Long QT syndrome-associated calmodulin mutations and their interactions with the Kv7.1 voltage-gated potassium channel

Introduction

Calmodulin (CaM) is a highly conserved mediator of calcium (Ca2+) dependent signalling. Its flexible structure allows CaM to bind and modulate many targets, including cardiac ion channels. Genotyping has revealed several CaM mutations associated with congenital disorders of heart rhythm, known as long QT-syndrome (LQTS). LQTS patients suffer from prolonged ventricular recovery times (QT-interval) which increases their risk of significant cardiac events. Loss of function KV7.1 mutations are the largest cause of LQTS, accounting for >50% of cases. CaM facilitates Ca2+-sensitivity to KV7.1 in producing IKs, Kv7.1 mutations which reduce CaM binding promote LQTS pathology. However, the effects of LQTS-associated CaM mutations on Kv7.1 function remain unknown.

Purpose

To determine the biophysical consequences of congenital LQTS-associated CaM mutations and how they alter modulation of Kv7.1 in producing the ventricular repolarising IKs current.

Methods

WT and mutant CaM proteins were recombinantly expressed and purified for biophysical characterisation. Using circular dichroism, secondary structures and thermostability of proteins were quantified. Isothermal titration calorimetry was used to quantitatively measure interactions between CaM proteins and binding sites of KV7.1 (Helix B). NMR was employed to study the conformations of target-bound WT and mutant proteins. Whole cell currents were determined using voltage clamp electrophysiology in HEK cells.

Results

Mutations significantly changed the thermostability and secondary structure distributions of CaM, and also caused site-dependent increases in susceptibility to protease digestion. CaM interacted with Helix B (KV7.1) via both Ca2+-dependent and independent mechanisms. Ca2+ dependent binding to Helix B was much higher affinity than Ca2+-independent binding, with >2000-fold reduction in dissociation constant measured. LQTS-CaM variants reduced Helix B affinity with the largest reductions found in EF-hand IV mutants. These mutants also adopted most distinct conformations when Helix B-bound. Calmodulation of the KV7.1 channel produced larger (IKs) currents without altering channel activation kinetics. IKs exhibited Ca2+-sensitivity, in response to increased cytosolic Ca2+, larger currents were generated. Modulation by CaM mutants reduced current density at systolic Ca2+-concentrations (1000 nM), within physiological time periods (0.35 s), revealing a direct QT-prolonging modulatory effect.

Conclusions

Provided here are mechanistic insights as to how LQTS-associated CaM variants contribute to electrical disease of the heart. Mutations in the highly conserved structure of CaM disrupt protein conformation and perturb complex formation with KV7.1. This results in aberrant Ca2+-sensitivity of Kv7.1, reducing IKs generation. This ultimately decreases the repolarisation capacity of cells and would extend the QT interval of myocytes.

Funding Acknowledgement

Type of funding sources: Foundation. Main funding source(s): British Heart Foundation Intermediate Basic Science Research Fellowship

Disease-associated calmodulin mutations disrupt L-type Ca2+ channel (Cav1.2) activity and CaMKIIdelta phosphorylation in long QT syndrome

Introduction

Long QT Syndrome (LQTS) is a major inherited arrhythmia syndrome that can cause sudden cardiac death. Using genome sequencing in human patients, mutations in the ubiquitous calcium (Ca2+) sensor protein calmodulin (CaM) have been associated to LQTS. CaM is an ion channel regulator and can modulate the activity of the voltage-gated calcium channel (Cav1.2) and Ca2+/CaM-dependent protein kinase II (CaMKIIδ), involved in cardiac muscle contraction. However the molecular mechanism by which CaM mutations contribute to irregular heartbeats remains unclear.

Methods

Interaction of CaM proteins with Cav1.2 and CaMKIId synthetic peptides (Cav1.2-NSCaTE51–68, Cav1.2-IQ1665–1685, Cav1.2-C1627–1652, CaMKIIδ294–315,) was investigated using Isothermal Titration Calorimetry (ITC) and X-ray crystallography. Whole-cell patch clamp electrophysiology was used to determine the effect of CaM mutations on L-type Ca2+ currents and Ca2+-dependent inactivation (CDI). CaMKIIδ phosphorylation activity was determined by western blot and fluorescence kinase assay.

