The electrophysiological effects of Tongyang Huoxue granules on the ignition phase during hypoxia/reoxygenation injury in sinoatrial node cells

Introduction

This study was undertaken to explore the potential therapeutic effects of Tongyang Huoxue Granules (TYHX) on sinoatrial node (SAN) dysfunction, a cardiac disorder characterized by impaired impulse generation or conduction. The research question addressed whether TYHX could positively influence SAN ion channel function, specifically targeting the sodium-calcium exchanger (I NCX) and L-type calcium channel (I CaL) of the SAN.

Methods

Sinoatrial node cells (SANCs) were isolated and cultured from neonatal Japanese big-eared white rabbits within 24 h of birth. The study encompassed five groups: Control, H/R (hypoxia/reoxygenation), H/R+100 μg/mL TYHX, H/R+200 μg/mL TYHX, and H/R+400 μg/mL TYHX. The H/R model, simulating hypoxia/reoxygenation stress, was induced within 5 days of culture. Whole-cell patch clamp technique was employed to record currents following a 3-min perfusion and stabilization period with TYHX.

Results

TYHX administration demonstrated improvements in the ignition phase of impaired SANCs. The half-maximal effective dose of TYHX, as determined by SANC beating frequency, was found to be 323.63 μg/mL. Inward current density of I NCX increased in response to TYHX (200 and 400 μg/mL), while TYHX enhanced I CaL current density in H/R SANCs, with 400 μg/mL exhibiting greater efficacy. Additionally, TYHX regulated the gating mechanisms of I CaL by right-shifting the steady-state inactivation curve and accelerating recovery from inactivation. Notably, TYHX increased the activation time constant under 200 and 400 μg/mL, prolonged the fast inactivation time constant τ1 with 400 μg/mL, and extended the slow inactivation time constant τ2 with 100 and 400 μg/mL.

Discussion and conclusion

The findings suggest that TYHX may hold promise as a therapeutic intervention for sinus node dysfunction, offering potential avenues for drug development aimed at safeguarding SAN function.

Metabolic syndrome inhibits store-operated Ca2+ entry and calcium-induced calcium-release mechanism in coronary artery smooth muscle

BACKGROUND AND PURPOSE

Metabolic syndrome causes adverse effects on the coronary circulation including altered vascular responsiveness and the progression of coronary artery disease (CAD). However the underlying mechanisms linking obesity with CAD are intricated. Augmented vasoconstriction, mainly due to impaired Ca²⁺ homeostasis in coronary vascular smooth muscle (VSM), is a critical factor for CAD. Increased calcium-induced calcium release (CICR) mechanism has been associated to pathophysiological conditions presenting persistent vasoconstriction while increased store operated calcium (SOC) entry appears to activate proliferation and migration in coronary vascular smooth muscle (VSM). We analyze here whether metabolic syndrome might alter SOC entry as well as CICR mechanism in coronary arteries, contributing thus to a defective Ca²⁺ handling and therefore accelerating the progression of CAD.

EXPERIMENTAL APPROACH

Measurements of intracellular Ca²⁺ ([Ca²⁺]_i) and tension and of Ca²⁺ channels protein expression were performed in coronary arteries (CA) from lean Zucker rats (LZR) and obese Zucker rats (OZR).

KEY RESULTS

SOC entry stimulated by emptying sarcoplasmic reticulum (SR) Ca²⁺ store with cyclopiazonic acid (CPA) was decreased and associated to decreased STIM-1 and Orai1 protein expression in OZR CA. Further, CICR mechanism was blunted in these arteries but Ca²⁺ entry through voltage-dependent L-type channels was preserved contributing to maintain depolarization-induced increases in [Ca²⁺]_i and vasoconstriction in OZR CA. These results were associated to increased expression of voltage-operated L-type Ca²⁺ channel alpha 1C subunit (Ca_V1.2) but unaltered ryanodine receptor (RyR) and sarcoendoplasmic reticulum Ca²⁺-ATPase (SERCA) pump protein content in OZR CA.

CONCLUSION AND IMPLICATIONS

The present manuscript provides evidence of impaired Ca²⁺ handling mechanisms in coronary arteries in metabolic syndrome where a decrease in both SOC entry and CICR mechanism but preserved vasoconstriction are reported in coronary arteries from obese Zucker rats. Remarkably, OZR CA VSM at this state of metabolic syndrome seemed to have developed a compensation mechanism for impaired CICR by overexpressing Ca_V1.2 channels.

