The rs13376333 risk allele mimics the effect of atrial fibrillation on SK-current density and afterdepolarizations in human atrial myocytes

Background

Various genome wide association studies have associated the single nucleotide polymorphism (SNP) rs13376333 with increased risk of atrial fibrillation. This SNP is intronic to the KCNN3 gene that codes for the small conductance calcium-activated potassium channel type 3 (SK3), but functional electrophysiological effects of the risk variant are not known.

Purpose

This study aimed to study the effects of atrial fibrillation and the rs13376333 risk SNP on the apamine sensitive SK-current and on afterdepolarizations in human atrial myocytes.

Methods

Myocytes were isolated from human right atrial samples, and subjected to perforated patch-clamp technique to measure the apamine (100 nM) sensitive SK-current elicited by a voltage-ramp protocol, spontaneous transient inward Na-Ca exchange currents (ITI) or afterdepolarizations at rest. To assess the impact of the rs13376333 risk variant, patients without atrial fibrillation were genotyped and divided into risk and control groups according to the genotype at rs13376333.

Results

At membrane potentials above −40 mV, the SK-current density increased with increasing membrane potentials. At +20mV the density was significantly smaller (0.18±0.04 pA/pF) in 5 patients with atrial fibrillation than in 24 patients without this arrhythmia (0.65±0.12 pA/pF, p<0.01). On the other hand, the ITI density was larger in 10 patients with than 23 without atrial fibrillation (0.71±0.16 vs. 0.30±0.03 pA/pF, p<0.01), supporting the notion that a smaller SK-current in myocytes from patients with atrial fibrillation cannot counter-balance the calcium-release induced ITI, resulting in a larger net ITI density and a higher incidence of detectable afterdepolarizations in these patients. In line with this notion, inhibition of the SK-current with 100 nM apamine increased the frequency of afterdepolarization between −85 and −75 mV from 1.2±0.3 to 2.5±0.5 events/min (p=0.03) in patients without atrial fibrillation. Analysis of the current-voltage relationship for the SK-current in atrial myocytes from patients with the rs13376333 risk variant showed that these patients presented a significantly smaller apamine-sensitive SK current (p<0.001, n=15). At +20 mV the SK-density was 60% smaller in patients carrying a rs13376333 risk allele (0.43±0.18 vs. 1.08±0.27 pA/pF) mimicking the depression of the SK-current observed in the patients with atrial fibrillation.

Conclusion

Both the rs13376333 risk allele and atrial fibrillation is associated with a reduced SK-current in human atrial myocytes, which may contribute to the higher incidence of afterdepolarizations in these conditions. Thus, restoration of SK-channel function in patients with a rs1337633 risk SNP or with atrial fibrillation may help reduce the incidence of afterdepolarizations in these patients.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Spanish Ministry of Science and Innovation

PDE1 inhibition and adenosine A2A receptor activation synergistically potentiates spontaneous calcium waves and transients in mouse ventricular myocytes

Background

The phosphodiesterase-1C (PDE1C) and the adenosine A2A receptor (A2AR) are located in a macromolecular cluster and expected to modulate cAMP-dependent regulation of the intracellular calcium homeostasis in cardiac myocytes. In this contect, atrial fibrillation has been associated with a high incidence of spontanoeus arrhythmogenic calcium waves caused by excessive A2AR activation. On the other hand, inhibition of PDE1C and A2AR activation has also been reported to produce a synergistic and protective effect on cardiomyocyte survival, but little is known about the corresponding effects on the intracellular calcium homeostasis.

Purpose

This study, therfore, aimed to investigate how PDE1 inhibition and/or A2AR activation modulate sarcoplasmic reticulum (SR) calcium homeostasis in mouse ventricular myocytes.

Methods

Mouse ventricular myocytes (47 cells from 20 mice) were loaded with the calcium indicator Rhod-2 and spontaneous calcium release events at rest or caffeine induced calcium transients were visualized using resonance-scanning confocal microscopy at a frame rate of 150 Hz. The SR calcium content was estimated as the ratio of the peak fluorescence and the fluoresence the end of a 10 mM caffeine pulse (20s). Calcium leak was estimated as ratio of the flourescence emmision at baseline before and at the end of the caffeine pulse.

