative leftward shift for the fetal compared with adult myocytes at different [Ca2+]i. The [Ca2+] for half-maximal activation (i.e., the calcium set points) at 0 mV were 8.8 and 4.7 microM for adult and fetal myocytes, respectively. Thus the increased BKCa current density in fetal myocytes appears to result from a lower calcium set point.

Oxygen-induced fetal pulmonary vasodilation is mediated by intracellular calcium activation of K(Ca) channels

O(2) sensing in fetal pulmonary artery smooth muscle is critically important in the successful transition to air breathing at birth. However, the mechanism by which the fetal pulmonary vasculature senses and responds to an acute increase in O(2) tension is not known. Isolated fetal pulmonary artery smooth muscle cells were kept in primary culture for 5-14 days in a hypoxic environment (20-30 mmHg). These cells showed a 25.1 +/- 1.7% decrease in intracellular calcium in response to an acute increase in O(2) tension. Low concentrations of caffeine (0.5 mM) and diltiazem also decreased intracellular calcium. The decrease in intracellular calcium concentration in response to increasing O(2) was inhibited by iberiotoxin and ryanodine. Freshly isolated fetal pulmonary artery smooth muscle cells exhibited "spontaneous transient outward currents," indicative of intracellular calcium spark activation of calcium-sensitive potassium channels. The frequency of spontaneous transient outward currents increased when O(2) tension was increased to normoxic levels. Increasing fetal pulmonary O(2) tension in acutely instrumented fetal sheep increased fetal pulmonary blood flow. Ryanodine attenuated O(2)-induced pulmonary vasodilation. This study demonstrates that fetal pulmonary vascular smooth muscle cells are capable of responding to an acute increase in O(2) tension and that this O(2) response is mediated by intracellular calcium activation of calcium-sensitive potassium channels.

Molecular basis of hypoxia-induced pulmonary vasoconstriction: role of voltage-gated K+ channels

The hypoxia-induced membrane depolarization and subsequent constriction of small resistance pulmonary arteries occurs, in part, via inhibition of vascular smooth muscle cell voltage-gated K+ (KV) channels open at the resting membrane potential. Pulmonary arterial smooth muscle cell KV channel expression, antibody-based dissection of the pulmonary arterial smooth muscle cell K+ current, and the O2 sensitivity of cloned KV channels expressed in heterologous expression systems have all been examined to identify the molecular components of the pulmonary arterial O2-sensitive KV current. Likely components include Kv2.1/Kv9.3 and Kv1.2/Kv1.5 heteromeric channels and the Kv3.1b alpha-subunit. Although the mechanism of KV channel inhibition by hypoxia is unknown, it appears that KV alpha-subunits do not sense O2 directly. Rather, they are most likely inhibited through interaction with an unidentified O2 sensor and/or beta-subunit. This review summarizes the role of KV channels in hypoxic pulmonary vasoconstriction, the recent progress toward the identification of KV channel subunits involved in this response, and the possible mechanisms of KV channel regulation by hypoxia.

Pulmonary vascular response to normoxia and K(Ca) channel activity is developmentally regulated

To address developmental regulation of pulmonary vascular O(2) sensing, we tested the hypotheses that 1) fetal but not adult pulmonary artery smooth muscle cells (PASMCs) can directly sense an acute increase in O(2), 2) Ca2+-sensitive K(+) (K(Ca)) channel activity decreases with maturation, and 3) PASMC K(Ca) channel expression decreases with maturation. We used fluorescence microscopy to confirm that fetal but not adult PASMCs are able to sense an acute increase in O(2) tension. Acute normoxia induced a 22 +/- 2% decrease in cytosolic Ca2+ concentration ([Ca2+](i)) in fetal PASMCs and no change in ([Ca2+](i)) in adult PASMCs (P < 0.01). The effects of K(+) channel antagonists were studied on fetal and adult PASMC ([Ca2+](i)). Iberiotoxin (10(-9) M) caused PASMC ([Ca2+](i)) to increase by 694 +/- 22% in the fetus and caused no change in adult PASMCs. K(Ca) channel expression and mRNA levels in distal pulmonary arteries from fetal and adult sheep were examined. Both K(Ca) channel protein and mRNA expression in the distal pulmonary vasculature decreased with maturation. We conclude that maturation-dependent changes in PASMC O(2) sensing render the fetal PASMCs uniquely sensitive to an acute increase in O(2) tension at a biologically critical time point.

Neonatal pulmonary hypertension prevents reorganisation of the pulmonary arterial smooth muscle cytoskeleton after birth

The pulmonary arterial smooth muscle cell (SMC) cytoskeleton was studied in tissue from 36 piglets aged from within 5 min of birth to 21 d of age, and in 8 adults. An additional 16 piglets were made pulmonary hypertensive by exposure to hypobaric hypoxia (50.8 kPa) for 3 d. In conduit intrapulmonary elastic arteries alpha, beta and gamma actin, the 204, 200 and 196 kDa myosin heavy chain (MHC) isoforms and vinculin were localised by immunohistochemistry. The total actin content, the proportion of monomeric to filamentous alpha and gamma actin and changes in the proportions of the MHC isoforms were determined biochemically. Dividing SMCs were localised and quantified using Ki-67. We found a transient reduction in immunohistochemical expression of gamma actin, 204 kDa MHC isoform and vinculin at 3 and 6 d in the inner media, associated with a transient increase in Ki-67 labelling. The actin content also decreased at 3 and 6 d (P < 0.05), but there was a postnatal, permanent increase in monomeric actin, first the alpha then the gamma isoform. The relative proportions of the MHC isoforms did not change between birth and adulthood in elastic pulmonary arteries but in muscular arteries the 200 kDa isoform increased between 14 d and adulthood. Pulmonary hypertension prevented both the immunohistochemical changes and the postnatal burst of SMC replication and prevented the transient postnatal reduction in actin content. These findings suggest that rapid remodelling of the actin cytoskeleton is an essential prerequisite of a normal postnatal fall in pulmonary vascular resistance.

Role of NO in recovery from neonatal hypoxic pulmonary hypertension

BACKGROUND

The management of sick newborn infants who have sustained a hypoxic insult is a common clinical problem but relatively little is known about the recovery process. The aim of this study was to investigate this process in newborn piglets.

METHODS

Thirty five newborn piglets were exposed to chronic hypobaric hypoxia for three days, either from birth, three or 14 days of age, and were allowed to recover for one, three, or six days. Control animals of relevant age were also studied. The heart weight ratio and pulmonary arterial muscularity were measured. Endothelial dependent and independent relaxation of the isolated intrapulmonary conduit arteries was determined in classical organ chamber studies, together with measurement of basal and stimulated cGMP accumulation.

RESULTS

After six days of recovery the hypoxia induced right ventricular hypertrophy and pulmonary arterial medial hypertrophy had decreased in all animals but values were still abnormal in the two younger age groups. Relaxation was still impaired during the first three days of recovery in all groups, had normalised by six days in the two youngest groups, but relaxation (both endothelium dependent and independent) remained impaired in older animals. In these older animals basal nitric oxide (NO) production and basal and stimulated cGMP accumulation was normal.

CONCLUSIONS

The recovery of the smooth muscle cells lags behind that of the endothelial cells. A normal stimulated increase in cGMP with reduced relaxation suggests an altered threshold for cGMP effected relaxation. These findings help to explain why some hypoxic infants require protracted NO therapy.