Ethnicity-specific myocardial remodelling in hypertensive heart disease by multi-parametric cardiovascular magnetic resonance

Background

Patients with systemic hypertension (HTN) of African ancestry (Afr-a) are at greater risk of incident heart failure (HF), hospitalisation and death than those of European ancestry (Eu-a). This has been related to higher prevalence of HTN-related target organ damage, including high level of circulating cardiac troponins, which is not fully explained by blood pressure level. Thus, one may speculate that Afr-a hypertensives have a higher tendency to develop myocardial damage in response to arterial afterload. However, myocardial composition differences between Afr-a and Eu-a hypertensives remain speculative.

Purpose

To investigate ethnic-specific differences in myocardial tissue composition in Eu-a and Afr-a hypertensives by multi-parametric cardiovascular magnetic resonance (CMR).

Methods

This cross-sectional study included 63 Afr-a and 47 Eu-a hypertensive patients. All patients underwent multi-parametric CMR (1.5-Tesla Aera, Siemens-Healthcare, Erlangen-Germany). Left (LV) and right ventricular (RV) volumes, mass and function, atrial dimensions, and myocardial tissue characterisation (including T1- and T2-mapping) were measured using a standardised imaging protocol, and post-processing recommendations from international scientific societies. Analysis was completed using a commercially available cardiac-software (CVI-42, Calgary-Canada). Central pulse-wave-velocity (PWV) between the ascending and proximal descending thoracic aorta was measured by high-temporal, resolution 2D phase-contrast velocity-encoded parasagittal cine images, using in-house MATLAB software.

Results

Although Afr-a were 5 years older than Eu-a hypertensives, cardiovascular risk factors, anthropometric, body composition and haemodynamic measures were similar between the two groups (Figure 1). Segmental PWV was greater in Afr-a than Eu-a patients (8.16±2.71 vs 6.97±2.82 m/s, P=0.044), underlying higher aortic stiffness in Afr-a hypertensives. Afr-a hypertensives also had greater LV mass and LV-mass/end-diastolic volume ratio than Eu-a (Figure 2), whilst no difference was observed in LV systolic/diastolic function. Native T1 relaxation time and synthetic extracellular volume were also similar between the two ethnicities, though T2 relaxation time was significantly higher in Afr-a hypertensives. Late gadolinium enhancement (LGE), a well-established metric of replacement fibrosis (scarring), was more prevalent in Afr-a than Eu-a hypertensives (14% vs 4%, P=0.001). In patients with LGE, the extent of LGE was higher in Afr-a than Eu-a hypertensives (Figure 2).

Conclusion

Afr-a hypertensives have higher arterial afterload, LV mass and remodelling than Eu-a, despite comparable mean blood pressure, body-mass-index, and body composition. These changes in LV structure and geometry were associated with higher T2 relaxation time, likely reflecting low-grade inflammation, as well as higher prevalence and extent of replacement myocardial fibrosis.

Funding Acknowledgement

Type of funding sources: None.

Racial differences of right ventricular remodelling in systemic hypertension unveiled by multiparametric cardiovascular magnetic resonance

Background

Patients with systemic hypertension (HTN) of African ancestry (Afr-a) are at greater risk of heart failure (HF), hospitalisation and death than those of European ancestry (Eu-a). Compelling evidence suggests that left ventricular (LV) remodelling and hypertrophy are more prevalent in Afr-a than Eu-a hypertensives due to either a high clustering of cardiovascular risk-factors and/or a difference in genetic background. Prior studies in Eu-a subjects have shown that uncomplicated HTN is associated with right ventricular (RV) hypertrophy and remodelling which may contribute to development of HF. However, the impact of ethnicity on RV remodelling in HTN remains speculative.

Purpose

To investigate the influence of ethnicity on RV remodelling/hypertrophy in patients with HTN using cardiovascular magnetic resonance (CMR).

Methods

In this cross-sectional study we included 16 Afr-a and 32 Eu-a age- and sex-matched healthy-volunteers, and 63 Afr-a and 47 Eu-a hypertensives. All participants underwent a CMR exam (1.5-Tesla, Aera, Siemens-Healthcare, Erlangen-Germany). LV and RV volumes, masses and function were measured according to the current recommendations. Blood pressure was recorded during the CMR.

Results

Age- and sex-matched Afr-a and Eur-a healthy-volunteers (37±10 vs 37±12 years, P=0.975; male 53% vs 44%; P=0.539) exhibited closely comparable LV and RV volumes, masses, and end-diastolic volume/mass ratios. In the HTN group, despite Afr-a hypertensives being roughly 5 years older than Eu-a, baseline characteristics including cardiovascular risk factors, mean blood pressure, body-mass-index, and body composition metrics were similar between the two groups (Figure 1). Afr-a hypertensives also had greater LV and RV masses and mass/end-diastolic volume ratios than Eur-a hypertensives (Figure 2). RV mass correlated with LV mass in both ethnic groups (r=0.593 in Eu-a and r=0.569 in Afr-a; both P<0.001). Multivariable linear regression analysis showed that RV mass was independently associated with African descendance after correction for major confounders including LV mass, biventricular volumes, and body composition.

Conclusion

Our findings support the notion that Afr-a and Eur-a healthy-volunteers have comparable left and right ventricular geometry and masses, arguing against genetic-determinate ventricular geometry and myocardial mass in this population. However, Afr-a individuals exhibit higher sensitivity to myocardial hypertrophy in response to HTN which translates into greater biventricular masses and remodelling, compared to Eu-a hypertensives.

Funding Acknowledgement

Type of funding sources: None.

465 Split-thickness Skin Graft Harvest under Local Anaesthetic: A Single Pass Technique

Adequate local anaesthetic, in harvesting a split thickness skin graft (SSG), involves multiple passes of a needle across the length and width of the marked donor site.