Results

The incidence of spontaneous calcium waves or transients increased from 0.7±0.3 events/min in control to 1.1±0.5 events/min and 3.4±1.0 events/min (p<0.01) after inhibition of PDE1 with the selective antagonist ITI-214 and the A2AR agonist CGS-21680 respectively. Concomitant PDE1 inhibition and A2AR activation synergistically increased the event frequency to 8.4±2.3 events/min (p<0.05). There were no significant differences in the corresponding SR calcium contents (4.4±0.3, 4.5±0.4, 4.7±0.4 and 4.1±0.2 for control, ITI-214, CGS-21680 and ITI-214+CGS-21680 respectively). However, there was a parallel increase in calcium leak from 8.7±4.2% in control to 21.8±9.8% with ITI-214, 17.6±7.2% with CGS-21680 and 37.5±11.1% when myocytes were exposed to both compounds (p<0.05), suggesting that the higher incidence of spontaneous calcium waves and transients observed upon PDE1 inhibition and/or A2AR activation is likely caused by a lowering of the threshold for spontaneous SR calcium release rather than by SR calcium overloading.

Conclusion

PDE1 inhibition exacerbates the incidence of spontaneous arrhythmogenic SR calcium release events in myocytes with high levels of A2AR activation, suggesting that PDE1 inhibition under these conditions increases the risk of arrhythmia.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Spanish Ministry of Science and Innovation

Walras, a novel lncRNA linked to cardiomyocyte cytoarchitecture and atrial fibrillation

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Junta de Andalucia

Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia in humans. Genetic and genomic analyses have recently demonstrated that the homeobox transcription factor Pitx2 plays a fundamental role regulating expression of distinct growth factors, microRNAs and ion channels leading to morphological and molecular alterations that promote the onset of AF. Here we address the plausible contribution of long non-coding (lnc)RNAs within the Pitx2>Wnt>miRNA signaling pathway. In silico analyses of annotated lncRNAs in the vicinity of the Pitx2, Wnt8 and Wnt11 chromosomal loci identified five novel lncRNAs with differential expression during cardiac development. Importantly, three of them, Walaa, Walras and Wallrd, are evolutionarily conserved in humans and displayed preferential atrial expression during embryogenesis. In addition, Walrad displayed moderate expression during embryogenesis but was more abundant in the right atrium. Walaa, Walras and Wallrd were distinctly regulated by Pitx2, Wnt8, and Wnt11, and Wallrd was severely elevated in conditional atrium-specific Pitx2-deficient mice. Furthermore, pro-arrhythmogenic and pro-hypertrophic substrate administration to primary cardiomyocyte cell cultures consistently modulate expression of these lncRNAs, supporting distinct modulatory roles of the AF cardiovascular risk factors in the regulation of these lncRNAs. Walras affinity pulldown assays revealed its association with distinct cytoplasmic and nuclear proteins previously involved in cardiac pathophysiology, while loss-of-function assays further support a pivotal role of this lncRNA in cytoskeletal organization. We propose that lncRNAs Walaa, Walras and Wallrd, distinctly regulated by Pitx2>Wnt>miRNA signaling and pro-arrhythmogenic and pro-hypertrophic factors, are implicated in atrial arrhythmogenesis, and Walras additionally in cardiomyocyte cytoarchitecture.

Treatment with beta-blockers normalizes RyR2 phosphorylation and calcium spark activity in atrial myocytes from patients with atrial fibrillation

Background

Atrial fibrillation has been associated with an increase in ryanodine receptor (RyR2) phosphorylation and local calcium release (calcium sparks). Carvedilol, a nonselective beta-adrenergic receptor blocker also inhibits the cardiac ryanodine receptor (RyR2), but it has been suggested that the enantiomer R-carvedilol only inhibits RyR2 activity and hence has the potential to inhibit calcium sparks without affecting RyR2 phosphorylation.

Purpose

This study aimed to determine the ability of the enantiomers R- and S-carvedilol to reverse RyR2 phosphorylation at s2808 and calcium sparks induced by the β2-adrenergic agonist fenoterol, in order to determine the relationship between RyR2 phosphorylation at s2808 and calcium spark frequency, and to assess the efficacy of R- and S-carvedilol.

