A maturational shift in pulmonary K+ channels, from Ca2+ sensitive to voltage dependent

Oxygen sensing, mitochondrial biology and experimental therapeutics for pulmonary hypertension and cancer

The homeostatic oxygen sensing system (HOSS) optimizes systemic oxygen delivery. Specialized tissues utilize a conserved mitochondrial sensor, often involving NDUFS2 in complex I of the mitochondrial electron transport chain, as a site of pO₂-responsive production of reactive oxygen species (ROS). These ROS are converted to a diffusible signaling molecule, hydrogen peroxide (H₂O₂), by superoxide dismutase (SOD2). H₂O₂ exits the mitochondria and regulates ion channels and enzymes, altering plasma membrane potential, intracellular Ca²⁺ and Ca²⁺-sensitization and controlling acute, adaptive, responses to hypoxia that involve changes in ventilation, vascular tone and neurotransmitter release. Subversion of this O₂-sensing pathway creates a pseudohypoxic state that promotes disease progression in pulmonary arterial hypertension (PAH) and cancer. Pseudohypoxia is a state in which biochemical changes, normally associated with hypoxia, occur despite normal pO₂. Epigenetic silencing of SOD2 by DNA methylation alters H₂O₂ production, activating hypoxia-inducible factor 1α, thereby disrupting mitochondrial metabolism and dynamics, accelerating cell proliferation and inhibiting apoptosis. Other epigenetic mechanisms, including dysregulation of microRNAs (miR), increase pyruvate dehydrogenase kinase and pyruvate kinase muscle isoform 2 expression in both diseases, favoring uncoupled aerobic glycolysis. This Warburg metabolic shift also accelerates cell proliferation and impairs apoptosis. Disordered mitochondrial dynamics, usually increased mitotic fission and impaired fusion, promotes disease progression in PAH and cancer. Epigenetic upregulation of dynamin-related protein 1 (Drp1) and its binding partners, MiD49 and MiD51, contributes to the pathogenesis of PAH and cancer. Finally, dysregulation of intramitochondrial Ca²⁺, resulting from impaired mitochondrial calcium uniporter complex (MCUC) function, links abnormal mitochondrial metabolism and dynamics. MiR-mediated decreases in MCUC function reduce intramitochondrial Ca²⁺, promoting Warburg metabolism, whilst increasing cytosolic Ca²⁺, promoting fission. Epigenetically disordered mitochondrial O₂-sensing, metabolism, dynamics, and Ca²⁺ homeostasis offer new therapeutic targets for PAH and cancer. Promoting glucose oxidation, restoring the fission/fusion balance, and restoring mitochondrial calcium regulation are promising experimental therapeutic strategies.

β1-Subunit of the calcium-sensitive potassium channel modulates the pulmonary vascular smooth muscle cell response to hypoxia

Accessory subunits associated with the calcium-sensitive potassium channel (BKCa), a major determinant of vascular tone, confer functional and anatomical diversity. The β1 subunit increases Ca2+ and voltagesensitivity of the BKCa channel and is expressed exclusively in smooth muscle cells. Evidence supporting the physiological significance of the β1 subunit includes the observations that murine models with deletion of the β1 subunit are hypertensive and that humans with a gain-of-function β1 mutation are at a decreased risk of diastolic hypertension. However, whether the β1 subunit of the BKCa channel contributes to the low tone that characterizes the normal pulmonary circulation or modulates the pulmonary vascular response to hypoxia remains unknown. To determine the role of the BKCa channel β1 subunit in the regulation of pulmonary vascular tone and the response to acute and chronic hypoxia, mice with deletion of the Kcnmb1 gene that encodes for the β1 subunit ( Kcnmb1-/-) were placed in chronic hypoxia (10% O2) for 21-24 days. In normoxia, right ventricular systolic pressure (RVSP) did not differ between Kcnmb1+/+ (controls) and Kcnmb1-/- mice. After exposure to either acute or chronic hypoxia, RVSP was higher in Kcnmb1-/- mice compared with Kcnmb1+/+ mice, without increased vascular remodeling. β1 subunit expression was predominantly confined to pulmonary artery smooth muscle cells (PASMCs) from vessels ≤ 150 µm. Peripheral PASMCs contracted collagen gels irrespective of β1 expression. Focal adhesion expression and Rho kinase activity were greater in Kcnmb1-/- compared with Kcnmb1+/+ PASMCs. Compromised PASMC β1 function may contribute to the heightened microvascular vasoconstriction that characterizes pulmonary hypertension.

Unique aspects of the developing lung circulation: structural development and regulation of vasomotor tone

This review summarizes our current knowledge on lung vasculogenesis and angiogenesis during normal lung development and the regulation of fetal and postnatal pulmonary vascular tone. In comparison to that of the adult, the pulmonary circulation of the fetus and newborn displays many unique characteristics. Moreover, altered development of pulmonary vasculature plays a more prominent role in compromised pulmonary vasoreactivity than in the adult. Clinically, a better understanding of the developmental changes in pulmonary vasculature and vasomotor tone and the mechanisms that are disrupted in disease states can lead to the development of new therapies for lung diseases characterized by impaired alveolar structure and pulmonary hypertension.

Hypoxia-dependent reactive oxygen species signaling in the pulmonary circulation: focus on ion channels

SIGNIFICANCE

An acute lack of oxygen in the lung causes hypoxic pulmonary vasoconstriction, which optimizes gas exchange. In contrast, chronic hypoxia triggers a pathological vascular remodeling causing pulmonary hypertension, and ischemia can cause vascular damage culminating in lung edema.