We describe a technique using hyaluronidase to uniformly anaesthetise an SSG donor site with one injection, in one pass of one needle. Mix 10mls 1% Lidocaine solution with Adrenaline 1:200,000 with 1 vial Hyaluronidase 1 Unit/ml solution. The mixture is buffered with 1 ml NaHCO3 to neutralise acidity and minimise pain. Mark out the SSG donor site Using a 27G long needle (sterican), enter perpendicular to the skin in the middle of the proximal aspect of the donor site. Inject some local anaesthetic subdermally, creating a mound. Change the angle of the needle to 180 degrees and continue to inject the remaining anaesthetic along one half of the width of the donor site. Using a rolled 4x4 swab, apply firm advancing pressure to distribute the mound across the remaining width and length of marked donor site. As the mound advances, the hyluronidase/anaesthetic mixture will distribute uniformly across the donor site within the same plane. The skin blanches secondary to the adrenaline during its distribution.

The technique described is a fast, reproducible way to improve patient comfort through the elimination of repeated passes of a needle, distribute the anaesthetic uniformly across the donor site, and facilitate the acquisition of an SSG of uniform thickness

The technique described is a fast, reproducible way to improve patient comfort through the elimination of repeated passes of a needle, distribute the anaesthetic uniformly across the donor site, and facilitate the acquisition of an SSG of uniform thickness

1345 A Punch Above the Rest: A High Yield Modification of The Diagnostic Nailbed Biopsy

Aim

Diagnostic nailbed biopsy is indicated for pathology including malignancy, persistent and suspected mycomial infection and rheumatological disease. Operatively, the challenge resides in establishing an equilibrium between harvesting ample histological sample for diagnostic value without the creation of a functionally and aesthetically unacceptable nail due to significant disruption of the nailbed. We illustrate a simple and reproducible biopsy technique through one small forgiving defect without disruption of surrounding soft tissue.

Method

The procedure is carried out under normal sterile conditions with local anaesthetic and a ring tourniquet. A 5mm punch biopsy is taken of the nail, adjacent to the eponychium, followed by a 4mm punch biopsy of the underlying nailbed at a 45-degree angle. The subungual surface of the nail is tangentially transected generating a mycomial sample and the nailbed is sent for histology. The remaining nail is replaced if not required for histology, and cyanoacrylate glue applied followed by a finger dressing.

Results

Figures show an example of how the nail biopsy is carried out. Biopsy sample and the resulting defect in the nail is also displayed.

Conclusions

The double punch technique reliably harvests adequate diagnostic specimens through one wound, without the disruption of adjacent nail plate, nailbed and perionychium. This is even more a priority when histology identifies no pathology requiring further operative resection. Whilst the nail undergoes its normal growth cycle post-operatively, aesthetically, the fingertip does not draw undue patient dissatisfaction once the dressing is removed in comparison to alternative biopsy techniques.

529 Safeguarding Skin Grafts: A 21st Century Algorithm for Fixation Techniques

Aim

Effective skin graft fixation is fundamental in preventing sheering forces, seroma and haematoma from compromising graft take. However, determining the ideal method of graft fixation remains a contentious subject. Currently, there is significant variation in fixation techniques used, based not only on clinical requirement, but also surgeon preference. Evidence-based recommendations are necessary to guide the decision-making process.

Method

We undertook a PRISMA-based assessment of the literature to define all fixation techniques and analyse their outcomes. Inclusion and exclusion criteria were composed. A search of Medline and Embase was performed, yielding 399 articles. After abstract screening, 96 were included for qualitative data analysis.

Results

Nine fixation techniques were identified: ‘tie over bolster’, ‘staple fixation’, ‘simple dressings’, ‘quilting sutures’, ‘re-look methods’, ‘foam sponge bolster’, ‘adhesive glues’, ‘negative pressure wound therapy’ and ‘less common techniques. We analyse the available evidence for each technique, identifying 13 studies with level I/II evidence. We summarise the research that underpins these nine categories, proposing an algorithm to facilitate technique selection based on anatomical and patient-specific factors.

Conclusions

An array of skin graft fixation techniques are used in plastic surgery, without clear guidelines. To our knowledge, this is the first time all fixation techniques have been defined. Our suggested algorithm is intended to aid surgeons in selecting an appropriate fixation technique and should be challenged by future research, particularly randomised control trials.

Large-scale, multi-vendor, multi-protocol, quality-controlled analysis of clinical cine CMR using artificial intelligence

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Advancing Impact Award scheme of the EPSRC Impact Acceleration Account at King’s College London

Background

Artificial intelligence (AI) has the potential to facilitate the automation of CMR analysis for biomarker extraction. However, most AI algorithms are trained on a specific input domain (e.g., scanner vendor or hospital-tailored imaging protocol) and lack the robustness to perform optimally when applied to CMR data from other input domains.

Purpose

To develop and validate a robust CMR analysis tool for automatic segmentation and cardiac function analysis which achieves state-of-the-art performance for multi-vendor short-axis cine CMR images. 

Methods

The current work is an extension of our previously published quality-controlled AI-based tool for cine CMR analysis [1]. We deployed an AI algorithm that is equipped to handle different image sizes and domains automatically - the ‘nnU-Net’ framework [2] - and retrained our tool using the UK Biobank (UKBB) cohort population (n = 4,872) and a large database of clinical CMR studies obtained from two NHS hospitals (n = 3,406). The NHS hospital data came from three different scanner types: Siemens Aera 1.5T (n = 1,419), Philips Achieva 1.5T and 3T (n = 1,160), and Philips Ingenia 1.5T (n = 827). The ‘nnU-net’ was used to segment both ventricles and the myocardium. The proposed method was evaluated on randomly selected test sets from UKBB (n = 488) and NHS (n = 331) and on two external publicly available databases of clinical CMRs acquired on Philips, Siemens, General Electric (GE), and Canon CMR scanners – ACDC (n = 100) [3] and M&Ms (n = 321) [4]. We calculated the Dice scores - which measure the overlap between manual and automatic segmentations - and compared manual vs AI-based measures of biventricular volumes and function. 

Results

 Table 1 shows that the Dice scores for the NHS, ACDC, and M&Ms scans are similar to those obtained in the highly controlled, single vendor and single field strength UKBB scans. Although our AI-based tool was only trained on CMR scans from two vendors (Philips and Siemens), it performs similarly in unseen vendors (GE and Canon). Furthermore, it achieves state-of-the-art performance in online segmentation challenges, without being specifically trained on these databases. Table 1 also shows good agreement between manual and automated clinical measures of ejection fraction and ventricular volume and mass.

