The Alaska blackfish (Dallia pectoralis) is a facultative air-breather endemic to northern latitudes where it remains active in winter under ice cover in cold hypoxic waters. To understand the ...changes in cellular Ca2+ cycling that allow the heart to function in cold hypoxic water, we acclimated Alaska blackfish to cold (5 °C) normoxia or cold hypoxia (2.1–4.2 kPa; no air access) for 5–8 weeks. We then assessed the impact of the acclimation conditions on intracellular Ca2+ transients (ΔCa2+i) of isolated ventricular myocytes and contractile performance of isometrically-contracting ventricular strips. Measurements were obtained at various contractile frequencies (0.2–0.6 Hz) in normoxia, during acute exposure to hypoxia, and reoxygenation at 5 °C. The results show that hypoxia-acclimated Alaska blackfish compensate against the depressive effects of hypoxia on excitation-contraction coupling by remodelling cellular ΔCa2+i to maintain ventricular contractility. When measured at 0.2 Hz in normoxia, hypoxia-acclimated ventricular myocytes had a 3.8-fold larger ΔCa2+i peak amplitude with a 4.1-fold faster rate of rise, compared to normoxia-acclimated ventricular myocytes. At the tissue level, maximal developed force was 2.1-fold greater in preparations from hypoxia-acclimated animals. However, maximal attainable contraction frequencies in hypoxia were lower in hypoxia-acclimated myocytes and strips than preparations from normoxic animals. Moreover, the inability of hypoxia-acclimated ventricular myocytes and strips to contract at high frequency persisted upon reoxygenation. Overall, the findings indicate that hypoxia alters aspects of Alaska blackfish cardiac myocyte Ca2+ cycling, and that there may be consequences for heart rate elevation during hypoxia, which may impact cardiac output in vivo.
•The air-breathing Alaska blackfish remains active under ice cover in hypoxic waters.•Maintained activity is supported by compensation of intracellular Ca2+ transients.•The compensation permits greater ventricular maximal developed force.•However, maximal attainable contraction frequencies are limited by hypoxia exposure.
We tested the hypothesis that both stretch-activated channels (SACs) and intracellular calcium (Ca 2+ i ) are important in the electrical response of single guinea-pig ventricular myocytes to axial ...stretch. Myocytes were attached
to carbon fibre transducers and stretched, sarcomere length increased by approximately 9 %, and there was a prolongation of
the action potential duration. Streptomycin, a blocker of SACs, had no effect upon the shortening, Ca 2+ i transients or action potentials of electrically stimulated, unstretched myocytes, at a concentration of 50 μ m , but at 40 μ m , prevented any stretch-induced increase in action potential duration. Under action potential clamp, stretch elicited a current
with a linear current-voltage relationship that was inward at membrane potentials negative to its reversal potential of â30
mV, in 10 of 24 cells tested, and was consistent with the activation of non-specific, cationic SACs. This current was not
seen in any stretched cells that were exposed to 40 μ m streptomycin. However, exposure of cells to 5 μ m BAPTA-AM, in order to reduce Ca 2+ i transients, also abolished stretch-induced prolongation of the action potential. We conclude that both SACs and Ca 2+ i are important in the electrical response of cardiac myocytes to stretch, and propose that stretch-induced changes in electrical
activity and Ca 2+ i may be linked by inter-dependent mechanisms.
Epifluorescence imaging using voltage-sensitive dyes has provided unique insights into cardiac electrical activity and arrhythmias. However, conventional dyes use blue-green excitation light, which ...has limited depth penetration.
The aim of this study was to demonstrate that combining a short and a long excitation wavelength using near-infrared (NIR) dyes allows for epifluorescence imaging of transmural electrophysiological properties in intact hearts.
Epifluorescence imaging was performed in rat hearts (N = 11) using DI-4-ANEPPS and the NIR dye DI-4-ANBDQBS. Activation and action potential duration (APD) patterns were investigated at 2 excitation wavelengths (530 and 660 nm) after epicardial stimulation at various cycle lengths (160 to 70 ms).
Optical action potential upstrokes acquired with 660-nm excitation of DI-4-ANBDQBS were significantly longer than upstrokes obtained with 530-nm excitation of DI-4-ANEPPS (P < .001). Comparison of activation maps showed counterclockwise rotation of isochrones consistent with a transmural rotation of myofibers. Pronounced APD modulation by the activation sequence was observed at both excitation wavelengths. Significantly prolonged APDs (P = .016) and steeper APD restitution curves were found with DI-4-ANBDQBS (660-nm excitation) when compared with DI-4-ANEPPS (530-nm excitation). Dual excitation wavelength experiments using solely DI-4-ANBDQBS yielded similar results. Monophasic action potential recordings showed prolonged APD and steeper APD restitution curves in the endocardium, indicating that 660-nm excitation provides a significant endocardial contribution to the signal. Three-dimensional computer simulations confirmed our findings.