Role of Orai1 and L-type CaV1.2 channels in Endothelin-1 mediated coronary contraction under ischemia and reperfusion

Ischemia and Reperfusion (I/R) injuries are associated with coronary artery hypercontracture. They are mainly originated by an exacerbated response to agonists released by endothelium such as Endothelin (ET-1), involving the alteration in intracellular calcium handling. Recent evidences have highlighted the implication of Store-Operated Calcium Channels (SOCC) in intracellular calcium homeostasis in coronary artery. However, little is known about the role of SOCC in the regulation of coronary vascular tone under I/R. The aim of this study was to evaluate the role of SOCC and l-type Ca²⁺ channels (LTCC) in coronary artery vasoconstriction originated by ET-1 in I/R. We used Left Anterior Descendent coronary artery (LAD) rings, isolated from Wistar rats, to study the contractility and intracellular Ca²⁺ concentration ([Ca²⁺]_i) under a simulated I/R protocol. We observed that responses to high-KCL induced depolarization and caffeine-induced Ca²⁺ release are attenuated in coronary artery under I/R. Furthermore, ET-1 addition in ischemia promotes transient and small rise of [Ca²⁺]_i and coronary vascular tone. Meanwhile, these effects are significantly potentiated during reperfusion. The resulting ET-1-induced vasoconstrictions and [Ca²⁺]_i increase were abolished by; GSK-7975A and gadolinium, inhibitors of SOCC; and nifedipine a widely used inhibitor of LTCC. Interestingly, using in situ Proximity Ligation Assay (PLA) in isolated coronary smooth muscle cells we found significant colocalization of LTCC Ca_V1.2 isoform with Orai1, the pore forming subunit of SOCC, and TRPC1 under I/R. Our data suggest that hypercontraction of coronary artery induced by ET-1 after I/R involves the co-activation of LTCC and SOCC, which colocalize significantly in the sarcolemma of coronary smooth muscle cells.

Mitochondrial transplantation attenuates hypoxic pulmonary vasoconstriction

Hypoxia triggers pulmonary vasoconstriction, however induces relaxation of systemic arteries such as femoral arteries. Mitochondria are functionally and structurally heterogeneous between different cell types. The aim of this study was to reveal whether mitochondrial heterogeneity controls the distinct responses of pulmonary versus systemic artery smooth muscle cells to hypoxia. Intact mitochondria were transplanted into Sprague-Dawley rat pulmonary artery smooth muscle cells in culture and pulmonary arteries in vitro. Mitochondria retained functional after transplantation. The cross transplantation of mitochondria between pulmonary and femoral artery smooth muscle cells reversed acute hypoxia-induced alterations in cell membrane potential, [Ca²⁺]_i signaling in smooth muscle cells and constriction or relaxation of arteries. Furthermore, the high or low amount of reactive oxygen species generation from mitochondria and their divergent (dis-)abilities in activating extracellular Ca²⁺-sensing receptor in smooth muscle cells were found to cause cell membrane potential depolarization, [Ca²⁺]_i elevation and constriction of pulmonary arteries versus cell membrane potential hyperpolarization, [Ca²⁺]_i decline and relaxation of femoral arteries in response to hypoxia, respectively. Our findings suggest that mitochondria necessarily determine the behaviors of vascular smooth muscle cells in response to hypoxia.

Regulation of Contractile Responses of Vascular Smooth Muscle Cells under Conditions of Hypoxia-Reoxygenation

We analyzed the effects of hypoxia and reoxygenation on changes in contractile activity in rat aortic smooth muscles. Both hypoxia and reoxygenation induced relaxation of smooth muscle cells precontracted with high-potassium Krebs solution (30 mM KCl) or α₁-adrenoceptor agonist phenylephrine. Vasodilation resulted from enhancement of potassium permeability of smooth muscle cell membranes caused by activation of voltage-gated potassium channels (triggered by both precontracting agents) or by opening of ATP-sensitive potassium channels (phenylephrine). In isolated smooth muscle cells, both hypoxia and inhibition of Na⁺,K⁺-ATPase with ouabain led to depletion of intracellular store of macroergic substances, reduced potassium concentration, and elevated the content of sodium ions.