RECENT ADVANCES

Regulation of ion channel expression and gating by cellular redox state is a widely accepted mechanism; however, it remains a matter of debate whether an increase or a decrease in reactive oxygen species (ROS) occurs under hypoxic conditions. Ion channel redox regulation has been described in detail for some ion channels, such as Kv channels or TRPC6. However, in general, information on ion channel redox regulation remains scant.

CRITICAL ISSUES AND FUTURE DIRECTIONS

In addition to the debate of increased versus decreased ROS production during hypoxia, we aim here at describing and deciphering why different oxidants, under different conditions, can cause both activation and inhibition of channel activity. While the upstream pathways affecting channel gating are often well described, we need a better understanding of redox protein modifications to be able to determine the complexity of ion channel redox regulation. Against this background, we summarize the current knowledge on hypoxia-induced ROS-mediated ion channel signaling in the pulmonary circulation.

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.

Characterisation of K+ channels in human fetoplacental vascular smooth muscle cells

Adequate blood flow through placental chorionic plate resistance arteries (CPAs) is necessary for oxygen and nutrient transfer to the fetus and a successful pregnancy. In non-placental vascular smooth muscle cells (SMCs), K(+) channels regulate contraction, vascular tone and blood flow. Previous studies showed that K(+) channel modulators alter CPA tone, but did not distinguish between effects on K(+) channels in endothelial cells and SMCs. In this study, we developed a preparation of freshly isolated CPASMCs of normal pregnancy and investigated K(+) channel expression and function. CPASMCs were isolated from normal human term placentas using enzymatic digestion. Purity and phenotype was confirmed with immunocytochemistry. Whole-cell patch clamp was used to assess K(+) channel currents, and mRNA and protein expression was determined in intact CPAs and isolated SMCs with RT-PCR and immunostaining. Isolated SMCs expressed α-actin but not CD31, a marker of endothelial cells. CPASMCs and intact CPAs expressed h-caldesmon and non-muscle myosin heavy chain-2; phenotypic markers of contractile and synthetic SMCs respectively. Whole-cell currents were inhibited by 4-AP, TEA, charybdotoxin and iberiotoxin implicating functional K(v) and BK(Ca) channels. 1-EBIO enhanced whole cell currents which were abolished by TRAM-34 and reduced by apamin indicating activation of IK(Ca) and SK(Ca) respectively. BK(Ca), IK(Ca) and SK(Ca)3 mRNA and/or protein were expressed in CPASMCs and intact CPAs. This study provides the first direct evidence for functional K(v), BK(Ca,) IK(Ca) and SK(Ca) channels in CPASMCs. These cells display a mixed phenotype implicating a dual role for CPASMCs in controlling both fetoplacental vascular resistance and vasculogenesis.

Function and regulation of large conductance Ca(2+)-activated K+ channel in vascular smooth muscle cells

Large conductance Ca(2+)-activated K(+) (BK(Ca)) channels are abundantly expressed in vascular smooth muscle cells. Activation of BK(Ca) channels leads to hyperpolarization of cell membrane, which in turn counteracts vasoconstriction. Therefore, BK(Ca) channels have an important role in regulation of vascular tone and blood pressure. The activity of BK(Ca) channels is subject to modulation by various factors. Furthermore, the function of BK(Ca) channels are altered in both physiological and pathophysiological conditions, such as pregnancy, hypertension and diabetes, which has dramatic impacts on vascular tone and hemodynamics. Consequently, compounds and genetic manipulation that alter activity and expression of the channel might be of therapeutic interest.

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.

Hypoxia-inducible factor-1α regulates KCNMB1 expression in human pulmonary artery smooth muscle cells

Previously, we observed that hypoxia increases the expression of the β1-subunit (KCNMB1) of the calcium-sensitive potassium channel (BK(Ca)). Herein, we elucidate the mechanism whereby hypoxia increases KCNMB1 expression in human pulmonary artery smooth muscle cells (hPASMC). In response to hypoxia, the expression of both the transcription factor hypoxia-inducible factor 1-α (HIF-1α) and KCNMB1 are increased. Knockdown of HIF-1α using a shRNA plasmid blocked the hypoxic induction of KCNMB1 expression. Chromatin immunoprecipitation (ChIP) demonstrated HIF-1α binding to three discrete regions of the human KCNMB1 promoter known to contain hypoxia response elements (HREs). A KCNMB1 promoter reporter assay combined with site-directed mutagenesis identified two adjacent HREs located between -3,540 bp and -3,311 bp that are essential for the hypoxic induction of KCNMB1 promoter activity. Furthermore, additional ChIP assays demonstrated recruitment of the HIF-1α transcriptional coactivator, p300, to this same promoter region. Treatment of hPASMC with the histone deacetylase inhibitor, trichostatin, prolonged the increase in KCNMB1 observed with hypoxia, suggesting that alterations in chromatin remodeling function to limit the hypoxic induction of KCNMB1. Finally, KCNMB1 knockdown potentiated the hypoxia-induced increase in cytosolic calcium in hPASMC, highlighting the contribution of the β1-subunit in modulating vascular SMC tone in response to acute hypoxia. In conclusion, HIF-1α increases KCNMB1 expression in response to hypoxia in hPASMC by binding to two HREs located at -3,540 to -3,311 of the KCNMB1 promoter. We speculate that selective modulation of KCNMB1 expression may serve as a novel therapeutic approach to address diseases characterized by an increase in vascular tone.

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.