Methods

Human right atrial myocytes were isolated and subjected to immunofluorescent labelling of total and s2808 phosphorylated RyR2, or loaded with fluo-4 and subjected to confocal calcium imaging. Beta-adrenergic receptors were first activated with 3μM fenoterol and then inhibited by different concentrations of carvedilol R- or S-enantiomers.

Results

Incubation of myocytes with fenoterol increased the s2808/RyR2 ratio from 0.32±0.03 to 0.66±0.05 (n=18, p<0.001). Incubation with 0.1, 0.3, 1 or 3μM R-carvedilol in the presence of fenoterol changed the s2808/RyR2 ratio to 0.64±0.05, 0.44±0.04, 0.34±0.07 and 0.28±0.05 (p<0.01) respectively. For comparison 3μM S-carvedilol reduced the s2808/RyR2 ratio to 0.23±0.06 in myocytes from 5 patients (p<0.01). Confocal calcium imaging revealed that fenoterol increased the spark density from 0.28±0.04 to 1.24±0.25 events/s/1000μm2 (n=9, p<0.01) and addition of 0.1, 0.3, or 1μM R-carvedilol changed the frequency to 1.32±0.52, 0.38±0.05, and 0.15±0.05 events/s/1000μm2 (p<0.01) respectively. Analysis of atrial myocytes from patients without atrial fibrillation revealed that the s2808/RyR2 ratio was similar in 25 patients treated with beta-blockers (0.39±0.04) and 57 that did not receive beta-blockers (0.44±0.03, p=0.33) while the s2808/RyR2 ratio was significantly smaller in 16 patients with atrial fibrillation receiving beta-blockers (0.43±0.08) than in 5 patients that did not (0.80±0.19, p<0.05).

Conclusions

R-carvedilol reverses the effects of beta-adrenergic stimulation on s2808 phosphorylation and calcium sparks in human atrial myocytes, and treatment with beta-blockers reduces excessive RyR2 phosphorylation at s2808 in patients with atrial fibrillation to levels observed in those without the arrhythmia, pointing to beta-adrenergic receptors as a target for controlling RyR2 phophorylation and activity in atrial fibrillation.

Funding Acknowledgement

Type of funding source: Public grant(s) – National budget only. Main funding source(s): Spanish Ministry of Science and Innovation & Spanish Ministry of Health and Consume

Pathological phosphorylation of the ryanodine receptor at s2808 increases the number of individual clusters activated per calcium spark and the calcium released per cluster

Background

Atrial fibrillation (AF) has been associated with an increase in ryanodine receptor (RyR2) phosphorylation and local calcium release (sparks), but it is not known how calcium dynamics of individual RyR2 clusters affect spark dimensions and properties.

Purpose

This study aimed to test the hypothesis that pathological alterations in the phosphorylation of individual RyR2 clusters at s2808 facilitate the fusion of spontaneous calcium release events from neighboring RyR2 clusters.

Methods

Cardiomyocytes from mice with GFP-tagged RyR2 or human right atrial tissue were subjected to confocal calcium imaging or immunofluorescent labelling of total and s2808 phosphorylated RyR2. Calcium signals were measured at a frame rate of 240 Hz in a 0.5 x 0.5 μm region of interest (ROI) for each GFP-tagged RyR2 cluster and spontaneous calcium release events were detected using a custom-made algorithm.