Aging Reduces L-Type Calcium Channel Current and the Vasodilatory Response of Small Mesenteric Arteries to Calcium Channel Blockers

Calcium channel blockers (CCBs) are widely used to treat cardiovascular disease (CVD) including hypertension. As aging is an independent risk factor for CVD, the use of CCBs increases with increasing age. Hence, this study was designed to evaluate the effect of aging on the sensitivity of small mesenteric arteries to L-type voltage-gated calcium channel (LTCC) blockers and also to investigate whether there was a concomitant change in calcium current density. Third order mesenteric arteries from male F344 rats, aged 2.5-3 months (young) and 22-26 months (old) were mounted on wire myograph to measure the tension during isometric contraction. Arteries were contracted with 100 mM KCl and were then relaxed in a cumulative concentration-response dependent manner with nifedipine (0.1 nM-1 μM), verapamil (0.1 nM-10 μM), or diltiazem (0.1 nM-10 μM). Relaxation-concentration response curves produced by cumulative concentrations of three different CCBs in arteries of old rats were shifted to the right with statistically significant IC50s. pIC50 ± s.e.m: (8.37 ± 0.06 vs. 8.04 ± 0.05, 7.40 ± 0.07 vs. 6.81 ± 0.04, and 6.58 ± 0.07 vs. 6.34 ± 0.06) in young vs. old. It was observed that the maximal contractions induced by phenylephrine and reversed by sodium nitroprusside were not different between young and old groups. However, Bay K 8644 (1 μM) increased resting tension by 23 ± 4.8% in young arteries and 4.7 ± 1.6% in old arteries. LTCC current density were also significantly lower in old arteries (-2.77 ± 0.45 pA/pF) compared to young arteries (-4.5 ± 0.40 pA/pF); with similar steady-state activation and inactivation curves. Parallel to this reduction, the expression of Cav1.2 protein was reduced by 57 ± 5% in arteries from old rats compared to those from young rats. In conclusion, our results suggest that aging reduces the response of small mesenteric arteries to the vasodilatory effect of the CCBs and this may be due to, at least in part, reduced current density of LTCC.

Soluble guanylate cyclase redox state under hypoxia or hypoxia/reoxygenation in isolated monkey coronary arteries

Hypoxia or hypoxia/reoxygenation impairs nitric oxide (NO)-mediated relaxation through the increase in superoxide generation in monkey coronary arteries. Soluble guanylate cyclase (sGC), the target enzyme of NO, has been shown to change from the NO-sensitive reduced form to the NO-insensitive oxidized/heme-free form under substantial oxidative stress, so the present study investigated whether hypoxia or hypoxia/reoxygenation influences sGC redox equilibrium. In isolated monkey coronary arteries without endothelium, the relaxation caused by the sGC stimulator BAY 41-2272 (Emax: 93.3% ± 2.2%) was somewhat impaired under hypoxia (Emax: 86.3% ± 2.6%) or hypoxia/reoxygenation (Emax: 86.1% ± 3.2%), whereas that by the sGC activator BAY 60-2770 (Emax: 86.0% ± 3.2%) was significantly augmented under hypoxia (Emax: 94.4% ± 1.3%) or hypoxia/reoxygenation (Emax: 95.5% ± 1.1%). In addition, cGMP formation in response to BAY 41-2272 and BAY 60-2770 was inhibited and stimulated, respectively, under hypoxia or hypoxia/reoxygenation. The effects of hypoxia or hypoxia/reoxygenation on BAY 41-2272- and BAY 60-2770-induced vasorelaxation were completely canceled by the treatment with the superoxide dismutase mimetic tempol. These findings suggest that sGC redox equilibrium in the coronary artery is shifted towards the NO-insensitive form under hypoxia or hypoxia/reoxygenation and that superoxide seems to play an important role in this shift.

Protection of vascular endothelial cells injured by angiotensin II and hypoxia in vitro by Ginkgo biloba (Ginaton)

The objective of this study was to explore the protective effect and the possible mechanism of Ginkgo biloba extract (Ginaton) on human vascular endothelial cells (VECs) injured by angiotensin II (Ang-II) and hypoxia. The human aortic VECs were divided into different groups to observe the changes in endothelin (ET), calcium concentration ([Ca(2+)]i), and mitochondrial membrane potential (MMP). The results showed that Ginaton had inhibited ET secretion induced by hypoxia and Ang-II (P < .01). the protection offered by Ginaton at mid (10 mg/mL) and low (5 mg/mL) concentrations was obviously better than that offered at high concentration (25 mg/mL). The [Ca(2+)]i increased and MMP decreased significantly in both hypoxia group and Ang-II group (P < .01); however, the changes in [Ca(2+)]i and MMP could be meliorated by Ginaton. This study suggested that Ginaton could effectively protect VECs against injury, and the dose used clinically would rather be low than too high for getting better results.