Conclusions

We show that our proposed AI-based tool, which combines training on a large-scale multi-domain CMR database with a state-of-the-art AI algorithm, allows us to robustly deal with routine clinical data from multiple centres, vendors, and field strengths. This is a fundamental step for the clinical translation of AI algorithms. Moreover, our method yields a range of additional metrics of cardiac function (filling and ejection rates, regional wall motion, and strain) at no extra computational cost.

Characterization of interactions between Nedd4 and beta and gammaENaC using surface plasmon resonance

Cell surface expression of the epithelial Na(+) channel ENaC is regulated by the ubiquitin ligase Nedd4. Binding of the WW domains of Nedd4 to the PY region in the carboxy tails of beta and gammaENaC, results in channel ubiquitination and degradation. Kinetic analysis of these interactions has been done using surface plasmon resonance. Synthetic peptides corresponding to the PY regions of beta and gammaENaC were immobilized on a sensor chip and "real-time" kinetics of their binding to recombinant WW proteins was determined. Specificity of the interactions was established by competition experiment, as well as by monitoring effects of a point mutation known to impair Nedd4/ENaC binding. These data provides the first determination of association, dissociation and equilibrium constants for the interactions between WW2 and beta or gammaENaC.

In vitro phosphorylation of COOH termini of the epithelial Na+ channel and its effects on channel activity inXenopus oocytes

Recent findings have suggested the involvement of protein phosphorylation in the regulation of the epithelial Na+ channel (ENaC). This study reports the in vitro phosphorylation of the COOH termini of ENaC subunits expressed as glutathione S-transferase fusion proteins. Channel subunits were specifically phosphorylated by kinase-enriched cytosolic fractions derived from rat colon. The phosphorylation observed was not mediated by the serum- and glucocorticoid-regulated kinase sgk. For the γ-subunit, phosphorylation occurred on a single, well-conserved threonine residue located in the immediate vicinity of the PY motif (T630). The analogous residue on β(S620) was phosphorylated as well. The possible role of γT630 and βS620 in channel function was studied in Xenopus laevis oocytes. Mutating these residues to alanine had no effect on the basal channel-mediated current. They do, however, inhibit the sgk-induced increase in channel activity but only in oocytes that were preincubated in low Na+ and had a high basal Na+ current. Thus mutating γT630 or βS620 may limit the maximal channel activity achieved by a combination of sgk and low Na+.

Aldosterone induces rapid apical translocation of ENaC in early portion of renal collecting system: possible role of SGK

Aldosterone controls sodium reabsorption and potassium secretion in the aldosterone-sensitive distal nephron (ASDN). Although clearance measurements have shown that aldosterone induces these transports within 30--60 min, no early effects have been demonstrated in vivo at the level of the apical epithelial sodium channel (ENaC), the main effector of this regulation. Here we show by real-time RT-PCR and immunofluorescence that an aldosterone injection in adrenalectomized rats induces alpha-ENaC subunit expression along the entire ASDN within 2 h, whereas beta- and gamma-ENaC are constitutively expressed. In the proximal ASDN portions only, ENaC is shifted toward the apical cellular pole and the apical plasma membrane within 2 and 4 h, respectively. To address the question of whether the early aldosterone-induced serum and glucocorticoid-regulated kinase (SGK) might mediate this apical shift of ENaC, we analyzed SGK induction in vivo. Two hours after aldosterone, SGK was highly induced in all segment-specific cells of the ASDN, and its level decreased thereafter. In Xenopus laevis oocytes, SGK induced ENaC activation and surface expression by a kinase activity-dependent mechanism. In conclusion, the rapid in vivo accumulation of SGK and alpha-ENaC after aldosterone injection takes place along the entire ASDN, whereas the translocation of alpha,beta,gamma-ENaC to the apical plasma membrane is restricted to its proximal portions. Results from oocyte experiments suggest the hypothesis that a localized activation of SGK may play a role in the mediation of ENaC translocation.

Regulation of sgk by aldosterone and its effects on the epithelial Na(+) channel

Aldosterone is the major corticosteroid regulating Na(+) absorption in tight epithelia and acts primarily by activating the epithelial Na(+) channel (ENaC) through unknown induced proteins. Recently, it has been reported that aldosterone induces the serum- and glucocorticoid-dependent kinase sgk and that coexpressing ENaC with this kinase in Xenopus laevis oocytes increases the amiloride-sensitive Na(+) current (Chen SY, Bhargava A, Mastroberardino L, Meijer OC, Wang J, Buse P, Firestone GL, Verrey F, and Pearce D. Proc Natl Acad Sci USA 96: 2514-2519, 1999). The present study was done to further characterize regulation of sgk by aldosterone in native mammalian epithelia and to examine its effect on ENaC. With both in vivo and in vitro protocols, an almost fivefold increase in the abundance of sgk mRNA has been demonstrated in rat kidney and colon but not in lung. Induction of sgk by aldosterone was detected in kidney cortex and medulla, whereas the papilla expressed a constitutively high level of the kinase. The increase in sgk mRNA was detected as early as 30 min after the hormonal application and was independent of de novo protein synthesis. The observed aldosterone dose-response relationships suggest that the response is mediated, at least in part, by occupancy of the mineralocorticoid receptor. Coexpressing sgk and ENaC in Xenopus oocytes evoked a fourfold increase in the amiloride-blockable Na(+) channel activity. A point mutation in the beta-subunit known to impair regulation of the channel by Nedd4 (Y618A) had no significant effect on the response to sgk.

An aldosterone regulated chicken intestine protein with high affinity to amiloride

The pattern of chicken intestine amiloride-binding proteins was determined using the photoreactive amiloride analogue 2'-methoxy-5'-nitrobenzamil (NMBA) and a polyclonal anti-amiloride antibody. At 10(-7)M, NMBA inhibits approximately 62% of the Na+ channel activity. At this concentration the amiloride analogue labels a number of membrane proteins, and in particular a 40-45 kDa polypeptide denoted ABP40. Incorporation of NMBA into ABP40 could be prevented by a 100-fold excess of benzamil, but not by a 1000-fold excess of 5-(N-ethyl-N-isopropyl)-amiloride. Labeling of ABP40 was intense in membranes derived from salt-deprived chickens and approximately 5-fold weaker in membranes from salt-repleted animals. Because of its small size, ABP40 is not likely to be an avian Na+ channel subunit, yet this amiloride-binding protein could be involved in the response to aldosterone.