Results

Calcium sparks recorded in 41 myocytes with GFP-tagged RyR2s was due to the spontaneous opening of a single RyR2 cluster in 91.2±2.2% of the cells and two neighbouring clusters in (6.2±1.6%) of the cells. Events with two clusters had bigger amplitude (0.14±0.01 vs. 0.10±0.01, p<0.05), were wider (1.43±0.03 vs. 1.13±0.04 μm, p<0.05), and lasted longer at half maximum (59.8±5.2 vs. 44.4±2.4 ms, p<0.01). Consequently, the calcium spark mass, measured as the time integral of the spark in each ROI increased from 9.2±1.6 for 1 cluster to 17.8±3.5 a.u. for 2 clusters (p<0.01). Interestingly, sparks lasted longer (79±5 vs. 61±4 ms, p<0.001) were wider (3.0±0.2 vs. 2.2±0.1 μm, p>0.001) and had bigger mass (31.5±3.3 vs. 21.9±3.3 a.u, p<0.01) in atrial myocytes from 21 patients with AF than in 27 without. Because phosphorylation of RyR2 clusters at s2808 (s2808/total RyR2) was higher in patients with than without AF (0.80±0.19 vs. 0.44±0.03, p<0.05), we tested how stimulation of RyR2 phosphorylation at s2808 with the β2-adrenergic agonist fenoterol (3μM) affected calcium release in individual RyR2 clusters. Fenoterol increased s2808 phosphoryaltion from 0.39±0.05 to 0.79±0.16 (p<0.05, n=9). It also increased the mass of sparks with 1 RyR2 cluster (from 9.2±1.1 to 16.0±2.3 a.u., p<0.01) and sparks with 2 clusters from 17.8±3.5 to 23.6±2.7 a.u. Moreover, it increased the fraction of sparks with 2 clusters from 6.2±1.6% to 19.3±3.3% (p<0.01) and sparks with 3 clusters reached 6.3±1.9% in the presence of fenoterol.

Conclusions

The calcium spark mass recorded in patients without AF is comparable to that recorded during activation of calcium release from one or two GFP-tagged RyR2 clusters. The larger mass and slower kinetics of sparks recorded in patients with AF is compatible with an increase in the calcium released from each RyR2 cluster and a 3-fold increase in sparks with 2 or 3 RyR2 clusters observed in GFP-tagged RyR2s when phosphorylation at s2808 is increased to levels observed in atrial fibrillation.

Funding Acknowledgement

Type of funding source: Public grant(s) – National budget only. Main funding source(s): Spanish Ministry of Science and Innovation; Generatlitat de Catalunya

P3830Carvedilol treatment diminishes spontaneous calcium release and electrical activity in human atrial myocytes

Background

Atrial fibrillation (AF) has been associated with an increase in spontaneous calcium release induced electrical activity, which could potentially be reversed by carvedilol, a nonselective beta-blocker that also inhibits the cardiac ryanodine receptor (RyR2). Interestingly the enantiomer R-carvedilol inhibits the RyR2 but not beta-adrenergic receptors, allowing it to effectively prevent calcium release-induced spontaneous electrical activity without inducing bradycardia and hypotension.

Purpose

The purpose of this study was to determine how carvedilol treatment affects calcium release-induced transient inward currents (ITI) in human atrial myocytes from patients with AF; and to test the effects of R-carvedilol on spontaneous calcium release in order to assess its therapeutical utility.

Methods

Human atrial myocytes were isolated from patients undergoing cardiac surgery and subjected to patch-clamp technique (n=60) or confocal calcium imaging (n=6). Beta-2 adrenergic receptors were activated with the selective agonist fenoterol (3μM) and 1μM R-carvedilol was used to inhibit spontaneous calcium release events.

Results

Recordings of calcium release-induced transient inward currents (ITI) revealed that carvedilol treatment reduced the ITI frequency in patients with AF from 2.2±0.4 events/min in untreated patients to 0.59±0.35 events/min (p<0.01), which was even lower than the incidence in patients without AF (1.0±0.1 events/min; p<0.01). To assess the effects of R-carvedilol, myocytes were first simulated with fenoterol. This increased the calcium spark frequency from 23±15 to 960±336 events/s/1000μm2 in 16 cells from 6 patients (p<0.05). This was due to an increase in the spark site density (from 0.50±0.24 to 12.1±2.4 sites/1000μm2, p<0.001) rather than in the firing rate (0.068±0.14 vs. 0.035±0.012 sparks/s in control, p=0.14). Fenoterol also increased the spark duration from 50.9±5.4 to 77.3±4.1ms (p<0.001) without affecting the amplitude. Importantly, fenoterol also induced global calcium release events such as calcium waves and transients (2.8±1.1 vs. 0 events/min in control, p<0.05). When R-carvedilol was added, the effects of fenoterol were abolished, reducing the incidence of calcium sparks to 69±51 events/s/1000μm2 (p<0.05), the spark site density to 1.68±1.04 sites/1000μm2 (p<0.01), the spark duration to 63.4±4.3ms (p<0.05), and calcium waves and transients were reduced to 0.21±0.14 events/min (p<0.05).