Results

Binding affinity of CaMKIId and Cav1.2 peptides to the LQTS-associated CaM variants was significantly reduced, up to 7-fold. Interestingly, the Cav1.2-IQ1665–1685 peptide showed a stronger binding, up to 2-fold, towards LQTS-CaM mutants. Crystal structures of Ca2+-CaM:CaMKIId294–315 showed structural alterations induced by LQTS associated mutations. In addition, we demonstrated that CaMKIIδ autophosphorylation and kinase activity can be significantly reduced by LQTS-associated CaM mutants. Electrophysiological examination of Cav1.2 function revealed that CaM mutations significantly impaired channel CDI, without affecting the voltage dependence of activation and inactivation.

Conclusions

These data demonstrate a strong correlation between LQTS-associated CaM mutations and Cav1.2 activity. We provide molecular insights into the diverse factors contributing to CaM-mediated arrhythmias.

Funding Acknowledgement

Type of funding sources: Foundation. Main funding source(s): British Heart Foundation Intermediate Basic Science Research Fellowship

Combined nestorone-testosterone gel suppresses serum gonadotropins to concentrations associated with effective hormonal contraception in men

BACKGROUND

Novel male-based contraceptives are needed to broaden family planning choices. A progestin, Nestorone^® (Nes) gel, plus a testosterone (T) gel suppresses sperm concentrations to levels associated with effective contraception in normal men. However, administration of two gels on different parts of the body daily is impractical.

OBJECTIVE

Compare the effectiveness of daily application of a single, combined 8.3 mg Nes-62.5 mg T gel (Nes-T) vs. 62.7 mg T gel to suppress serum FSH and LH concentrations to ≤1.0 IU/L (a threshold associated with suppression of sperm concentrations to ≤1 million and effective contraception) and to compare the pharmacokinetics of serum Nes and T concentrations between the gel groups.

DESIGN

We conducted a 28-day, double-blind, controlled trial of 44 healthy men randomized to daily Nes-T or T gel with measurement of hormones at baseline, treatment, and recovery and during 24-h pharmacokinetic studies on days 1 and 28 of treatment.

RESULTS

Of the subjects who met pre-defined inclusion criteria, 84% of the Nes-T group suppressed serum gonadotropin concentrations to ≤1.0 IU/L at days 21-28 vs. 16.7% in the T group (p < 0.001). On day 1, Nes concentrations rose significantly above baseline by 2 h and continued to rise up to 24 h after Nes-T gel application. Nes concentrations were not detectable in the T group. Serum total T concentrations rose and were significantly higher in the T gel group compared to the Nes-T group at 24 h on day 1 and days 11, 14, and 21 (p < 0.01). There were no serious adverse events in either group. About 80% of the subjects reported satisfaction with both gels.

CONCLUSION

Daily Nes-T gel effectively and safely suppresses serum gonadotropins and is acceptable to most men. It should be studied further in efficacy trials of hormonal male contraception.

Preventing secondary exposure to women from men applying a novel nestorone/testosterone contraceptive gel

BACKGROUND

Testosterone (T)/Nestorone (NES) combination gel is a potential transdermal male contraceptive that suppresses gonadotropins and spermatogenesis. Transfer of transdermal T from men to women can be prevented by washing or covering application sites with clothing.

OBJECTIVES

We hypothesized that showering or wearing a shirt over gel application sites would prevent secondary exposure of T and NES to a woman after close skin contact.

MATERIALS AND METHODS

Twelve healthy male and 12 healthy female participants were recruited. Men applied T/NES 62 mg/8 mg gel to their shoulders and upper arms. Two hours after application, female partners rubbed the application site for 15 min. Exposure in the female partner was assessed under three conditions: a shirt covered the application site; the man showered prior to skin contact; or without intervention to reduce transfer. Serum T and NES concentrations were measured by LC-MS/MS in serial blood samples for 24 h after gel exposure.