Acute simvastatin inhibits K ATP channels of porcine coronary artery myocytes

Background

Statins (3-hydroxy-3-methyl-glutaryl coenzyme A (HMG-CoA) reductase inhibitors) consumption provides beneficial effects on cardiovascular systems. However, effects of statins on vascular K_ATP channel gatings are unknown.

Methods

Pig left anterior descending coronary artery and human left internal mammary artery were isolated and endothelium-denuded for tension measurements and Western immunoblots. Enzymatically-dissociated/cultured arterial myocytes were used for patch-clamp electrophysiological studies and for [Ca²⁺]_i, [ATP]_i and [glucose]_o uptake measurements.

Results

The cromakalim (10 nM to 10 µM)- and pinacidil (10 nM to 10 µM)-induced concentration-dependent relaxation of porcine coronary artery was inhibited by simvastatin (3 and 10 µM). Simvastatin (1, 3 and 10 µM) suppressed (in okadaic acid (10 nM)-sensitive manner) cromakalim (10 µM)- and pinacidil (10 µM)-mediated opening of whole-cell K_ATP channels of arterial myocytes. Simvastatin (10 µM) and AICAR (1 mM) elicited a time-dependent, compound C (1 µM)-sensitive [³H]-2-deoxy-glucose uptake and an increase in [ATP]_i levels. A time (2–30 min)- and concentration (0.1–10 µM)-dependent increase by simvastatin of p-AMPKα-Thr¹⁷² and p-PP2A-Tyr³⁰⁷ expression was observed. The enhanced p-AMPKα-Thr¹⁷² expression was inhibited by compound C, ryanodine (100 µM) and KN93 (10 µM). Simvastatin-induced p-PP2A-Tyr³⁰⁷ expression was suppressed by okadaic acid, compound C, ryanodine, KN93, phloridzin (1 mM), ouabain (10 µM), and in [glucose]_o-free or [Na⁺]_o-free conditions.

Conclusions

Simvastatin causes ryanodine-sensitive Ca²⁺ release which is important for AMPKα-Thr¹⁷² phosphorylation via Ca²⁺/CaMK II. AMPKα-Thr¹⁷² phosphorylation causes [glucose]_o uptake (and an [ATP]_i increase), closure of K_ATP channels, and phosphorylation of AMPKα-Thr¹⁷² and PP2A-Tyr³⁰⁷ resulted. Phosphorylation of PP2A-Tyr³⁰⁷ occurs at a site downstream of AMPKα-Thr¹⁷² phosphorylation.

Protective Effects of Ginaton on Vascular Endothelial Cells Injured by Angiotensin II and Hypoxia In Vitro

The objective of this study was to explore the protective effect and possible mechanism of Ginkgo biloba extract (Ginaton) on human vascular endothelial cells (VECs) injured by angiotensin II (Ang-II) and hypoxia. The human aortic VECs were divided into different groups to observe the changes in endothelin (ET), intracellular calcium concentration ([Ca2+]i), and mitochondrial membrane potential (MMP). The results showed that Ginaton had inhibited ET secretion induced by hypoxia and Ang-II ( P < .01); the protective effects of mid (10 mg/mL) and low concentrations (5 mg/mL) of Ginaton was obviously higher than that of the high concentration (25 mg/mL); [Ca2+]i increased and MMP decreased significantly in both the hypoxia and the Ang-II groups ( P < .01); however, the changes in [Ca2+]i and MMP could be meliorated by Ginaton. This study suggested that Ginaton could effectively protect VECs against injury, and lower dose would be used clinically rather than the higher dose for obtaining better results.