An index of early left ventricular filling that combined with pulsed Doppler peak E velocity may estimate capillary wedge pressure

OBJECTIVES

This study sought to determine the applicability of the combined information obtained from transmitral Doppler flow and color M-mode Doppler flow propagation velocities for estimating pulmonary capillary wedge pressure.

BACKGROUND

Although Doppler-derived measurements of left ventricular (LV) filling have been applied to determine left atrial pressure, their accuracy has been limited by the variable effect of ventricular relaxation in these indexes. Recently, flow propagation velocity measured by color M-mode Doppler echocardiography has been suggested as an index of ventricular relaxation.

METHODS

We studied 45 patients admitted to the intensive care unit who underwent invasive hemodynamic monitoring. We measured peak early (E) and late (A) transmitral Doppler velocities, E/A ratio and flow propagation velocity (vp) and compared them by linear regression with pulmonary capillary wedge pressure (pw).

RESULTS

We found a modest positive correlation between pw and E (r = 0.62, p < 0.001) and the E/A ratio (r = 0.52, p < 0.001) and a negative correlation between pw and vp (r = -0.34, p = 0.02). By stepwise linear regression, only E and vp were statistically significant predictors of pw. However, the E/vp ratio provided the best estimate of pw (r = 0.80, p < 0.001; pw = 5.27 x [E/vp] + 4.6, SEE 3.1 mm Hg).

CONCLUSIONS

The ratio of component velocity (E) over the color M-mode propagation velocity during early LV filling, by correcting for the effect of LV relaxation, provides a better estimate of pw than standard measurements of transmitral Doppler flow.

Cloning and induction by low NaCl intake of avian intestine Na+ channel subunits

The alpha-subunit of the highly Na(+)-selective amiloride-blockable channel (ENaC) was cloned from chicken lower intestine. The deduced amino acid sequence of the avian clone exhibits -60% identity to the previously cloned mammalian and amphibian alpha-subunits. It also maintains the same hydropathy profile and structural motifs. These include two transmembrane domains separated by a large extracellular loop, four extracellular N-glycosylation sites, a cysteine-rich box in the extracellular domain, and a proline-rich stretch at the carboxy terminus. Xenopus oocytes injected with cRNA transcribed from this clone express a small amiloride-blockable Na+ conductance. Degenerate primers have been used to amplify two other related products. Sequence homology indicates that one of them is the beta-subunit, whereas the other appears to represent a closely related but different transcript. Regulation of the mRNA corresponding to these clones was examined in chickens fed normal and low-NaCl rations. The low-salt diet evoked an approximately fourfold increase in the abundance of mRNA coding for the alpha-subunit, presumably through an increase in plasma aldosterone. The beta- and "beta-like" transcripts were even more strongly affected. The current data provide additional information on sequence conservation in the growing ENaC family and demonstrate that the avian intestine channel is strongly induced by varying NaCl intake.

Inhibition of amiloride-sensitive Na+ channel by isothiouronium derivatives

The effects on the amiloride-blockable Na+ channel of a family of recently synthesized isothiouronium derivatives were measured in plasma membrane vesicles from rat distal colon. Some of these derivatives act as high-affinity Na(+)-like antagonists on the Na(+)-K(+)-adenosinetriphosphatase. One of the reagents tested, 1-bromo-2,4,6-tris(isothiouronium methyl)-benzene tribromide (Br-TITU), was found to be a potent blocker of the Na+ channel. At neutral pH, Br-TITU rapidly inhibits the channel mediated 22Na+ uptake, with an inhibition constant of 94 +/- 39 nM. The inhibition observed is specific and reversible. 1,3-Dibromo-2,4,6-tris(isothiouronium methyl)benzene tribromide and Br-TITU derivatives with methyl and phenyl substitutions on the isothiouronium moiety were much less effective blockers. Incubation of cells with Br-TITU at alkaline (but not neutral) pH produces irreversible inactivation of channels, possibly due ot covalent modification of a lysine residue. This inactivation can be attenuated by amiloride but not by Na+. Thus Br-TITU may be a useful reagent in identifying essential residues of the channel protein.

Aldosterone induction and epithelial distribution of CHIF

CHIF is a recently cloned, corticosteroid-induced gene which evokes K+ channel activity in oocytes (B. Attali, H. Latter, N. Rachamim, and H. Garty. Proc. Natl. Acad. Sci. USA 92: 6092-6096, 1995). To further characterize the possible role of this gene in epithelial ion transport, we have studied its epithelial distribution and hormonal induction. Northern hybridizations indicate that the zonal distribution of CHIF mRNA in kidney is: papilla >>medulla>> cortex. High levels of CHIF were also detected in a primary culture from inner medullary collecting duct (IMCD). Perfusing rats with < 20 nM aldosterone through osmotic minipumps evoked a 22.4 +/- 1.9-fold increase in colonic CHIF. A significant increase was observed 3 h after administrating the corticosteroid, but maximal response was detected only after a 72-h incubation. This response appears to be mineralocorticoid specific; perfusing or injecting rats with maximal doses of dexamethasone did not evoke a further increase in CHIF mRNA. In contrast, high levels of CHIF are expressed in kidney papilla and IMCD primary culture, irrespective of corticosteroid treatment. Thus, like the apical Na+ channel and the H(+)-K(+)-adenosinetriphosphatase, CHIF is mineralocorticoid induced in the colon but constitutively expressed in kidney.