MAIN OUTCOMES

Change in female serum T and NES levels as measured by average concentration over 24 h (C_avg ).

RESULTS

Median female serum T C_avg was 23.9 ng/dL (interquartile range, 19.3, 33.9) with the shirt barrier and 26.7 ng/dL (20.7, 33.9) after showering, which was higher than baseline 20.9 ng/dL (16.7, 25.0), both p < 0.03) but lower than without intervention (58.2 ng/dL [30.9, 89.1], both p < 0.01). Female serum NES C_avg and maximum concentration were below the lower limit of quantification with the shirt barrier and after showering, but increased without intervention in six of 12 women (maximum concentration <60 pg/mL). Men had lower average serum NES levels after showering (47 pg/ml [20, 94] compared to no intervention (153.3 pg/mL [51, 241], p < 0.02).

CONCLUSION

Secondary transfer of T and NES occurs after intensive skin contact with the gel application site. Secondary transfer is decreased by a shirt barrier or showering before contact.

Caveolar disruption causes contraction of rat femoral arteries via reduced basal NO release and subsequent closure of BKCa channels

Background and Purpose. Caveolae act as signalling hubs in endothelial and smooth muscle cells. Caveolar disruption by the membrane cholesterol depleting agent methyl-β-cyclodextrin (M-β-CD) has various functional effects on arteries including (i) impairment of endothelium-dependent relaxation, and (ii) alteration of smooth muscle cell (SMC) contraction independently of the endothelium. The aim of this study was to explore the effects of M-β-CD on rat femoral arteries. Methods. Isometric force was measured in rat femoral arteries stimulated to contract with a solution containing 20 mM K(+) and 200 nM Bay K 8644 (20 K/Bay K) or with one containing 80 mM K(+)(80 K). Results. Incubation of arteries with M-β-CD (5 mM, 60 min) increased force in response to 20 K/Bay K but not that induced by 80 K. Application of cholesterol saturated M-β-CD (Ch-MCD, 5 mM, 50 min) reversed the effects of M-β-CD. After mechanical removal of endothelial cells M-β-CD caused only a small enhancement of contractions to 20 K/Bay K. This result suggests M-β-CD acts via altering release of an endothelial-derived vasodilator or vasoconstrictor. When nitric oxide synthase was blocked by pre-incubation of arteries with L-NAME (250 µM) the contraction of arteries to 20 K/Bay K was enhanced, and this effect was abolished by pre-treatment with M-β-CD. This suggests M-β-CD is inhibiting endothelial NO release. Inhibition of large conductance voltage- and Ca(2+)-activated (BKCa) channels with 2 mM TEA(+) or 100 nM Iberiotoxin (IbTX) enhanced 20 K/Bay K contractions. L-NAME attenuated the contractile effect of IbTX, as did endothelial removal. Conclusions. Our results suggest caveolar disruption results in decreased release of endothelial-derived nitric oxide in rat femoral artery, resulting in a reduced contribution of BKCa channels to the smooth muscle cell membrane potential, causing depolarisation and contraction.

TRPV4 and K(Ca) ion channels functionally couple as osmosensors in the paraventricular nucleus

BACKGROUND AND PURPOSE

Transient receptor potential vanilloid type 4 (TRPV4) and calcium-activated potassium channels (KCa ) mediate osmosensing in many tissues. Both TRPV4 and KCa channels are found in the paraventricular nucleus (PVN) of the hypothalamus, an area critical for sympathetic control of cardiovascular and renal function. Here, we have investigated whether TRPV4 channels functionally couple to KCa channels to mediate osmosensing in PVN parvocellular neurones and have characterized, pharmacologically, the subtype of KCa channel involved.

EXPERIMENTAL APPROACH

We investigated osmosensing roles for TRPV4 and KCa channels in parvocellular PVN neurones using cell-attached and whole-cell electrophysiology in mouse brain slices and rat isolated PVN neurons. Intracellular Ca(2+) was recorded using Fura-2AM. The system was modelled in the NEURON simulation environment.