Endothelium-dependent control of vascular tone during early postnatal and juvenile growth

Endothelial dysfunction can develop at an early age in children with risk factors for cardiovascular disease. A clear understanding of the nature of this dysfunction and how it can worsen over time requires detailed information on the normal growth-related changes in endothelial function on which the pathological changes are superimposed. This review summarizes our current understanding of these normal changes, as derived from studies in four different mammalian species. Although the endothelium plays an important role in controlling vascular tone from birth onward, the vasoactive molecules that mediate this control often change during postnatal or juvenile growth. The specifics of this transition to an adult endothelial cell phenotype can vary depending on the vascular bed. During growth, the contribution of nitric oxide to endothelium-dependent dilation generally increases in the lung, cerebral cortex, and skeletal muscle, but decreases in the intestine. Endothelial capacity for release of other vasoactive factors (e.g., cyclooxygenase products, hydrogen peroxide, carbon monoxide) can also increase or decrease during growth. Although these changes have been well documented, there is less information on their underlying cellular or molecular events. Further research is required to clarify these mechanisms, and to evaluate the functional significance of such shifts in endothelial phenotype.

NADPH oxidases and reactive oxygen species at different stages of chronic hypoxia-induced pulmonary hypertension in newborn piglets

Recently, we reported that reactive oxygen species (ROS) generated by NADPH oxidase (NOX) contribute to aberrant responses in pulmonary resistance arteries (PRAs) of piglets exposed to 3 days of hypoxia (Am J Physiol Lung Cell Mol Physiol 295: L881-L888, 2008). An objective of the present study was to determine whether NOX-derived ROS also contribute to altered PRA responses at a more advanced stage of pulmonary hypertension, after 10 days of hypoxia. We further wished to advance knowledge about the specific NOX and antioxidant enzymes that are altered at early and later stages of pulmonary hypertension. Piglets were raised in room air (control) or hypoxia for 3 or 10 days. Using a cannulated artery technique, we found that treatments with agents that inhibit NOX (apocynin) or remove ROS [an SOD mimetic (M40403) + polyethylene glycol-catalase] diminished responses to ACh in PRAs from piglets exposed to 10 days of hypoxia. Western blot analysis showed an increase in expression of NOX1 and the membrane fraction of p67phox. Expression of NOX4, SOD2, and catalase were unchanged, whereas expression of SOD1 was reduced, in arteries from piglets raised in hypoxia for 3 or 10 days. Markers of oxidant stress, F(2)-isoprostanes, measured by gas chromatography-mass spectrometry, were increased in PRAs from piglets raised in hypoxia for 3 days, but not 10 days. We conclude that ROS derived from some, but not all, NOX family members, as well as alterations in the antioxidant enzyme SOD1, contribute to aberrant PRA responses at an early and a more progressive stage of chronic hypoxia-induced pulmonary hypertension in newborn piglets.

Mechanisms of oxygen sensing: a key to therapy of pulmonary hypertension and patent ductus arteriosus

Specialized tissues that sense acute changes in the local oxygen tension include type 1 cells of the carotid body, neuroepithelial bodies in the lungs, and smooth muscle cells of the resistance pulmonary arteries and the ductus arteriosus (DA). Hypoxia inhibits outward potassium current in carotid body type 1 cells, leading to depolarization and calcium entry through L-type calcium channels. Increased intracellular calcium concentration ([Ca+ +]i) leads to exocytosis of neurotransmitters, thus stimulating the carotid sinus nerve and respiration. The same K+ channel inhibition occurs with hypoxia in pulmonary artery smooth muscle cells (PASMCs), causing contraction and providing part of the mechanism of hypoxic pulmonary vasoconstriction (HPV). In the SMCs of the DA, the mechanism works in reverse. It is the shift from hypoxia to normoxia that inhibits K+ channels and causes normoxic ductal contraction. In both PA and DA, the contraction is augmented by release of Ca+ + from the sarcoplasmic reticulum, entry of Ca+ + through store-operated channels (SOC) and by Ca+ + sensitization. The same three 'executive' mechanisms are partly responsible for idiopathic pulmonary arterial hypertension (IPAH). While vasoconstrictor mediators constrict both PA and DA and vasodilators dilate both vessels, only redox changes mimic oxygen by having directly opposite effects on the K+ channels, membrane potential, [Ca(++)]i and tone in the PA and DA. There are several different hypotheses as to how redox might alter tone, which remain to be resolved. However, understanding the mechanism will facilitate drug development for pulmonary hypertension and patent DA.

Reactive oxygen species from NADPH oxidase contribute to altered pulmonary vascular responses in piglets with chronic hypoxia-induced pulmonary hypertension

Our main objective was to determine whether reactive oxygen species (ROS), such as superoxide (O(2)(-)) and hydrogen peroxide (H(2)O(2)), contribute to altered pulmonary vascular responses in piglets with chronic hypoxia-induced pulmonary hypertension. Piglets were raised in either room air (control) or hypoxia for 3 days. The effect of the cell-permeable superoxide dismutase mimetic (SOD; M40403) and/or PEG-catalase (PEG-CAT) on responses to acetylcholine (ACh) was measured in endothelium-intact and denuded pulmonary resistance arteries (PRAs; 90-to-300-microm diameter). To determine whether NADPH oxidase is an enzymatic source of ROS, PRA responses to ACh were measured in the presence and absence of a NADPH oxidase inhibitor, apocynin (APO). A Western blot technique was used to assess expression of the NADPH oxidase subunit, p67phox. A lucigenin-derived chemiluminescence technique was used to measure ROS production stimulated by the NADPH oxidase substrate, NADPH. ACh responses, which were dilation in intact control arteries but constriction in both intact and denuded hypoxic arteries, were diminished by M40403, PEG-CAT, the combination of M40403 plus PEG-CAT, as well as by APO. Although total amounts were not different, membrane-associated p67phox was greater in PRAs from hypoxic compared with control piglets. NADPH-stimulated lucigenin luminescence was nearly doubled in PRAs from hypoxic vs. control piglets. We conclude that ROS generated by NADPH oxidase contribute to the aberrant pulmonary arterial responses in piglets exposed to 3 days of hypoxia.