Aldosterone-induced increase in the abundance of Na+ channel subunits

The highly selective, amilorideblockable Na+ channel is a major target to the natriferic action of the mineralocorticoid aldosterone. This rat epithelial Na+ channel (rENaC) has been recently cloned from colon and is composed of three homologous subunits denoted alpha-, beta-, and gamma-rENaC (C. M. Canessa, L. Schild, G. Buell, B. Thorens, L. Gautschi, J.-D. Horisberger, and B. C. Rossier. Nature Lond. 367: 463-467, 1994). We have tested the effects of corticosteroids on the abundance of mRNA coding for each subunit in kidney cortex and distal colon. Chronic treatment of rats with aldosterone or dexamethasone evoked in kidney cortex a small induction of alpha-rENaC and no change in beta- and gamma-rENaC. In distal colon, however, beta- and gamma-rENaC were strongly induced by either aldosterone or dexamethasone, whereas alpha-rENaC was constitutively expressed. Most of the aldosterone-induced increase in beta- and gamma-rENaC mRNA took place during 3-24 h after plasma aldosterone was elevated. A similar differential induction of rENaC subunits in kidney and colon was also evoked by a Na(+)-free diet. The effects of salt deprivation were reversed by resalinating rats with a half time of < 2 h, suggesting a high turnover rate of at least beta- and gamma-rENaC. The data are consistent with the possibility that induction of channel subunits contributes to the chronic but not the acute response to aldosterone in the colon. Such a mechanism is not likely to play a major role in cortical collecting ducts.

Dexamethasone enhances expression of mitochondrial oxidative phosphorylation genes in rat distal colon

Dexamethasone and aldosterone are major activators of Na+ reabsorption in tight epithelia. The genes whose expression mediates the steroid actions are mostly unknown. To identify such genes, we performed differential screening of a rat colon cDNA library with total 32P-labeled cDNA probes reverse transcribed from steroid-stimulated and steroid-depleted poly(A)+ RNA. Several cDNAs whose corresponding mRNA is enhanced two- to threefold after dexamethasone injection were identified. Partial sequencing indicated that four of them code for subunits of cytochrome-c oxidase and 16S mitochondrial mRNA. The dexamethasone-induced increase in mitochondrial RNA abundance could not be mimicked by a low-salt diet, found to increase plasma aldosterone from 1.0 +/- 0.1 to 12.8 +/- 1.4 nM. Induction of mitochondrial genes by adrenal steroids may serve to prevent limitation of transport by the ATP supply to the Na(+)-K+ pump under conditions of maximal stimulation of Na+ transport.

Effects of corticoid agonists and antagonists on apical Na+ permeability of toad urinary bladder

Effects of RU-28362 (glucocorticoid agonist), RU-38486 (glucocorticoid antagonist), and RU-26752 (mineralocorticoid antagonist) on the apical Na+ permeability of toad bladder were measured and correlated with occupancies of cytosolic type I (mineralocorticoid) and type II (glucocorticoid) receptors. Effects of the above steroids were measured in whole bladders, plasma membrane vesicles, and RNA-injected Xenopus oocytes. RU-38486 was found to fully displace aldosterone from type II receptors without affecting type I occupancy. Under these conditions, RU-38486 inhibited approximately 35% of the effect of aldosterone measured in the whole tissue and isolated membranes. Unexpectedly, oocytes injected with RNA from tissue stimulated with aldosterone plus RU-38486 expressed channel activity that was much higher than the sum of activities induced by either steroid alone. RU-28362 and RU-26752 at concentrations sufficient to fully occupy both receptors had only partial agonistic and antagonistic effects, respectively. The results suggest that at least one-third of the natriferic action of aldosterone measured in the amphibian urinary bladder is mediated by the glucocorticoid receptor. However, some of the effects observed cannot be accounted for by a simple receptor occupancy-response scheme.

An epithelial high-affinity amiloride-binding site, different from the Na+ channel

Specific binding of the radioactive amiloride analogues [3H]phenamil and [3H]benzamil was studied in plasma membrane from chicken lower intestine. A single population of sites whose affinities and specificities towards pyrazinecarboxamides roughly resemble those of the epithelial Na+ channel, was identified. However, a matched comparison of pyrazinecarboxamide binding and Na+ transport inhibition revealed substantial differences between the high-affinity [3H]phenamil-binding site detected, and the site whose occupancy by phenamil blocks Na+ transport. First, 5-(N-ethyl-N-isopropyl)-amiloride was found to displace bound [3H]phenamil at concentrations that are at least 10-fold lower than those needed to block the channel. Second, the rates at which [3H]phenamil associates and dissociates from this site are lower than the rates at which Na+ channels are inhibited and reactivated, under similar conditions. A site with high affinity to both amiloride and 5-(N-ethyl-N-isopropyl)-amiloride was detected also in membranes from other epithelia. We conclude that tight epithelia contain a major high-affinity amiloride receptor other than the Na(+)-conducting channel, the Na+/H+ antiport or the Na+/Ca2+ exchanger. This site could be associated with a pool of nonconducting channels, another (but structurally related) channel, or a totally unrelated protein.

Expression of amiloride-sensitive Na+ channels of hen lower intestine in Xenopus oocytes: electrophysiological studies on the dependence of varying NaCl intake

Epithelial Na+ channels were incorporated into the plasma membrane of Xenopus laevis oocytes after micro-injection of RNA from hen lower intestinal epithelium (colon and coprodeum). The animals were fed either a normal poultry food which contained NaCl (HS), or a similar food devoid of NaCl (LS). Oocytes were monitored for the expression of amiloride-sensitive sodium channels by measuring membrane potentials and currents. Oocytes injected with poly(A)+RNA prepared from HS animals or non-injected control oocytes showed no detectable sodium currents, whereas oocytes injected with LS-poly(A)+RNA had large amiloride-blockable sodium currents. These currents were almost completely saturated by sodium concentrations of 20 mM with a Km of about 2.6 mM sodium. Amiloride (10 microM) inhibits the expressed sodium channels entirely and examination of dose response relationships yielded a half-maximal inhibition concentration (Ki) of 120 nM amiloride. I-V difference curves in the presence or absence of sodium or amiloride (10 microM) indicate a potential dependence of the sodium transport which can be described by the Goldman equation. When Na+ is replaced by K+, no amiloride response was detected indicating a high selectivity for Na+ over K+. These results provide strong evidence that intestinal Na+ channels are regulated by dietary salt intake on the RNA level.