KEY RESULTS

Hypotonic saline reduced action current frequency in hypothalamic slices; a response mimicked by TRPV4 channel agonists 4αPDD (1 μM) and GSK1016790A (100 nM), and blocked by inhibitors of either TRPV4 channels (RN1734 (5 μM) and HC067047 (300 nM) or the low-conductance calcium-activated potassium (SK) channel (UCL-1684 30 nM); iberiotoxin and TRAM-34 had no effect. Our model was compatible with coupling between TRPV4 and KCa channels, predicting the presence of positive and negative feedback loops. These predictions were verified using isolated PVN neurons. Both hypotonic challenge and 4αPDD increased intracellular Ca(2+) and UCL-1684 reduced the action of hypotonic challenge.

CONCLUSIONS AND IMPLICATIONS

There was functional coupling between TRPV4 and SK channels in parvocellular neurones. This mechanism contributes to osmosensing in the PVN and may provide a novel pharmacological target for the cardiovascular or renal systems.

Characteristics associated with suppression of spermatogenesis in a male hormonal contraceptive trial using testosterone and Nestorone(®) gels

Development of a male hormonal contraceptive has been challenging ascribable to the failure to adequately suppress spermatogenesis in 5-10% of men. Methods to identify incomplete suppressors early in treatment might identify men most responsive to male hormonal contraceptives. We hypothesized that serum hormone and gonadotropin concentrations after 4 weeks of transdermal treatment with testosterone and Nestorone in a contraceptive trial would be associated with suppression of sperm concentrations to <1 million/mL after 24 weeks. Indeed, luteinizing hormone or follicle-stimulating hormone concentrations greater than 1 IU/L after 4 weeks of transdermal testosterone/nestorone treatment were 97% sensitive for predicting failure to suppress spermatogenesis after 24 weeks of treatment. Serum nestorone concentrations were significantly associated with suppression, but serum testosterone concentrations were not. Early suppression of gonadotropins is associated with, but does not ensure, adequate suppression of spermatogenesis. This information may allow for rapid identification of non-responders in male hormonal contraceptive trials.

Evidence for involvement of A-kinase anchoring protein in activation of rat arterial K(ATP) channels by protein kinase A

1. We have investigated the possible role of A-kinase anchoring proteins (AKAPs) in protein kinase A (PKA) signalling to ATP-sensitive K+ (K(ATP)) channels of rat isolated mesenteric arterial smooth muscle cells using whole-cell patch clamp and peptides that inhibit PKA-AKAP binding. 2. Intracellular Ht31 peptide (20 microM), which inhibits the PKA-AKAP interaction, blocked K(ATP) current activation by either dibutyryl cAMP or calcitonin gene-related peptide. Ht31-proline (20 microM), which does not inhibit PKA binding to AKAP, did not block K(ATP) current activation. 3. Ht31 reduced K(ATP) current activated by pinacidil and also prevented its inhibition by Rp-cAMPS, effects consistent with Ht31 blocking steady-state K(ATP) channel activation by PKA. However, Ht31 did not prevent K(ATP) current activation by the catalytic subunit of PKA. 4. An antibody to the RII subunit of PKA showed localization of PKA near to the cell membrane. Our results provide evidence that both steady-state and receptor-driven activation of K(ATP) channels by PKA involve the localization of PKA by an AKAP.

Targeting of an A kinase-anchoring protein, AKAP79, to an inwardly rectifying potassium channel, Kir2.1

Protein kinase A (PKA) is targeted to discrete subcellular locations close to its intended substrates through interaction with A kinase-anchoring proteins (AKAPs). Ion channels represent a diverse and important group of kinase substrates, and it has been shown that membrane targeting of PKA through association with AKAPs facilitates PKA-mediated phosphorylation and regulation of several classes of ion channel. Here, we investigate the effect of AKAP79, a membrane-associated multivalent-anchoring protein, upon the function and modulation of the strong inwardly rectifying potassium channel, Kir2.1. Functionally, the presence of AKAP79 enhanced the response of Kir2.1 to elevated intracellular cAMP, suggesting a requirement for a pool of PKA anchored close to the channel. Antibodies directed against a hemagglutinin epitope tag on Kir2.1 coimmunoprecipitated AKAP79, indicating that the two proteins exist together in a complex within intact cells. In support of this, glutathione S-transferase fusion proteins of both the intracellular N and C domains of Kir2.1 isolated AKAP79 from cell lysates, while glutathione S-transferase alone failed to interact with AKAP79. Together, these findings suggest that AKAP79 associates directly with the Kir2.1 ion channel and may serve to anchor kinase enzymes in close proximity to key channel phosphorylation sites.