Conclusions

Carvedilol treatment reduces the ITI frequency in patients with AF to levels below that observed at baseline in patients without AF. Furthermore, the non-beta-blocking R-carvedilol enantiomer abolishes spontaneous calcium release events induced by beta-2 adrenergic stimulation in human atrial myocytes, proposing a therapeutical utility for this compound in patients with AF linked to excessive spontaneous calcium release.

Acknowledgement/Funding

SAF2017-88019; Marato2015-20-30; SGR2017-1769; CIBERCV

P1233Differential effects of five risk variants for atrial fibrillation at the 4q25 region on L-type calcium current and transient inward currents in human atrial myocytes

Background

An increasing number of single nucleotide polymorphisms (SNPs) at the chromosomal region 4q25 have been associated with risk of atrial fibrillation (AF) and we have recently reported that carriers of the rs13143308 risk variant have an increased incidence of spontaneous calcium release-induce transient inward currents (ITI). However, it is not known if different 4q25 variants have similar effects.

Purpose

This study aimed to compare the effects of five SNPs at 4q25 on L-type calcium current (ICa) and ITI frequency, features that are altered in patients with AF.

Methods

To avoid confounding effects of AF on calcium homeostasis, atrial samples from 63 patients without AF were genotyped and divided into groups according to the genotype of the SNPs rs1448818, rs6817105, rs13143308, rs6843082, rs3853443 ordered according to their location and identified by the three last digits + an R for risk or N for normal variants. ICa density and ITI frequency were measured with perforated patch clamp technique in atrial myocytes from these patients.

Results

Three SNPs 818, 308 and 443 segregated independently of the genotype at the other loci. The 105 and 082 loci always co-segregated with 308 but never together. The ICa density was smaller in carriers of 818R and 443N variants (−1.6±0.3pA/pF, p=0.01) or 818N and 443R variants (−1.6±0.4pA/pF, p=0.02) than in patients with 818N and 443N variants (−3.2±0.4pA/pF), independently of the genotype at 105, 308 and 082 (these loci did not affect ICa). In contrast, to this, the ITI frequency was increased only in myocytes from patients carrying 105R, 308R and 082N (1.4±0.2events/min, p<0.001) or 105N, 308R and 082R (1.6±0.5events/min, p=0.002) when compared to patients with 105N, 308N and 082N (0.36±0.09events/min) independently of the genotypes at 818 and 443, or when compared to patients without risk at any of the five loci (0.55±0.30events/min). The table shows schematically the qualitative effects of the different risk variants.

Risk variant rs1448818C rs6817105C rs13143308T rs6843082T rs3853445C ICa Decreased Unchanged Unchanged Unchanged Decreased ITI Unchanged Increased Increased Increased Unchanged

Conclusion

Different SNPs at the chromosomal region 4q25 are associated with differential pathological changes in intracellular calcium homeostasis. Risk variants at rs1448818 or rs3853445 cause loss of ICa without affecting ITI frequency while a risk variant at rs13143308 elevates the ITI frequency without affecting ICa. These findings afford a framework for stratification of pharmacological therapy based on the functional effects of the 4q25 risk variants

Acknowledgement/Funding

SAF2017-88019; Marato2015-20-30; SGR2017-1769; CIBERCV

Spatial localization of ryanodine receptors in human cardiac cells

We present a novel image processing method to determine the location of ryanodine receptors in cardiac cells. A semi-automatic manual validation by an expert has been used in order to establish the performance of the segmentation method. The approach provides high accuracy under different experimental conditions and it is robust to common sources of noise including experimental, molecular and biological fluctuations.