Oxidant and redox signaling in vascular oxygen sensing: implications for systemic and pulmonary hypertension

It has been well known for >100 years that systemic blood vessels dilate in response to decreases in oxygen tension (hypoxia; low PO2), and this response appears to be critical to supply blood to the stressed organ. Conversely, pulmonary vessels constrict to a decrease in alveolar PO2 to maintain a balance in the ventilation-to-perfusion ratio. Currently, although little question exists that the PO2 affects vascular reactivity and vascular smooth muscle cells (VSMCs) act as oxygen sensors, the molecular mechanisms involved in modulating the vascular reactivity are still not clearly understood. Many laboratories, including ours, have suggested that the intracellular calcium concentration ([Ca2+]i), which regulates vasomotor function, is controlled by free radicals and redox signaling, including NAD(P)H and glutathione (GSH) redox. In this review article, therefore, we discuss the implications of redox and oxidant alterations seen in pulmonary and systemic hypertension, and how key targets that control [Ca2+]i, such as ion channels, Ca2+ release from internal stores and uptake by the sarcoplasmic reticulum, and the Ca2+ sensitivity to the myofilaments, are regulated by changes in intracellular redox and oxidants associated with vascular PO2sensing in physiologic or pathophysiologic conditions.

Blunted hypoxic pulmonary vasoconstriction in experimental neonatal chronic lung disease

RATIONALE

Neonatal chronic lung disease (CLD), caused by prolonged mechanical ventilation (MV) with O(2)-rich gas, is the most common cause of long-term hospitalization and recurrent respiratory illness in extremely premature infants. Recurrent episodes of hypoxemia and associated ventilator adjustments often lead to worsening CLD. The mechanism that causes these hypoxemic episodes is unknown. Hypoxic pulmonary vasoconstriction (HPV), which is partially controlled by O(2)-sensitive voltage-gated potassium (K(v)) channels, is an important adaptive response to local hypoxia that helps to match perfusion and ventilation in the lung.

OBJECTIVES

To test the hypothesis that chronic lung injury (CLI) impairs HPV.

METHODS

We studied preterm lambs that had MV with O(2)-rich gas for 3 weeks and newborn rats that breathed 95%-O(2) for 2 weeks, both of which resulted in airspace enlargement and pulmonary vascular changes consistent with CLD.

MEASUREMENTS AND MAIN RESULTS

HPV was attenuated in preterm lambs with CLI after 2 weeks of MV and in newborn rats with CLI after 2 weeks of hyperoxia. HPV and constriction to the K(v)1.x-specific inhibitor, correolide, were preferentially blunted in excised distal pulmonary arteries (dPAs) from hyperoxic rats, whose dPAs exhibited decreased K(v)1.5 and K(v)2.1 mRNA and K(+) current. Intrapulmonary gene transfer of K(v)1.5, encoding the ion channel that is thought to trigger HPV, increased O(2)-sensitive K(+) current in cultured smooth muscle cells from rat dPAs, and restored HPV in hyperoxic rats.

CONCLUSIONS

Reduced expression/activity of O(2)-sensitive K(v) channels in dPAs contributes to blunted HPV observed in neonatal CLD.

Perinatal hypoxia triggers alterations in K+ channels of adult pulmonary artery smooth muscle cells

Adverse events during the perinatal period, like hypoxia, have been associated with adult diseases. In pulmonary vessels, K(+) channels play an important role in the regulation of vascular tone. In the fetus, Ca(2+)-activated K(+) channels (K(Ca)) are predominant, whereas from birth voltage-gated K(+) channels (K(V)) prevail in the adult. We postulated that perinatal hypoxia could alter this maturational shift and influence regulation of pulmonary vascular tone in relation to K(+) channels in adulthood. We evaluated the effects of perinatal hypoxia on K(V) and K(Ca) channels in the adult main pulmonary artery (PA) using a murine model. Electrophysiological measurements showed a greater outward current in PA smooth muscle cells of mice born in hypoxia than in controls. In controls, only K(V) channels contributed to this current, whereas in mice born in hypoxia both K(V) and K(Ca) channels were implicated. K(V) channel activity was even higher in mice born in hypoxia than in controls. Therefore, perinatal hypoxia results in increased K(Ca) and K(V) channel activity in adult PA. Moreover, PA of adults born in hypoxia displayed higher large-conductance K(Ca) alpha-subunit and K(V)1.5 alpha-subunit protein expression than controls. Interestingly, relaxation induced by nitric oxide (NO) donors [S-nitroso-N-acetyl-D,l-penicillamine, 2-(N,N-diethylamino)-diazenolate-2-oxide] in isolated PA of control mice was not mediated by K(Ca) channels and only slightly by K(V) channels, whereas following perinatal hypoxia both K(Ca) and K(V) channels contributed to this relaxation. Thus perinatal hypoxia results in altered expression and activity of different K(+) channels in the adult main PA, which could contribute to modifications of pulmonary vasoreactivity.