PrimeScreen, a brief dietary screening tool: reproducibility and comparability with both a longer food frequency questionnaire and biomarkers

OBJECTIVE

Diet is an important determinant of health outcomes, but physicians have few ways to identify persons with suboptimal diets. The purposes of this study were to examine the reproducibility of a short dietary assessment questionnaire (PrimeScreen) and to compare its results with those of a longer food frequency questionnaire and with plasma levels of selected nutrients.

DESIGN

Each subject completed two PrimeScreen questionnaires at an interval of 2 weeks and one full length, 131-item, semiquantitative food frequency questionnaire (SFFQ), and had a sample of blood drawn. We compared the PrimeScreen with two reference standards, the SFFQ and plasma levels of selected nutrients.

SETTING

A large managed care organization in New England.

SUBJECTS

A total of 160 men and women, aged 19-65 years, participated.

RESULTS

For foods and food groups, the mean correlation coefficient (r) was 0.70 for reproducibility and 0.61 for comparability with the SFFQ. For nutrients, the mean r was 0.74 for reproducibility and 0.60 for comparability with the SFFQ. No substantial differences were evident by sex, race, body mass index, occupation or education. Correlation coefficients for the comparison of vitamin E, beta-carotene and lutein/zeaxanthin intakes from the PrimeScreen with plasma levels were 0.33, 0.43 and 0.43, respectively. These values were similar to those comparing the SFFQ with plasma levels. The median time to complete PrimeScreen was 5 min; 87% of participants required fewer than 10 min.

CONCLUSIONS

A quick way to assess quality of diet among adults, PrimeScreen has adequate reproducibility and its results compare well with a longer food frequency questionnaire and biomarkers.

The possible role of a disulphide bond in forming functional Kir2.1 potassium channels

The role of two cysteine residues--Cys122 and Cys154--in the structure of the strong inward rectifier K+ channel, Kir2.1, has been investigated using site-directed mutagenesis and electrophysiology. Such cysteine residues are conserved across the inward rectifier family and may be expected to form a crucial disulphide bond. Our experiments show that when the cysteines are absent, the protein is expressed, but the channels are not functional, suggesting that the disulphide bond is essential for correct channel assembly. However, reducing agents applied extracellularly have little effect on current amplitude in wild-type, so that, once the channel is assembled correctly in the membrane, the disulphide bonds are no longer essential for function. Molecular modelling suggests that a disulphide bond is formed--this may be either an intra- or an inter-subunit.

Direct block of native and cloned (Kir2.1) inward rectifier K+ channels by chloroethylclonidine

1. We have investigated the inhibition of inwardly rectifying potassium channels by the alpha-adrenergic agonist/antagonist chloroethylclonidine (CEC). We used two preparations; two-electrode voltage-clamp of rat isolated flexor digitorum brevis muscle and whole-cell patch-clamp of cell lines transfected with Kir2.1 (IRK1). 2. In skeletal muscle and at a membrane potential of -50 mV, chloroethylclonidine (CEC), an agonist at alpha2-adrenergic receptors and an antagonist at alpha1x-receptors, was found to inhibit the inward rectifier current with a Ki of 30 microM. 3. The inhibition of skeletal muscle inward rectifier current by CEC was not mimicked by clonidine, adrenaline or noradrenaline and was not sensitive to high concentrations of alpha1-(prazosin) or alpha2-(rauwolscine) antagonists. 4. The degree of current inhibition by CEC was found to vary with the membrane potential (approximately 70% block at -50 mV c.f. approximately 10% block at -190 mV). The kinetics of this voltage dependence were further investigated using recombinant inward rectifier K+ channels (Kir2.1) expressed in the MEL cell line. Using a two pulse protocol, we calculated the time constant for block to be approximately 8 s at 0 mV, and the rate of unblock was described by the relationship tau=exp((Vm+149)/22) s. 5. This block was effective when CEC was applied to either the inside or the outside of patch clamped cells, but ineffective when a polyamine binding site (aspartate 172) was mutated to asparagine. 6. The data suggest that the clonidine-like imidazoline compound, CEC, inhibits inward rectifier K+ channels independently of alpha-receptors by directly blocking the channel pore, possibly at an intracellular polyamine binding site.