Dual excitation wavelength epifluorescence allows detecting transmural heterogeneity in intact hearts. It therefore has the potential to become an important tool in experimental cardiac electrophysiology.
Mechanoelectric feedback (MEF) describes the modulation of electrical activity by mechanical activity. This may occur via the activation of mechanosensitive ion channels (MSCs). MEF has not ...previously been investigated in fish ventricular tissue even though fish can greatly increase ventricular end diastolic volume during exercise which should therefore provide a powerful mechanical stimulus for MEF.
When the ventricles of extrinsically paced, isolated working trout hearts were dilated by increasing afterload, monophasic action potential (MAP) duration was significantly shortened at 25% repolarisation, unaltered at 50% repolarisation and significantly lengthened at 90% repolarisation. This observation is consistent with the activation of cationic non-selective MSCs (MSC(NS)s). We then cloned the trout ortholog of TRPC1, a candidate MSC(NS) and confirmed its presence in the trout heart.
Our results have validated the use of MAP technology for the fish heart and suggest that, in common with amphibians and mammals, MEF operates in fish ventricular myocardium, possibly via the activation of mechanosensitive TRPC1 ion channels.
Spatial dispersion of repolarization is known to play an important role in arrhythmogenesis. Electrotonic modulation of repolarization by the activation sequence has been observed in some species and ...tissue preparations, but to varying extents. Our study sought to determine the mechanisms underlying species- and tissue-dependent electrotonic modulation of repolarization in ventricles. Epi-fluorescence optical imaging of whole rat hearts and pig left ventricular wedges were used to assess epicardial spatial activation and repolarization characteristics. Experiments were supported by computer simulations using realistic geometries. Tight coupling between activation times (AT) and action potential duration (APD) were observed in rat experiments but not in pig. Linear correlation analysis found slopes of -1.03 ± 0.59 and -0.26 ± 0.13 for rat and pig, respectively (p < 0.0001). In rat, maximal dispersion of APD was 11.0 ± 3.1 ms but dispersion of repolarization time (RT) was relatively homogeneous (8.2 ± 2.7, p < 0.0001). However, in pig no such difference was observed between the dispersion of APD and RT (17.8 ± 6.1 vs. 17.7 ± 6.5, respectively). Localized elevations of APD (12.9 ± 8.3%) were identified at ventricular insertion sites of rat hearts both in experiments and simulations. Tissue geometry and action potential (AP) morphology contributed significantly to determining influence of electrotonic modulation. Simulations of a rat AP in a pig geometry decreased the slope of AT and APD relationships by 70.6% whereas slopes were increased by 75.0% when implementing a pig AP in a rat geometry. A modified pig AP, shortened to match the rat APD, showed little coupling between AT and APD with greatly reduced slope compared to the rat AP. Electrotonic modulation of repolarization by the activation sequence is especially pronounced in small hearts with murine-like APs. Tissue architecture and AP morphology play an important role in electrotonic modulation of repolarization.
Abstract Fish myocytes continue to develop active tension when stretched to sarcomere lengths (SLs) on the descending limb of the mammalian length–tension relationship. A greater length-dependent ...activation in fish than mammals could account for this because the increase in Ca2+ sensitivity may overcome the tendency for force to fall due to reduced cross-bridge availability at SLs above optimal myofilament overlap. We stretched skinned fish and rat ventricular myocytes over a wide range of SLs, including those on the descending limb of the mammalian length–tension relationship. We found that fish myocytes developed greater active tension than rat myocytes at physiological Ca2+ concentrations at long SLs as a result of a higher Ca2+ sensitivity and a steeper relationship between Ca2+ sensitivity and SL. We also investigated the diastolic properties of fish and rat myocytes at long SLs by measuring titin-based passive tension, titin isoform expression and titin phosphorylation. Fish myocytes produced higher titin-based passive tension despite expressing a higher proportion of a long N2BA-like isoform (38.0 ± 2% of total vs 0% in rat). However, titin phosphorylation in fish myocytes was lower than in rat, which may explain some of the difference in passive tension between species. The high level of titin-based passive tension and the differential phosphorylation of sarcomeric proteins in fish myocytes may contribute to the enhanced length-dependent activation and underlie the extended range of in vivo stroke volumes found in fish compared with mammals.