Expression of the amiloride-blockable Na+ channel by RNA from control versus aldosterone-stimulated tissue

The amiloride-blockable Na+ channel was expressed in Xenopus oocytes injected with total RNA isolated from the toad urinary bladder. This system was used to investigate mechanisms that mediate the natriferic action of aldosterone. Incubation of the epithelium with aldosterone for 3 h doubled its channel activity but did not increase the ability of isolated RNA to express functional channels in oocytes. A 20-h incubation with the hormone produced an additional increase of Na+ transport across the intact epithelium and also augmented the channel activity expressed in oocytes by nearly 10-fold. The data are in agreement with our model that aldosterone enhances the apical Na+ permeability of tight epithelia by a short term activation of pre-existing channels, followed by chronic induction of new channel protein. Blocking methyl transfer reactions, previously shown to inhibit the natriferic action of aldosterone in tight epithelia, did not alter the basal or aldosterone-induced response in oocytes.

NaCl-dependent expression of amiloride-blockable Na+ channel in Xenopus oocytes

RNA was isolated from chicken lower intestine (both colon and coprodeum) and injected into Xenopus oocytes. 22Na+ fluxes measured after 1-4 days demonstrated the induction of an amiloride-blockable pathway. The Na+ transporter expressed by the exogenous RNA had a high affinity to amiloride (inhibitory constant less than 0.1 microM), but was insensitive to ethylisopropyl amiloride, i.e., it is likely to be the apical Na+ channel. Functional channels were readily expressed in oocytes injected with RNA derived from chickens fed a low-NaCl diet. On the other hand, no channel activity was detected in oocytes injected with RNA isolated from chickens fed a high-NaCl diet. Thus the previously reported regulation of transport by the dietary NaCl intake involves modulations in the level of mRNA that codes either for the Na+ channel or a posttranscriptional regulator of the channel.

Guanosine nucleotide-dependent activation of the amiloride-blockable Na+ channel

Effects of guanosine nucleotides on the epithelial Na+ channel were studied in apical membrane vesicles derived from the toad bladder epithelium. Trapping 10 microM guanosine-5'-O-(thiotriphosphate) (GTP gamma S) in vesicles evoked two- to fourfold increase in the amiloride-sensitive (Na+ channel-mediated) 22Na+ uptake. The nucleotide had no significant effect on the amiloride-insensitive 22Na+ uptake or the valinomycin-mediated 86Rb+ uptake in the same membranes. The stimulatory action of GTP gamma S was mimicked by 5'-guanylylimidiodiphosphate (GppNHp) and could at least partly be reversed by guanosine-5'-O-(thiodiphosphate) (GDP beta S) (10-fold excess). GTP itself and adenosine-5'-O-(thiotriphosphate) (ATP gamma S) had no sustained effect on Na+ transport in vesicles. Thus it appears that the epithelial Na+ channel is directly or indirectly regulated by the occupancy of a guanosine-specific site, probably the alpha subunit of a G protein. The possibility that GTP gamma S acts indirectly by activating a membrane-bound, GTP-dependent enzyme the product of which modulates the channel conductance was assessed by measuring 22Na+ fluxes in membrane vesicles prepared to contain products of such enzymes. None of the reagents tested [adenosine 3',5' cyclic monophosphate (cAMP), guanosine 3',5' cyclic monophosphate (cGMP), inositol 1,4,5-trisphosphate (IP3), and diacylglycerol (DAG)] increased the tracer flux in vesicles or altered its response to GTP gamma S.

Conductive sodium pathway with low affinity to amiloride in LLC-PK1 cells and other epithelia

Electrical potential driven 22Na+ fluxes were measured in membrane vesicles prepared from a number of cultured and naturally occurring epithelia. In all preparations a rheogenic pathway blocked by 200 microM (but not by 1.5 microM) amiloride was noted. This transporter was characterized in membranes prepared from cultured LLC-PK1 cells. In this preparation more than 50% of the rheogenic 22Na+ uptake was blocked by amiloride (IC50 approximately 30 microM), phenamil (IC50 approximately 66 microM), or ethylisopropylamiloride (IC50 approximately 5 microM). This amiloride-sensitive flux was not seen if the vesicles were partially depolarized by external Na+ or K+. It could not be driven by a pH gradient, did not require the presence of Ca2+, sugars, or amino acids, and showed little dependence on temperature (25 versus 0 degrees C). The data suggest the existence of an epithelial amiloride-blockable Na+ transporter different from the previously characterized Na+ channel, Na+/H+ and Na+/Ca2+ exchangers, and the Na+-hexose co-transporter. In rat kidney cortex membranes prepared by Mn2+ precipitation, this transporter is primarily located in the brush-border fraction.

Aldosterone increases the apical Na+ permeability of toad bladder by two different mechanisms

The aldosterone-induced augmentation of Na+ transport in toad bladder was analyzed by comparing the hormonal actions on the transepithelial short-circuit current and on the amiloride-sensitive 22Na+ uptake in isolated membrane vesicles. Incubating bladders with 0.5 microM aldosterone for 3 hr evoked more than a 2-fold increase of the short-circuit current (because of the activation or insertion of apical amiloride-blockable channels) but had no effect on the amiloride-sensitive Na+ transport in apical vesicles derived from the treated tissue. A longer incubation (e.g., 6 hr) produced an additional augmentation of the short-circuit current, which was accompanied by about a 3-fold increase of the channel activity in isolated membranes. The stimulatory effect of aldosterone sustained in vesicles was inhibited by the antagonist spironolactone (present at 1000-fold excess) and the protein synthesis inhibitor cycloheximide (1 microM). In addition, triiodothyronine and butyrate, previously reported to partly inhibit the aldosterone-induced increase in short-circuit current, blocked the hormonal effect in vesicles. It is suggested that aldosterone elevates the apical Na+ permeability of target epithelia by two different mechanisms: a relatively fast effect (less than or equal to 3 hr), which is insensitive to triiodothyronine or butyrate and is not sustained by the isolated membrane, and a slower or later (greater than 3 hr) response blocked by these reagents, which is preserved by the isolated membrane. The data also indicate that these processes are mediated by different nuclear receptors.

Characterization of cAMP-induced activation of epithelial sodium channels

Incubating toad bladder with 10 mU/ml vasopressin increases the amiloride-blockable Na+ flux in membrane vesicles derived from the epithelial cells by about twofold. This stimulation is further enhanced by 3-isobutyl-1-methylxanthine and can be mimicked by 8-bromoadenosine 3', 5'-cyclic monophosphate. Thus the natriferic action of cAMP involves a sustained change of the apical membrane preserved by the isolated vesicles. The possibility that transport is modulated by direct phosphorylation/dephosphorylation of the Na+ channel was tested. Trapping purified cAMP-dependent protein kinase, cAMP, and ATP in apical vesicles failed to alter Na+ transport even though the enzyme proved active and could phosphorylate intravesicular proteins. Trapping several phosphatases partially purified from toad bladder in vesicles was ineffective as well. These data suggest that the cAMP-induced increase in Na+ conductance involves processes other than phosphorylation of the channel protein or direct channel-cAMP interaction.