Heterogeneity of hypoxia-mediated decrease in I(K(V)) and increase in [Ca2+](cyt) in pulmonary artery smooth muscle cells

Hypoxic pulmonary vasoconstriction is caused by a rise in cytosolic Ca(2+) ([Ca(2+)](cyt)) in pulmonary artery smooth muscle cells (PASMC) via multiple mechanisms. PASMC consist of heterogeneous phenotypes defined by contractility, proliferation, and apoptosis as well as by differences in expression and function of various genes. In rat PASMC, hypoxia-mediated decrease in voltage-gated K(+) (Kv) currents (I(K(V))) and increase in [Ca(2+)](cyt) were not uniformly distributed in all PASMC tested. Acute hypoxia decreased I(K(V)) and increased [Ca(2+)](cyt) in approximately 46% and approximately 53% of PASMC, respectively. Using combined techniques of single-cell RT-PCR and patch clamp, we show here that mRNA expression level of Kv1.5 in hypoxia-sensitive PASMC (in which hypoxia reduced I(K(V))) was much greater than in hypoxia-insensitive cells (in which hypoxia negligibly affected I(K(V))). These results demonstrate that 1) different PASMC express different Kv channel alpha- and beta-subunits, and 2) the sensitivity of a PASMC to acute hypoxia partially depends on the expression level of Kv1.5 channels; hypoxia reduces whole-cell I(K(V)) only in PASMC that express high level of Kv1.5. In addition, the acute hypoxia-mediated changes in [Ca(2+)](cyt) also vary in different PASMC. Hypoxia increases [Ca(2+)](cyt) only in 34% of cells tested, and the different sensitivity of [Ca(2+)](cyt) to hypoxia was not related to the resting [Ca(2+)](cyt). An intrinsic mechanism within each individual cell may be involved in the heterogeneity of hypoxia-mediated effect on [Ca(2+)](cyt) in PASMC. These data suggest that the heterogeneity of PASMC may partially be related to different expression levels and functional sensitivity of Kv channels to hypoxia and to differences in intrinsic mechanisms involved in regulating [Ca(2+)](cyt).

Chronic intrauterine pulmonary hypertension increases capacitative calcium entry in fetal pulmonary artery smooth muscle cells

Oxygen causes perinatal pulmonary dilatation. Although fetal pulmonary artery smooth muscle cells (PA SMC) normally respond to an acute increase in oxygen (O2) tension with a decrease in cytosolic calcium ([Ca2+]i), an acute increase in O2 tension has no net effect on [Ca(2+)](i) in PA SMC derived from lambs with chronic intrauterine pulmonary hypertension (PHTN). The present experimental series tests the hypothesis that an acute increase in O2 tension decreases capacitative calcium entry (CCE) in normal, but not hypertensive, fetal PA SMC. PA SMC were isolated from late-gestation fetal lambs after either ligation of the ductus arteriosus (PHTN) or sham (control) operation at 127 days gestation. PA SMC were isolated from the distal PA (>or=4th generation) and maintained under hypoxic conditions ( approximately 25 Torr) in primary culture. After fura 2 loading, apparent [Ca2+]i in PA SMC was determined as the ratio of 340- to 380-nm fluorescence intensity. Under both hypoxic and normoxic conditions, cyclopiazonic acid (CPA) increased [Ca2+]i more in PHTN than in control PA SMC. CCE was determined in PA SMC under hypoxic and normoxic conditions, after superfusion with zero extracellular Ca2+ and intracellular store depletion with CPA, followed by superfusion with Ca2+-containing solution, in the presence of the voltage-operated calcium channel blockade. CCE was increased in PHTN compared with control PA SMC under conditions of both acute and sustained normoxia. Transient receptor potential channel gene expression was greater in control compared with PHTN PA SMC. PHTN may compromise perinatal pulmonary vasodilation, in part, by modulating PA SMC CCE.

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.

Segmental heterogeneity in the mechanism of sodium nitroprusside-induced relaxation in ovine pulmonary artery

Segmental heterogeneity in relaxation response to nitric oxide (NO) was examined using NO donor sodium nitroprusside (SNP) in second- (medium) and fourth-generation (small) ovine isolated intralobar pulmonary arteries. In vessels precontracted with serotonin, NO donors SNP and S-nitroso-N-acetylpenicillamine (SNAP) were more potent in relaxing medium, in comparison to the small, arteries. Soluble guanylyl cyclase (sGC) inhibitor [1,2,4]oxadiazolo-[4,3-a]quinoxaline-1-one (ODQ 3 microM) caused a profound inhibition of SNP relaxation in small as compared with medium-sized arteries. However, both basal and SNP (10 microM)-stimulated intracellular cyclic guanosine monophosphate (cGMP) content was identical in these 2 arterial segments. The Na,K-ATPase inhibitor ouabain (1 microM) had a marked inhibitory effect on SNP-mediated relaxation in both segments. There was no segmental difference in SNP (10 microM)-stimulated plasma membrane Na,K-ATPase activity and ouabain-sensitive Rb-uptake. 4-AP (1 mM), a relatively selective inhibitor of Kv channels, decreased the potency of SNP relaxation by about 10-fold in the medium-sized vessels. On the other hand, 4-AP was without effect on the vasodilator potency of SNP in small vessels. Interestingly, in the presence of 4-AP, SNP was equipotent in dilating both medium (pD2 = 5.80 +/- 0.07; Emax = 84 +/- 1.6%, n = 7) and small (pD2 = 5.74 +/- 0.15; Emax = 83 +/- 2.5%, n = 7) pulmonary arteries. In conclusion, the results of the present study suggest that Kv channels determine the segmental heterogeneity of NO-mediated relaxation in ovine pulmonary artery.