Multilevel analysis of calcium dynamics in stimulated cultures of cardiomyocytes

We present an automatic method to characterize calcium activity in a culture of cardiac cells from a sequence of microscopy fluorescence images. The approach quantifies both the response of each individual cell in the culture to external field stimulation and the propagation properties of calcium wave-fronts, thus providing complementary information at different physiological levels. The technique classifies the response of each cell as regular or irregular based on a set of dynamical and morphological features of the calcium transients. Isochronal maps were constructed from the local activation times across the culture, and the front propagation was classified as planar or non-planar. The method has been applied to a set of 35 experiments, and the results indicate a significative connection between irregular behavior at the single-cell level and irregular front propagation.

Identification of intracellular calcium dynamics in stimulated cardiomyocytes

We have developed an automatic method for the analysis and identification of dynamical regimes in intracellular calcium patterns from confocal calcium images. The method allows the identification of different dynamical patterns such as spatially concordant and discordant alternans, irregular behavior or phase-locking regimes such as period doubling or halving. The method can be applied to the analysis of different cardiac pathologies related to anomalies at the cellular level such as ventricular reentrant arrhythmias.

Umbilical cord blood-derived stem cells spontaneously express cardiomyogenic traits

BACKGROUND

Umbilical cord blood (UCB) has been widely used for hematopoietic stem cell transplantation. The UCB-derived stem cells (UCBSCs) have been proposed as an alternative to bone marrow (BM)-derived mesenchymal stem cells (MSCs) for cardiac cell-based therapy. Herein we studied whether UCBSCs spontaneously exhibit cardiac-specific markers in vitro.

METHODS

Human UCBSCs were isolated, expanded, and phenotyped by flow cytometry, quantitative RT-PCR, and immunofluorescence. Cell pluripotency and proliferation were also assessed by adipogenic and osteogenic media and in growth assays.

RESULTS

Among 25 analyzed UCB, 16% of cases afforded primary culture satisfactory generation of UCBSCs. Duplication time (Td) of cultures was 2.16 +/- 0.06 days. The cells were strongly positive for CD105 (18.5 +/- 0.14), CD44 (27 +/- 2.8), CD166 (13 +/- 9), CD29 (59 +/- 9.4), CD90 (60 +/- 11) and consistently negative for CD117 (1.2 +/- 0.1), CD106 (1.1 +/- 0), CD34 (1.2 +/- 0.2), CD14 (1 +/- 0), and CD45 (1 +/- 0), consistent with a mesenchymal lineage. Adipogenesis and osteogenesis of cells resulted in low accumulation of intracellular lipid droplets and high deposition of calcium. The UCBSCs showed gene transcripts for alpha-actinin, connexin (Cx)-43, SERCA-2, and stromal cell-derived factor (SDF)-1alpha. At the protein level, the cells abundantly expressed alpha-actinin, Cx-43, SERCA-2 and SDF-1alpha. In contrast, these cells did not express the cardiac transcription factors GATA-4, Tbx5, and Nkx2.5, nor the sarcomeric proteins beta-myosin heavy chain (beta-MyHC) or cardiac troponin I (cTnI).

CONCLUSIONS

Human UCBSCs may represent an alternative source of stem cells for myocardial-cell replacement. These cells can be highly expanded. They spontaneously express proteins of paramount importance for cardiovascular regeneration, such as Cx-43, SERCA-2, and SDF-1alpha.

Identification of cardiomyogenic lineage markers in untreated human bone marrow-derived mesenchymal stem cells

BACKGROUND

Recent reports refute the classic paradigm by which human heart is unable to repair itself following disease or injury. Cardiac and noncardiac stem cells with cardiac regeneration potential have been documented. We studied whether untreated mesenchymal stem cells express markers of cardiomyogenic lineage in vitro.

METHODS

Mesenchymal stem cells were obtained from human iliac crest marrow aspirates. Cells were isolated and characterized using flow cytometry by surface expression of CD105, CD166, CD29, CD44, CD14, and CD34. To evaluate their cardiomyogenic potential, presence of cardiac proteins (cardiac troponin I, sarcomeric alpha-actinin, beta myosin heavy chain (beta-MyHC), connexin-43, and SERCA-2), and transcription factors (GATA-4) were assessed.