Sodium-dependent inhibition of the epithelial sodium channel by an arginyl-specific reagent

Effects of the arginyl- and lysyl-specific reagent phenylglyoxal (PGO) on the epithelial Na+ channel were evaluated by measuring the amiloride-blockable 22Na+ fluxes in membrane vesicles derived from the toad bladder epithelium. Incubating whole cells or isolated membranes with PGO readily and irreversibly blocked the channel-mediated tracer flux. Na+ ions present during the interaction of membranes with PGO could protect channels from inactivation by PGO. This effect required the presence of Na+ at the luminal side of the membrane and was characterized by an IC50 of 79 mM Na+. Amiloride, too, could desensitize channels to PGO, but its effect was significant only when whole cells were interacted with the protein-modifying reagent. The data are compatible with a model in which the conductive path of the channel contains a functional arginine, possibly forming a salt bridge with a carboxylic group, which is involved in Na+ translocation and amiloride binding. It was also shown that the augmentation of transport induced by incubating whole cells in Ca2+-free solution (Garty, H., and Asher, C. (1985) J. Biol. Chem. 260, 8330-8335) involves the activation or recruitment of channels that are not vulnerable to PGO prior to incubation.

Sodium channels in membrane vesicles from cultured toad bladder cells

Electrical potential-driven 22Na+ fluxes were measured in membrane vesicles prepared from TBM-18(c123) cells (a clone of the established cell line TB-M). Fifty to seventy percent of the tracer uptake in vesicles derived from cells that were cultivated on a porous support were blocked by the diuretic amiloride. The amiloride inhibition constant was less than 0.1 microM, indicating that this flux is mediated by the apical Na+-specific channels. Vesicles prepared from cells that were not grown on a porous support exhibited much smaller amiloride-sensitive fluxes. Two Ca2+-dependent processes that down-regulate the channel conductance and were previously identified in native epithelia were found in the cultured cells as well. Vesicles isolated from cells that were preincubated with 5 X 10(-7) M aldosterone for 16-20 h exhibited higher amiloride-sensitive conductance than vesicles derived from control, steroid-depleted cells. Thus membrane derived from TBM-18(c123) cells can be used to characterize the epithelial Na+ channel and its hormonal regulation.

Effects of amiloride analogues on Na+ transport in toad bladder membrane vesicles. Evidence for two electrogenic transporters with different affinities toward pyrazinecarboxamides

Most of the electrical potential-driven 22Na+ uptake in toad bladder membrane vesicles can be blocked by the diuretic amiloride. Analysis of the amiloride inhibition curve indicates the presence of two pathways with low and high affinities to the diuretic (Garty, H. (1984) J. Membr. Biol. 82, 269-279). The selectivity of these pathways to amiloride was explored by comparing the inhibition curve of this diuretic with those of 10 of its structural analogues. The relative potencies of various amiloride-like compounds as blockers of the flux component with high affinity to amiloride were in good agreement with the structure-activity relationships elucidated from transepithelial short-circuit current measurements. Thus, this pathway is most probably the apical Na+-specific channel. The other pathway with lower affinity to the diuretic was relatively insensitive to modifications of the amiloride molecule, and the structure-activity relationships measured for the inhibition of this pathway were different from those reported for any other amiloride-blockable process. Other experiments have established that the Na+ flux with low affinity to amiloride is electrogenic and is not mediated by a Na+/H+ or Na+/Ca2+ exchanger, Na+-hexose cotransporter, or the Na+/K+-ATPase. The data indicate that tracer flux measurements in toad bladder membrane vesicles monitor, in addition to the well-characterized apical Na+ channels, another amiloride-blockable electrogenic Na+ transporter. This pathway could be responsible for the basolateral amiloride-blockable Na+ conductance recently observed in nystatin-treated bladders (Garty, H., Warncke, J., and Lindemann, B. (1987) J. Membr. Biol. 95, 91-103).

Direct inhibition of epithelial Na+ channels by a pH-dependent interaction with calcium, and by other divalent ions

Direct inhibitory effects of Ca2+ and other ions on the epithelial Na+ channels were investigated by measuring the amiloride-blockable 22Na+ fluxes in toad bladder vesicles containing defined amounts of mono- and divalent ions. In agreement with a previous report (H.S. Chase, Jr., and Q. Al-Awqati, J. Gen. Physiol. 81:643-666, 1983) we found that the presence of micromolar concentrations of Ca2+ in the internal (cytoplasmic) compartment of the vesicles substantially lowered the channel-mediated fluxes. This inhibition, however, was incomplete and at least 30% of the amiloride-sensitive 22Na+ uptake could not be blocked by Ca2+ (up to 1 mM). Inhibition of channels could also be induced by millimolar concentrations of Ba2+, Sr2+, or VO2+, but not by Mg2+. The Ca2+ inhibition constant was a strong function of pH, and varied from 0.04 microM at pH 7.8 to greater than 10 microM at pH 7.0. Strong pH effects were also demonstrated by measuring the pH dependence of 22Na+ uptake in vesicles that contained 0.5 microM Ca2+. This Ca2+ activity produced a maximal inhibition of 22Na+ uptake at pH greater than or equal to 7.4 but had no effect at pH less than or equal to 7.0. The tracer fluxes measured in the absence of Ca2+ were pH independent over this range. The data is compatible with the model that Ca2+ blocks channels by binding to a site composed of several deprotonated groups. The protonation of any one of these groups prevents Ca2+ from binding to this site but does not by itself inhibit transport. The fact that the apical Na+ conductance in vesicles, can effectively be modulated by minor variations of the internal pH near the physiological value, raises the possibility that channels are being regulated by pH changes which alter their apparent affinity to cytoplasmic Ca2+, rather than, or in addition to changes in the cytoplasmic level of free Ca2+.