Oxygen tension modulates the expression of pulmonary vascular BKCa channel alpha- and beta-subunits

At birth, the lung environment changes from low to relatively high O(2) tension. Pulmonary blood flow increases and pulmonary artery pressure decreases. Recent data suggest that pulmonary vascular calcium-sensitive K(+) channel (BK(Ca)) activation mediates perinatal pulmonary vasodilation. Although BK(Ca) channel expression is developmentally regulated, the molecular mechanisms responsible for BK(Ca) expression remain unknown. We tested the hypothesis that the low-O(2) tension environment of the normal fetus modulates BK(Ca) channel expression. We analyzed BK(Ca) expression under conditions of hypoxia and normoxia both in vitro and in vivo. BK(Ca) alpha-subunit mRNA expression increased twofold in ovine pulmonary artery smooth muscle cell (PASMC) primary cultures maintained in hypoxia. In vivo, BK(Ca) expression was similarly affected by hypoxia. When adult Sprague-Dawley rats were placed in hypobaric hypoxic chambers for 3 wk, hypoxic animals showed an increase of threefold in the expression of BK(Ca) alpha- and more than twofold in the expression of BK(Ca) beta(1)-subunit mRNA. Immunochemical staining was consistent with the genetic data. To assess transcriptional activation of the beta-subunit of the BK(Ca), both BK(Ca) beta(1)- and beta(2)-subunit luciferase (K(Ca) beta:luc(+)) reporter genes were constructed. Hypoxia increased PASMC K(Ca) beta(1):luc(+) reporter expression by threefold and K(Ca) beta(2):luc(+) expression by 35%. Fetal PASMC treated with the hypoxia-inducible factor-1 mimetic deferoxamine showed a 63 and 41% increase in BK(Ca) channel alpha- and beta(1)-subunit expression, respectively. Together, these results suggest that oxygen tension modulates BK(Ca) channel subunit mRNA expression, and the regulation is, at least in part, at the transcriptional level.

Hypoxic pulmonary vasoconstriction: role of ion channels

Acute hypoxia induces pulmonary vasoconstriction and chronic hypoxia causes structural changes of the pulmonary vasculature including arterial medial hypertrophy. Electro- and pharmacomechanical mechanisms are involved in regulating pulmonary vasomotor tone, whereas intracellular Ca(2+) serves as an important signal in regulating contraction and proliferation of pulmonary artery smooth muscle cells. Herein, we provide a basic overview of the cellular mechanisms involved in the development of hypoxic pulmonary vasoconstriction. Our discussion focuses on the roles of ion channels permeable to K(+) and Ca(2+), membrane potential, and cytoplasmic Ca(2+) in the development of acute hypoxic pulmonary vasoconstriction and chronic hypoxia-mediated pulmonary vascular remodeling.

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.

Opposite effects of redox status on membrane potential, cytosolic calcium, and tone in pulmonary arteries and ductus arteriosus

At birth, associated with the rise in oxygen tension, the pulmonary arteries (PA) dilate and the ductus arteriosus (DA) constricts. Both PA and DA constrict with vasoconstrictors and dilate with vasodilators. They respond in a contrary manner only to changes in oxygen tension. We hypothesized that the effects of changes in oxygen are mediated by changes in redox status. Consequently, we tested whether a reducing agent, DTT, and an oxidizing agent, dithionitrobenzoic acid (DTNB), would have opposite effects on a major oxygen signaling pathway in the PA and DA smooth muscle cells (SMCs), the sequence of change in potassium current (IK), membrane potential (Em), cytosolic calcium, and vessel tone. Under normoxic conditions, DTT constricted adult and fetal resistance PA rings, whereas in DA rings DTT acted as a potent vasodilator. In normoxia, voltage-clamp measurements showed inhibition of IK by DTT in PASMCs and, in contrast, activation in DASMCs. Consequently, DTT depolarized fetal and adult PASMCs and hyperpolarized DASMCs. [Ca2+]i was increased by DTT in fetal and adult PASMCs and decreased in DASMCs. Under hypoxic conditions, DTNB constricted DA rings and caused vasodilatation in fetal PA rings. DTNB inhibited IK and depolarized the cell membrane in DASMCs. In contrast, activation of IK and hyperpolarization was seen in PASMCs. Thus the same redox signal can elicit opposite effects on IK, Em, cytosolic calcium, and vascular tone in resistance PA and the DA. These observations support the concept that redox changes could signal the opposite effects of oxygen in the PA and DA.

Chronic intrauterine pulmonary hypertension compromises fetal pulmonary artery smooth muscle cell O2 sensing

To test the hypothesis that chronic intrauterine pulmonary hypertension (PHTN) compromises pulmonary artery (PA) smooth muscle cell (SMC) O2 sensing, fluorescence microscopy was used to study the effect of an acute increase in Po2 on the cytosolic Ca2+ concentration ([Ca2+]i) of chronically hypoxic subconfluent monolayers of PA SMC in primary culture. PA SMCs were derived from fetal lambs with PHTN due to intrauterine ligation of the ductus arteriosus. Acute normoxia decreased [Ca2+]i in control but not PHTN PA SMC. In control PA SMC, [Ca2+]i increased after Ca2+-sensitive (KCa) and voltage-sensitive (Kv) K+ channel blockade and decreased after diltiazem treatment. In PHTN PA SMC, KCa blockade had no effect, whereas Kv blockade and diltiazem increased [Ca2+]i. Inhibition of sarcoplasmic reticulum Ca2+ ATPase activity caused a greater increase in [Ca2+]i in controls compared with PHTN PA SMC. Conversely, ryanodine caused a greater increase of [Ca2+]i in PHTN compared with control PA SMC. KCa channel mRNA is decreased and Kv channel mRNA is unchanged in PHTN PA SMC compared with controls. We conclude that PHTN compromises PA SMC O2 sensing, alters intracellular Ca2+ homeostasis, and changes the predominant ion channel that determines basal [Ca2+]i from KCa to Kv.