The dependence of Ag+ block of a potassium channel, murine kir2.1, on a cysteine residue in the selectivity filter

Externally applied Ag+ (100-200 nM) irreversibly blocked the strong inwardly rectifying K+ channel, Kir2.1. Mutation to serine of a cysteine residue at position 149 in the pore-forming H5 region of Kir2.1 abolished Ag+ blockage. To determine how many of the binding sites must be occupied by Ag+ before the channel is blocked, we measured the rate of channel block and found that our results were best fitted assuming that only one Ag+ ion need bind to eliminate channel current. We tested our hypothesis further by constructing covalently linked dimers and tetramers of Kir2.1 in which cysteine had been replaced by serine in one (dimer) or three (tetramer) of the linked subunits. When expressed, these constructs yielded functional channels with either two (dimer) or one (tetramer) cysteines per channel at position 149. Blockage in the tetramer was complete after sufficient exposure to 200 nM Ag+, a result that is also consistent with only one Ag+ being required to bind to Cys149 to block fully. The rate of development of blockage was 16 times slower than in wild-type channels; the rate was 4 times slower in channels formed from dimers.

The selectivity filter of a potassium channel, murine kir2.1, investigated using scanning cysteine mutagenesis

We have produced a structural model of the pore-forming H5 (or P) region of the strong inward rectifier K+ channel, Kir2.1, based initially on an existing molecular model of the pore region of the voltage-gated K+ channel, Kv1.3. Cysteine-scanning mutagenesis and subsequent blockage by Ag+ was used to test our model by determining the residues in H5 whose side chains line the ion conduction pathway. Mutations made in eight positions within the highly conserved H5 region resulted in apparently non-functional channels. Constructing covalently linked dimers, which carry a cysteine substitution in only one of the linked subunits, rescued six of these mutants; a covalently linked tetramer, carrying a cysteine substitution on only one of the linked subunits, rescued a further mutant. Our results using the dimers and tetramers suggest that residues Thr141, Thr142, Ile143, Tyr145, Phe147 and Cys149 are accessible to externally applied Ag+ (100-200 nM) and therefore that their side chains line the channel pore. We conclude that the topology of the Kir pore is similar, but not identical, to that of Kv channels. Additionally, the molecular model suggests that selectivity may be conferred both by aromatic residues (Tyr145 and Phe147) via cation-pi interactions and by backbone carbonyl groups (Thr142 and Gly144).

The use of the rat LAP LCR and promoter for the high-level constitutive expression of K+ channel cDNAs in a rat liver cell line

Locus control regions (LCRs) are cis-acting elements that confer position-independent and copy-number-dependent expression upon associated genes in transgenic mice. Here we show the second example of the use of an LCR (the rat LAP LCR) in a stable expression vector system, used here in conjunction with the rat liver (NRLM) cell line. Non-transfected NRLM cells are electrically silent and highly suitable for patch clamp electrophysiology. We report reliable constitutive expression from two different K+ channel cDNAs; the voltage-gated rat clone Kv3.4 and the inward rectifier mouse clone Kir2.1. We further show that constitutive expression levels are stable for at least 8 weeks from initial recording.