Ca2+-induced down-regulation of Na+ channels in toad bladder epithelium

Regulation of epithelial Na+ channels was investigated by measuring the amiloride-blockable 22Na+ fluxes in apical membrane vesicles, derived from cells exposed to various treatments. Maximal amiloride-blockable 22Na+ uptake into vesicles was obtained if the cells were preincubated at 25 degrees C in a Ca2+-free [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA) solution. Including 10(-5) M Ca2+ in the cell incubating medium blocked nearly all of the amiloride-sensitive flux in vesicles, even though the Ca2+ was removed before homogenization of the cells. This Ca2+-dependent inhibition of Na+ channels could be induced in whole cells only; incubating cell homogenates with Ca2+ had no effect on the transport in vesicles. The dose-response relationships of this effect were measured by equilibrating cell aliquots with various Ca2+-EGTA buffers, preparing membrane vesicles (in the absence of Ca2+ ions), and assaying them for amiloride-sensitive Na+ permeability. It was found that the Ca2+ blockage is highly cooperative (Hill coefficient of nearly 4) and is characterized by an inhibition constant which varies between 6.4 X 10(-8) to 8.15 X 10(-6)M Ca2+. Thus, it is likely that the above process is involved in the physiological control of Na+ transport. The Ca2+-dependent transport changes were not affected by the calmodulin inhibitor trifluoperasine, vanadate (VO3-), phorbol ester, colchicine, cytochalasin B, 3-deazaadenosine, and 8-bromo-cAMP. Vanadyl (VO2+) ions, on the other hand, produced a "Ca2+-like" inhibition of transport.

Ca2+-dependent, temperature-sensitive regulation of Na+ channels in tight epithelia. A study using membrane vesicles

Na+ fluxes were measured in toad bladder microsomes. Under favorable conditions, 60-90% of the tracer uptake was blocked by amiloride (Ki = 2.3 X 10(-8) M), i.e. mediated by the apical Na+-specific channels. Vesicles derived from cells maintained at 0 degrees C exhibited relatively small amiloride-sensitive fluxes. However, incubating the scraped cells at 25 degrees C prior to homogenization induced a nearly 5-fold increase of the amiloride-blockable flux in vesicles. This activation was fairly slow (t 1/2 = 5-10 min), irreversible, and strongly dependent on the incubation temperature. On the other hand, the Na+-specific apical conductance measured in mounted bladders was only slightly affected by the incubation temperature. The above activation process could be observed only in Ca2+-free EGTA-containing solutions. Adding Ca2+ (1 mM) to the cell suspension and subsequently removing it before homogenization blocked almost completely the amiloride-sensitive tracer uptake in the vesicles. The data are compatible with the model that the epithelial Na+ channels are down-regulated by a Ca2+-dependent reaction. The incubation of scraped, somewhat permeabilized cells in a Ca2+-free solution releases channels from this down-regulation and increases the Na+ conductance in a temperature-dependent process. The regulation of channels appears to involve a cytoplasmic factor which induces a stable modification of the apical membrane, preserved by the isolated vesicle.

Extracellular pH, transmembrane distribution and cytotoxicity of chlorambucil

The effects of extracellular pH (6.2 to 7.3) on uptake and cytotoxicity of the weak acid anti-tumor drug chlorambucil were investigated. Decreasing extracellular pH from 7.3 to 6.5 had a negligible effect on the intracellular pH of Chinese hamster V79 fibroblasts, thus resulting in the formation of a transmembrane pH gradient (intracellular alkaline). Addition of high concentrations of acetate or bicarbonate partially collapsed the pH gradient. Chlorambucil (pKa = 5.8) behaved as a weak acid with enhanced accumulation and cytotoxicity at extracellular pH less than 7.0. As predicted for a weak acid, partial collapse of the transmembrane pH gradient decreased both uptake and cell killing. Since the interstitial pH of micrometastases and solid tumors of many cancers is low relative to normal tissues, these results have potential implications for both in vitro drug testing and in vivo therapy.

The removable quadhelix appliance

The introduction of a preformed removable quadhelix appliance with curved rotation sheaths for insertion and removal has overcome many of the clinical problems associated with the fixed quadhelix appliance. The design, fitting and activation of the appliance are described. Various applications and modifications are discussed.

The effects of synthetic locust adipokinetic hormone on dispersed locust fat body cell preparations: cAMP induction, lipid mobilization, and inhibition of protein synthesis

A procedure for the preparation of functional cells from adult locust fat bodies by collagenase treatment has been developed. The high variability of replicates encountered when whole fat bodies are incubated in vitro is greatly reduced in incubation of the dispersed cells. Synthetic locust adipokinetic hormone (AKH) (40 nM) stimulated release of lipids from the dispersed fat body cells at a rate comparable to that observed using whole fat bodies in vitro. Synthetic AKH elevated cAMP levels sixfold in dispersed cells. In addition, AKH inhibited protein synthesis to a maximum of 50-70% in a concentration-dependent manner. None of these actions of AKH required the presence of locust hemolymph components. These results demonstrate the utility of the isolated locust fat body cells for investigating hormonal action in vitro.

Determination of locust vitellogenin by radioimmunoassay with [3H]Propionyl-vitellogenin

A simple procedure for radiolabeling of locust vitellogenin is described. This procedure involves coupling of [3H]propionyl succinimidate to purified vitellogenin with high yield and specific activity. Using this radiolabeled analog, a specific and sensitive radioimmunoassay was developed for determining locust vitellogenin content, with a lower detection limit of 1 ng. [3H]Propionyl-vitellogenin binds completely to rabbit anti-vitellogenin (locust) and can be completely competed out by locust vitellogenin. The structural similarity of locust vitellogenin with that of locust egg vitellin, male locust lipophorin (a diglyceride-carrying lipoprotein), Xenopus laevis vitellogenin, and chicken egg yolk lipovitellin was examined with this RIA procedure. Comparable binding competition was obtained with locust vitellin only. Male locust lipophorin, Xenopus vitellogenin, and chicken lipovitellin did not inhibit vitellogenin binding at concentrations 1000-fold greater than that of locust vitellogenin. The use of this RIA in determination of vitellogenin synthesis in vivo and in vitro, using isolated fat body preparations, is described.