Contribution of the K(Ca) channel to membrane potential and O2 sensitivity is decreased in an ovine PPHN model

Ca2+-sensitive K+ (K(Ca)) channels play an important role in mediating perinatal pulmonary vasodilation. We hypothesized that lung K(Ca) channel function may be decreased in persistent pulmonary hypertension of the newborn (PPHN). To test this hypothesis, pulmonary artery smooth muscle cells (PASMC) were isolated from fetal lambs with severe pulmonary hypertension induced by ligation of the ductus arteriosus in fetal lambs at 125-128 days gestation. Fetal lambs were killed after pulmonary hypertension had been maintained for at least 7 days. Age-matched, sham-operated animals were used as controls. PASMC K+ currents and membrane potentials were recorded using amphotericin B-perforated patch-clamp techniques. The increase in whole cell current normally seen in response to normoxia was decreased (333.9 +/- 63.6% in control vs. 133.1 +/- 16.0% in hypertensive fetuses). The contribution of the K(Ca) channel to the whole cell current was diminished in hypertensive, compared with control, fetal PASMC. In PASMC from hypertensive fetuses, a change from hypoxia to normoxia caused no change in membrane potential compared with a -14.6 +/- 2.8 mV decrease in membrane potential in PASMC from control animals. In PASMC from animals with pulmonary hypertension, 4-aminopyridine (4-AP) caused a larger depolarization than iberiotoxin, whereas in PASMC from control animals, iberiotoxin caused a larger depolarization than 4-AP. These data confirm the hypothesis that the contribution of the K(Ca) channel to membrane potential and O2 sensitivity is decreased in an ovine model of PPHN, and this may contribute to the abnormal perinatal pulmonary vasoreactivity associated with PPHN.

Mechanisms of bradykinin-mediated dilation in newborn piglet pulmonary conducting and resistance vessels

Bradykinin (BK) is a potent dilator of the perinatal pulmonary circulation. We investigated segmental differences in BK-induced dilation in newborn pig large conducting pulmonary artery and vein rings and in pressurized pulmonary resistance arteries (PRA). In conducting pulmonary arteries and veins, BK-induced relaxation is abolished by endothelial disruption and by inhibition of nitric oxide (NO) synthase with nitro-L-arginine (L-NA). In PRA, two-thirds of the dilation response is L-NA insensitive. Charybdotoxin plus apamin and depolarization with KCl abolish the L-NA-insensitive dilations, findings that implicate the release of endothelium-derived hyperpolarizing factor (EDHF). However, endothelium-disrupted PRA retain the ability to dilate to BK but not to ACh or A-23187. In endothelium-disrupted PRA, dilation was inhibited by charybdotoxin. Thus in PRA, BK elicits dilation by multiple and duplicative signaling pathways. Release of NO and EDHF contributes to the response in endothelium-intact PRA; in endothelium-disrupted PRA, dilation occurs by direct activation of vascular smooth muscle calcium-dependent potassium channels. Redundant signaling pathways mediating pulmonary dilation to BK may be required to assure a smooth transition to extrauterine life.

Fetal and adult cerebral artery K(ATP) and K(Ca) channel responses to long-term hypoxia

High-altitude long-term hypoxia (LTH) alters cerebral vascular contractile and relaxation responses in both fetus and adult. We tested the hypotheses that LTH-mediated vascular responses were secondary to altered K+ channel function and that in the fetus these responses differ from those of the adult. In middle cerebral arteries (MCA) from both nonpregnant adult and fetal (approximately 140 days gestation) sheep, which were either acclimatized to high altitude (3,820 m) or sea-level controls, we measured norepinephrine (NE)-induced contractions and intracellular Ca2+ concentration ([Ca2+]i) simultaneously, in the presence or absence of different K+ channel openers or blockers. In adult MCA, LTH was associated with approximately 20% decrease in NE-induced tension and [Ca2+]i, with a significant increase in Ca2+ sensitivity. In contrast, in fetal MCA, LTH failed to affect significantly NE-induced contraction or [Ca2+]i but significantly decreased the ATP-sensitive K+ (K(ATP)) channel and Ca2+-activated K+ (K(Ca)) channel-mediated relaxation. The significant effect of K(ATP) and K(Ca) channel activators on the relaxation responses and the fact that K+ channels play a key role in myogenic tone support the hypotheses that K+ channels play an important role in hypoxia-mediated responses. These results also support the hypothesis of significant developmental differences with maturation from fetus to adult.

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.

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.

Molecular mechanisms of phenotypic plasticity in smooth muscle cells

Morphological, functional, molecular and cell biology studies have revealed a striking multifunctional nature of individual smooth muscle cells (SMC). SMCs manifest phenotypic plasticity in response to changes in environment and functional requirements, acquiring a range of structural and functional properties bounded by two extremes, called "synthetic" and "contractile." Each phenotypic state is characterized by expression of a unique set of structural, contractile, and receptor proteins and isoforms that correlate with differing patterns of gene expression. Recent studies have identified signaling pathways and transcription factors (e.g., RhoA GTPase/ROCK, also known as Rho kinase, and serum response factor) that regulate the transcriptional activities of genes encoding proteins associated with the contractile apparatus. Mechanical plasticity of contractile-state smooth muscle further extends SMC functional diversity. This may also be regulated, in part, by the RhoA GTPase/ROCK pathway, via reorganization of cytoskeletal and contractile proteins. Future studies that define transcriptional and posttranscriptional mechanisms of SMC plasticity are necessary to fully understand the role of SMC in the pathogenesis and morbidity of human diseases of the airways, vasculature, and gastrointestinal tract.