The properties and distribution of inward rectifier potassium currents in pig coronary arterial smooth muscle

1. Whole-cell potassium currents were studied in single smooth muscle cells enzymatically isolated from pig coronary arteries. 2. In cells isolated from small diameter branches of the left anterior descending coronary artery (LAD), an inward rectifier potassium current (IK(IR)) was identified, which was inhibited by extracellular barium ions, suggesting the presence of inward rectifier potassium (KIR) channels. 3. The conductance for IK(IR) measured in 6, 12, 60 and 140 mM extracellular potassium was a function of membrane potential and the extracellular potassium concentration. 4. On hyperpolarization, IK(IR) activated along an exponential time course with a time constant that was voltage dependent. 5. Inward rectifier current was compared in cells isolated from coronary vessels taken from different points along the vascular tree. Current density was greater in cells isolated from small diameter coronary arteries; at -140 mV it was -20.5 +/- 4.4 pA pF-1 (n = 23) in 4th order branches of the LAD, but -0.8 +/- 0.2 pA pF-1 (n = 11) in the LAD itself. 6. In contrast to IK(IR), there was little effect of arterial diameter on the density of voltage-dependent potassium current; densities at +30 mV were 12.8 +/- 1.3 pA pF-1 (n = 19) in 4th order branches and 17.4 +/- 3.1 pA pF-1 (n = 11) in the LAD. 7. We conclude that KIR channels are present in pig coronary arteries, and that they are expressed at a higher density in small diameter arteries. The presence of an enhanced IK(IR) may have functional consequences for the regulation of cell membrane potential and tone in small coronary arteries.

Activation of ATP-dependent K+ channels by hypoxia in smooth muscle cells isolated from the pig coronary artery

1. The perforated patch technique with amphotericin B was used to record whole-cell currents activated by hypoxia in smooth muscle cells, isolated enzymatically from pig coronary arteries. 2. Superfusion with hypoxic solution (O2 partial pressure, 25-40 mmHg) activated an inward current at -60 mV in 143 mM extracellular K+. The reversal potential of the current induced by hypoxia shifted with extracellular [K+] as expected for a K+ current, while its current-voltage relation was consistent with the channels showing little voltage dependence. 3. The hypoxia-induced current was inhibited by glibenclamide (10 microM), but was unaffected by charybdotoxin (50 nM). 4. In whole-cell recordings at -60 mV in 143 mM K+ solution, openings of single channels passing a current close to -2 pA could sometimes be detected in normoxic solution. Openings became more frequent during the onset of the response to hypoxia, when several levels could be detected. Channels with a similar conductance were activated by hypoxia in cell-attached patches. 5. Our results suggest that hypoxia activates ATP-dependent K+ channels. We discuss possible mechanisms by which this activation may occur.

Adenosine-activated potassium current in smooth muscle cells isolated from the pig coronary artery

1. The perforated patch technique with nystatin or amphotericin was used to record whole cell currents activated by adenosine in smooth muscle cells isolated enzymatically from pig coronary arteries. 2. Adenosine (5-40 microM) activated an outward current at a holding potential of 0 mV in 5 mM [K+]o and an inward current at -60 mV in 143 mM [K+]o. The dependence of the reversal potential for the adenosine-activated current on [K+]o suggests that it flows through K+ channels, while its current-voltage relation is consistent with the channels showing little voltage dependence. 3. The adenosine-activated current was inhibited by the sulphonylurea glibenclamide (5 microM) and by phencyclidine (5 microM). It was unaffected by charybdotoxin (50 nM) or apamin (100 nM), blockers of large and small conductance Ca(2+)-activated K+ channels respectively. 4. At -60 mV in 143 mM K+ solution, openings of single channels passing a current of just over -2 pA could sometimes be detected in the absence of adenosine. Openings became more frequent after the application of adenosine, with several levels then being detected. Openings of channels with a larger conductance were sometimes also seen in the presence of adenosine. Fluctuation analysis gave somewhat lower estimates of unitary current than did direct measurements. 5. The effect of adenosine could be mimicked by the A1 receptor agonist CCPA (2-chloro-N6-cyclopentyladenosine), while the A2 agonist CGS 21680 (2-p-(2-carboxethyl)phenethylamino-5'-N-ethylcarboxamido adenosine hydrochloride) was without effect. The response to adenosine was inhibited by the A1 antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine), but was unaffected by the A2 antagonist CGS 15943A (5-amino-9-chloro-2-(2-furanyl)-1,2,4- triazolo[1,5-C]quinazoline monomethanesulphonate). 6. Our results suggest that adenosine acts at an A1 receptor to activate K+ channels. We consider it most likely that these are ATP-dependent K+ channels. We discuss the mechanism by which K+ channel activation may lead to hyperpolarization and so vasorelaxation.