Changes in skeletal muscle volume induce localized sarcoplasmic reticulum (SR) Ca(2+) release (LCR) events, which are sustained for many minutes, suggesting a possible signaling role in plasticity or ...pathology. However, the mechanism by which cell volume influences SR Ca(2+) release is uncertain. In the present study, rat flexor digitorum brevis fibers were superfused with isoosmotic Tyrode's solution before exposure to either hyperosmotic (404 mOsm) or hypoosmotic (254 mOsm) solutions, and the effects on cell volume, membrane potential (E(m)), and intracellular Ca(2+) (Ca(2+)(i)) were determined. To allow comparison with previous studies, solutions were made hyperosmotic by the addition of sugars or divalent cations, or they were made hypoosmotic by reducing NaCl(o). All hyperosmotic solutions induced a sustained decrease in cell volume, which was accompanied by membrane depolarization (by 14-18 mV; n = 40) and SR Ca(2+) release. However, sugar solutions caused a global increase in Ca(2+)(i), whereas solutions made hyperosmotic by the addition of divalent cations only induced LCR. Decreasing osmolarity induced an increase in cell volume and a negative shift in E(m) (by 15.04 +/- 1.85 mV; n = 8), whereas Ca(2+)(i) was unaffected. However, on return to the isoosmotic solution, restoration of cell volume and E(m) was associated with LCR. Both global and localized SR Ca(2+) release were abolished by the dihydropyridine receptor inhibitor nifedipine by sustained depolarization of the sarcolemmal or by the addition of the ryanodine receptor 1 inhibitor tetracaine. Inhibitors of the Na-K-2Cl (NKCC) cotransporter markedly inhibited the depolarization associated with hyperosmotic shrinkage and the associated SR Ca(2+) release. These findings suggest (1) that the depolarization that accompanies a decrease in cell volume is the primary event leading to SR Ca(2+) release, and (2) that volume-dependent regulation of the NKCC cotransporter contributes to the observed changes in E(m). The differing effects of the osmotic agents can be explained by the screening of fixed charges by divalent ions.
The book suggests that culturally responsive and sustaining education should be the guiding principle in our schools, and that community partnerships be developed in a similar light. Although many of ...the chapters focus on specific content or places, a transdisciplinary problem and project-based experiential critical pedagogy is an ultimate goal. This necessitates developing awareness, advocacy and action / engagement regarding issues of race, ethnicity, gender, ability, choice, and culture to promote equity and social justice. The stories included in this collection are those of educators in a variety of contexts, but always through a public education framing. The stories come from educators at all levels of public education who are currently practicing in one of the most diverse urban areas of the U.S. Their experiences serve to provide hope for transformational change in education where the priority is truly equity and social justice for all. The idea is to provide voices of these brave educators who are striving to address equity and social justice issues is schools, education, and society -- on their teaching and in the students' learning.
Mechanisms associated with right ventricular (RV) hypertension and arrhythmias are less understood than those in the left ventricle (LV). The aim of our study was to investigate whether and by what ...mechanisms a proarrhythmic substrate exists in a rat model of RV hypertension and hypertrophy. Rats were injected with monocrotaline (MCT; 60 mg/kg) to induce pulmonary artery hypertension or with saline (CON). Myocardial levels of mRNA for genes expressing ion channels were measured by real-time RT-PCR. Monophasic action potential duration (MAPD) was recorded in isolated Langendorff-perfused hearts. MAPD restitution was measured, and arrhythmias were induced by burst stimulation. Twenty-two to twenty-six days after treatment, MCT animals had RV hypertension, hypertrophy, and decreased ejection fractions compared with CON. A greater proportion of MCT hearts developed sustained ventricular tachycardias/fibrillation (0.83 MCT vs. 0.14 CON). MAPD was prolonged in RV and less so in the LV of MCT hearts. There were decreased levels of mRNA for K(+) channels. Restitution curves of MCT RV were steeper than CON RV or either LV. Dispersion of MAPD was greater in MCT hearts and was dependent on stimulation frequency. Computer simulations based on ion channel gene expression closely predicted experimental changes in MAPD and restitution. We have identified a proarrhythmic substrate in the hearts of MCT-treated rats. We conclude that steeper RV electrical restitution and rate-dependant RV-LV action potential duration dispersion may be contributing mechanisms and be implicated in the generation of arrhythmias associated with in RV hypertension and hypertrophy.
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