Tonic arterial contraction mediated by L-type Ca2+ channels requires sustained Ca2+ influx, G protein-associated Ca2+ release, and RhoA/ROCK activation

KCl-evoked sustained contraction requires L-type Ca(2+) channel activation, metabotropic Ca(2+) release from the sarcoplasmic reticulum (mechanism denoted calcium channel-induced Ca(2+) release) and RhoA/Rho associated kinase activation. Although high K(+) solutions are used to depolarize myocytes, these solutions can stimulate other signaling pathways such as those triggered by the activation of muscarinic and purinergic receptors. The present study examines the functional role of calcium channel-induced Ca(2+) release under pharmacological activation of L-type Ca(2+) channel without significant membrane depolarization. It also analyzes the role of the "steady-state" Ca(2+) influx through L-type Ca(2+) channels on myocyte sustained contraction. Measurement of contractility in arterial rings was done on a vessel myograph. Membrane potential was measured by fluorescence techniques loading intact myocytes with a membrane potential sensitive dye, and a reversible permeabilization method was used to load myocytes in intact arteries with GDPβS and Ca(v)1.2 siRNA. Application of an L-type Ca(2+) channel agonist, without effect on membrane potential, evoked sustained contraction via G-protein induced Ca(2+) release from the sarcoplasmic reticulum and RhoA/Rho associated kinase activation. Tonic myocyte contractions mediated by L-type Ca(2+) channel activation required sustained Ca(2+) influx through the channels and Ca(2+) uptake by the sarcoplasmic reticulum. Because L-type Ca(2+) channels participate in numerous pathophysiological processes mediated by maintained arterial contraction, our data could help to optimize therapeutic treatment of arterial vasospasm.

Metabotropic regulation of RhoA/Rho-associated kinase by L-type Ca2+ channels

Sustained vascular smooth muscle contraction can be mediated by several mechanisms, including the influx of extracellular Ca(2+) through L-type voltage-gated Ca(2+) channels (LTCCs) and by RhoA/Rho-associated kinase (ROCK)-dependent Ca(2+) sensitization of the contractile machinery. Conformational changes in the LTCC following depolarization can also trigger an ion-independent metabotropic pathway that involves G protein/phospholipase C activation, giving rise to inositol 1,4,5-trisphosphate synthesis and subsequent Ca(2+) release from the sarcoplasmic reticulum (SR) (calcium channel-induced Ca(2+) release or calcium channel-induced calcium release [CCICR]). In this review, we summarize recent data suggesting that LTCC activation and subsequent metabotropic Ca(2+) release from the SR participate in depolarization-evoked RhoA/ROCK activity and sustained arterial contraction. During protracted depolarizations, refilling of the SR stores by a residual influx of extracellular Ca(2+) through LTCCs helps maintain RhoA activity and contractile activation. These findings suggest that CCICR plays a major role in tonic vascular smooth muscle contraction, providing a link between membrane depolarization-induced LTCC activation and metabotropic Ca(2+) release and RhoA/ROCK stimulation.

Urocortin-2 induces vasorelaxation of coronary arteries isolated from patients with heart failure

1. Urocortin-2 (Ucn2) is a vasoactive peptide belonging to the corticotrophin-releasing factor (CRF) family that has potent cardiovascular actions. It has been suggested that Ucn2 participates in the pathophysiology of heart failure. However, little is known about the mechanisms underlying the action of Ucn2 in human coronary arteries. The aim of the present study was to assess the effects of Ucn2 on the vascular tone of human coronary arteries dissected from heart failure patients. 2. Human coronary arteries were dissected from the hearts of patients subjected to orthotopic heart transplantation. Coronary arteries were obtained from 17 patients with heart failure due to dilated cardiomyopathy of ischaemic origin in Stage III-IV of the New York Heart Association classification. Changes in tone were measured in arterial rings using force transducers. 3. Application of increasing concentrations of Ucn2 (5-20 nmol/L) to arterial rings precontracted with agonists induced dose-dependent relaxation of the coronary artery, which was independent of endothelial cell activation. Furthermore, the inhibition of the adenylyl cyclase by MDL-12 (100 nmol/L) and protein kinase A (PKA) by H89 (1 μmol/L) prevented Ucn2-mediated relaxation of coronary artery rings. 4. The results of the present study suggest that, in heart failure patients, Ucn2 could be useful in modulating coronary artery circulation independent of endothelial integrity through mechanisms that involve adenylyl cyclase activation and PKA stimulation. The findings warrant further investigation of the role of Ucn2 in circulatory regulation and its potential therapeutic application in heart disease.