Evidence for a Na(+)-Cl(-)-H(+)-HCO3- exchange system in the mammalian lens

36Cl- efflux was studied in the isolated rat lens under two conditions that are known to decrease internal pH. The first follows exposure to a pulse of ammonium chloride (50 mM) and the second accompanies exposure to an acidified propionate (20 mM) solution. Under acidifying conditions, a stimulation in 36Cl- efflux was observed, that was abolished on removing external Na+ and also on removing external Cl- and HCO3-. In the absence of external Cl-, the presence of HCO3- (16 mM) resulted in an increase in 36Cl- efflux during internal acidification. In the absence of internal acidification, the addition of 0.1 mM dibutyrylcAMP or 0.5 mM IBMX to the external medium produced a rapid increase in 36Cl- efflux. This stimulation was reduced by 0.2 mM SITS. Neither cAMP or IBMX had any significant effect on the electrical resistance of the lens membranes. It is suggested that a coupled SITS-sensitive, Na(+)-Cl(-)-H(+)-HCO3- exchange mechanism is activated when the lens internal pH falls and further that cAMP may play a role in regulating this mechanism.

Na(+)-HCO3- symport in the sheep cardiac Purkinje fibre

1. Intracellular pH (pHi) was recorded in isolated sheep cardiac Purkinje fibres using liquid sensor ion-selective microelectrodes in conjunction with conventional (3 M-KCl) microelectrodes (to record membrane potential). 2. In HEPES-buffered solution (pH0 7.4), pHi recovery from an intracellular acid load (20 mM-NH4Cl removal) was blocked by 1 mM-amiloride, consistent with the inhibition of Na(+)-H+ exchange. Replacement of the HEPES buffer with CO2-HCO3- caused a transient acidosis followed by an amiloride-resistant recovery of pHi to more alkaline levels (n = 43). This implies the presence of a HCO3(-)-dependent pHi regulatory mechanism. 3. Comparison of the membrane potential with the equilibrium potential for HCO3- ions (EHCO3) estimated during amiloride-resistant pHi recovery, showed that for polarized fibres (membrane potential Em approximately -80 mV), there was a net outward electrochemical driving force for HCO3- ions. Hence the amiloride-resistant pHi recovery cannot be explained in terms of passive HCO3- influx through membrane channels. 4. Removal of external Na+ (Na0+ replaced by N-methyl-D-glucamine) inhibited HCO3(-)-dependent pHi recovery, whereas removal of external Cl- (leading to depletion of internal Cl-; Cl0- replaced by glucuronate) or short-term removal of extracellular K+ had no inhibitory effect. We suggest that a Na(+)-HCO3- co-influx causes the recovery. Replacement of external Na+ with Li+ greatly reduced HCO3(-)-dependent pHi recovery indicating that Li0+ cannot readily substitute for Na0+ on the co-transport. 5. The stilbene drug DIDS (4,4-diisothiocyano-stilbene-disulphonic acid, 500 microM) slowed HCO3(-)-dependent pHi recovery. 6. Depolarization of the membrane potential in high K0+ (44.5 mM) solution or with 5 mM-BaCl2 had no effect upon the rate of HCO3(-)-sensitive pHi recovery. This observation, when coupled with the fact that activation of HCO3(-)-dependent pHi recovery was associated with no consistent change of membrane potential, suggests that the Na(+)-HCO3- co-influx is electroneutral and voltage insensitive. 7. HCO3(-)-dependent pHi recovery was unaffected by the Na(+)-K(+)-2Cl- co-transport inhibitor, bumetanide (150 microM). 8. The contribution of Na(+)-H+ exchange and Na(+)-HCO3- co-transport to net acid efflux was assessed. At a pHi of 6.6, we estimate that the co-transport should account for 20% of total acid equivalent efflux.(ABSTRACT TRUNCATED AT 400 WORDS)