Cerebral artery K(ATP)- and K(Ca)-channel activity and contractility: changes with development

The present study was designed to test the hypothesis that in cerebral arteries of the fetus, ATP-sensitive (K(ATP)) and Ca(2+)-activated K(+) channels (K(Ca)) play an important role in the regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) and that this differs significantly from that of the adult. In main branch middle cerebral arteries (MCA) from near-term fetal ( approximately 140 days) and nonpregnant adult sheep, simultaneously we measured norepinephrine (NE)-induced responses of vascular tension and [Ca(2+)](i) in the absence and presence of selective K(+)-channel openers/blockers. In fetal MCA, in a dose-dependent manner, both the K(ATP)-channel opener pinacidil and the K(Ca)-channel opener NS 1619 significantly inhibited NE-induced tension [negative logarithm of the half-maximal inhibitory concentration (pIC(50)) = 5.0 +/- 0.1 and 8.2 +/- 0.1, respectively], with a modest decrease of [Ca(2+)](i). In the adult MCA, in contrast, both pinacidil and NS 1619 produced a significant tension decrease (pIC(50) = 5.1 +/- 0.1 and 7.6 +/- 0.1, respectively) with no change in [Ca(2+)](i). In addition, the K(Ca)-channel blocker iberiotoxin (10(-7) to 10(-6) M) resulted in increased tension and [Ca(2+)](i) in both adult and fetal MCA, although the K(ATP)-channel blocker glibenclamide (10(-7) to 3 x 10(-5) M) failed to do so. Of interest, administration of 10(-7) M iberiotoxin totally eliminated vascular contraction and increase in [Ca(2+)](i) seen in response to 10(-5) M ryanodine. In precontracted fetal cerebral arteries, activation of the K(ATP) and K(Ca) channels significantly decreased both tension and [Ca(2+)](i), suggesting that both K(+) channels play an important role in regulating L-type channel Ca(2+) flux and therefore vascular tone in these vessels. In the adult, K(ATP) and the K(Ca) channels also appear to play an important role in this regard; however, in the adult vessel, activation of these channels with resultant vasorelaxation can occur with no significant change in [Ca(2+)](i). These channels show differing responses to inhibition, e.g., K(Ca)-channel inhibition, resulting in increased tension and [Ca(2+)](i), whereas K(ATP)-channel inhibition showed no such effect. In addition, the K(Ca) channel appears to be coupled to the sarcoplasmic reticulum ryanodine receptor. Thus differences in plasma membrane K(+)-channel activity may account, in part, for the differences in the regulation of contractility of fetal and adult cerebral arteries.

Fetal rabbit pulmonary artery smooth muscle cell response to ryanodine is developmentally regulated

To study developmental changes in intracellular calcium handling in pulmonary artery smooth muscle cells (PASMCs), cells were isolated from distal and proximal pulmonary arteries from rabbits at different developmental stages: juvenile (4-6 wk old), newborn (<48 h), and full-term fetal. Isolated PASMCs were studied using the calcium-sensitive dye fura 2. Cells from each age group responded to caffeine with an increase in calcium; however, ryanodine (50 microM) only increased calcium in fetal distal PASMCs. The ryanodine-induced increase was due to influx of extracellular calcium because it was blocked by removal of extracellular calcium or by diltiazem. The calcium-sensitive potassium (K(Ca)) channel blocker iberiotoxin produced a transient increase in calcium in the fetal distal PASMCs, which could be inhibited by prior application of ryanodine. Conversely, the ryanodine response was inhibited if iberiotoxin was given first. With the use of electrophysiology and confocal microscopy, fetal PASMCs were shown to exhibit spontaneous transient outward currents and calcium sparks, respectively. These observations suggest that ryanodine-sensitive release of calcium from the sarcoplasmic reticulum and K(Ca) channels act together to control intracellular calcium only in fetal distal PASMCs.

Voltage-gated K(+)-channel activity in ovine pulmonary vasculature is developmentally regulated

To examine mechanisms underlying developmental changes in pulmonary vascular tone, we tested the hypotheses that 1) maturation-related changes in the ability of the pulmonary vasculature to respond to hypoxia are intrinsic to the pulmonary artery (PA) smooth muscle cells (SMCs); 2) voltage-gated K(+) (K(v))-channel activity increases with maturation; and 3) O(2)-sensitive Kv2.1 channel expression and message increase with maturation. To confirm that maturational differences are intrinsic to PASMCs, we used fluorescence microscopy to study the effect of acute hypoxia on cytosolic Ca(2+) concentration ([Ca(2+)](i)) in SMCs isolated from adult and fetal PAs. Although PASMCs from both fetal and adult circulations were able to sense an acute decrease in O(2) tension, acute hypoxia induced a more rapid and greater change in [Ca(2+)](i) in magnitude in PASMCs from adult compared with fetal PAs. To determine developmental changes in K(v)-channel activity, the effects of the K(+)-channel antagonist 4-aminopyridine (4-AP) were studied on fetal and adult PASMC [Ca(2+)](i). 4-AP (1 mM) caused PASMC [Ca(2+)](i) to increase by 94 +/- 22% in the fetus and 303 +/- 46% in the adult. K(v)-channel expression and mRNA levels in distal pulmonary arteries from fetal, neonatal, and adult sheep were determined through the use of immunoblotting and semiquantitative RT-PCR. Both Kv2.1-channel protein and mRNA expression in distal pulmonary vasculature increased with maturation. We conclude that there are maturation-dependent changes in PASMC O(2) sensing that may render the adult PASMCs more responsive to acute hypoxia.