Metabotropic regulation of RhoA/Rho-associated kinase by L-type Ca2+ channels: new mechanism for depolarization-evoked mammalian arterial contraction

BACKGROUND

Sustained vascular smooth muscle contraction is mediated by extracellular Ca(2+) influx through L-type voltage-gated Ca(2+) channels (VGCC) and RhoA/Rho-associated kinase (ROCK)-dependent Ca(2+) sensitization of the contractile machinery. VGCC activation can also trigger an ion-independent metabotropic pathway that involves G-protein/phospholipase C activation, inositol 1,4,5-trisphosphate synthesis, and Ca(2+) release from the sarcoplasmic reticulum (calcium channel-induced Ca(2+) release). We have studied the functional role of calcium channel-induced Ca(2+) release and the inter-relations between Ca(2+) channel and RhoA/ROCK activation.

METHODS AND RESULTS

We have used normal and genetically modified animals to study single myocyte electrophysiology and fluorimetry as well as cytosolic Ca(2+) and diameter in intact arteries. These analyses were complemented with measurement of tension and RhoA activity in normal and reversibly permeabilized arterial rings. We have found that, unexpectedly, L-type Ca(2+) channel activation and subsequent metabotropic Ca(2+) release from sarcoplasmic reticulum participate in depolarization-evoked RhoA/ROCK activity and sustained arterial contraction. We show that these phenomena do not depend on the change in the membrane potential itself, or the mere release of Ca(2+) from the sarcoplasmic reticulum, but they require the simultaneous activation of VGCC and the downstream metabotropic pathway with concomitant Ca(2+) release. During protracted depolarizations, refilling of the stores by a residual extracellular Ca(2+) influx through VGCC helps maintaining RhoA activity and sustained arterial contraction.

CONCLUSIONS

These findings reveal that calcium channel-induced Ca(2+) release has a major role in tonic vascular smooth muscle contractility because it links membrane depolarization and Ca(2+) channel activation with metabotropic Ca(2+) release and sensitization (RhoA/ROCK stimulation).

Short communication: genetic ablation of L-type Ca2+ channels abolishes depolarization-induced Ca2+ release in arterial smooth muscle

RATIONALE

In arterial myocytes, membrane depolarization-induced Ca(2+) release (DICR) from the sarcoplasmic reticulum (SR) occurs through a metabotropic pathway that leads to inositol trisphosphate synthesis independently of extracellular Ca(2+) influx. Despite the fundamental functional relevance of DICR, its molecular bases are not well known.

OBJECTIVE

Biophysical and pharmacological data have suggested that L-type Ca(2+) channels could be the sensors coupling membrane depolarization to SR Ca(2+) release. This hypothesis was tested using smooth muscle-selective conditional Ca(v)1.2 knockout mice.

METHODS AND RESULTS

In aortic myocytes, the decrease of Ca(2+) channel density was paralleled by the disappearance of SR Ca(2+) release induced by either depolarization or Ca(2+) channel agonists. Ca(v)1.2 channel deficiency resulted in almost abolition of arterial ring contraction evoked by DICR. Ca(2+) channel-null cells showed unaltered caffeine-induced Ca(2+) release and contraction.

CONCLUSION

These data suggest that Ca(v)1.2 channels are indeed voltage sensors coupled to the metabolic cascade, leading to SR Ca(2+) release. These findings support a novel, ion-independent, functional role of L-type Ca(2+) channels linked to intracellular signaling pathways in vascular myocytes.

The role of ion channels in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is an important mechanism by which localized flow of blood in small resistance pulmonary arteries is matched to alveolar ventilation. This chapter discusses the role of several potassium and calcium channels in HPV, both in enhancing calcium influx into smooth muscle cells (SMCs) and in stimulating the release of calcium from the sarcoplasmic reticulum, thus increasing cytosolic calcium. The increase in calcium sensitivity caused by hypoxia is reviewed in Chapter 19. Particular attention is paid to the activity of the L-type calcium channels which increase calcium influx as a result of membrane depolarization and also increase calcium influx at any given membrane potential in response to hypoxia. In addition, activation of the L-type calcium channel may, in the absence of any calcium influx, cause calcium release from the sarcoplasmic reticulum. Many of these mechanisms have been reported to be involved in both HPV and in normoxic contraction of the ductus arteriosus.