RESULTS

Mesenchymal stem cells expressed CD105 (4.25 +/- 0.35), CD166 (27.83 +/- 1.89), and CD29 (9.4 +/- 0.57) and were negative for CD34, CD14, and CD45. In absence of additional stimuli in the culture media, these cells expressed connexin-43, alpha-actinin, and GATA-4, and were negative for SERCA-2, cardiac troponin I, and beta-MyHC.

CONCLUSIONS

Human adult mesenchymal stem cells spontaneously exhibit markers of cardiac phenotype in vitro. In the appropiate myocardial environment, these cells may transdifferentiate into mature cardiomyocytes.

Otilonium bromide inhibits muscle contractions via L-type calcium channels in the rat colon

The aim of this study is to evaluate in vitro the effect of otilonium bromide (OB) on the mechanical and electrical activities of the rat colonic smooth muscle using muscle bath, microelectrodes and patch-clamp techniques. Otilonium bromide dose dependently inhibited the spontaneous activity (logIC(50) +/- SE: -5.31 +/- 0.05). This effect was not modified by TTX (10(-6) mol L(-1)). Cyclic depolarizations were abolished by OB (10(-4) mol L(-1)). Electrical field stimulation induced inhibitory junction potentials (IJPs) followed by a depolarization with superimposed spikes causing a contraction. In the presence of OB (10(-4) mol L(-1)) IJPs were recorded, but spikes and contractions were abolished. Otilonium bromide (3 x 10(-6) mol L(-1)) inhibited inward current obtained in isolated cells (amphotericin perforated patch technique). The otilonium-sensitive current amplitude was maximal (75pA) around 0 mV. The effect of different doses of OB was tested by depolarizing cells from -70 mV to 0 mV. OB dose dependently inhibited the inward current with an EC(50) of 885 nmol L(-1). Abolishment of the otilonium-sensitive current by 3 x 10(-6) mol L(-1) nifedipine confirmed that it was an L-type Ca(2+) current. Our results show that OB inhibits the spontaneous and triggered muscular contractions. This effect is produced by the inhibition of muscular action potentials carried by L-type calcium current, confirming the spasmolytic properties of OB.

The function of the sarcoplasmic reticulum is not inhibited by low temperatures in trout atrial myocytes

The effect of temperature on sarcoplasmic reticulum (SR) Ca(2+) uptake and release was measured in trout atrial myocytes using the perforated patch-clamp technique. Depolarization of the myocyte for 10 s to different membrane potentials (V(m)) induced SR Ca(2+) uptake. The relationship between V(m) and SR Ca(2+) uptake was not significantly changed by lowering the experimental temperature from 21 to 7 degrees C, and the relationship between total cytosolic Ca(2+) and SR Ca(2+) uptake was similar at the two temperatures with a pooled V(max) = 66 amol/pF and K(0.5) = 4 amol/pF. Quantification of the Ca(2+) release from the SR elicited by 10-ms depolarizations to different V(m) showed an increasing SR Ca(2+) release at more positive V(m) between -50 and +10 mV, whereas SR Ca(2+) release stagnated between +10 and +50 mV. Lowering of the temperature did not affect this relationship significantly, giving an SR Ca(2+) release of 1.71 and 1.54 amol/pF at 21 and 7 degrees C, respectively. Furthermore, clearance of the SR Ca(2+) content slowed down inactivation of the L-type Ca(2+) current at both temperatures (the fast time constant increased significantly from 10.4 +/- 1.9 to 15.0 +/- 2.0 ms at 21 degrees C and from 38 +/- 15 to 73 +/- 24 ms at 7 degrees C). Thus the SR has the capacity to remove the entire Ca(2+) transient at physiologically relevant stimulation frequencies at both 21 and 7 degrees C, although it is estimated that ~40% of the total Ca(2+) transient is liberated from and reuptaken by the SR with continuous stimulation at 0.5 Hz independently of the experimental temperature.

Characterization of the relationship between Na+ -Ca2+ exchange rate and cytosolic calcium in trout cardiac myocytes

The whole-cell patch-clamp technique combined with rapid caffeine (CAF) applications was used to measure Na+-Ca2+ exchange (NCX) currents (I(NCX)). The rate of Ca2+ extrusion and the amount of Ca2+ extruded from the cell upon a rapid CAF exposure were obtained from I(NCX) and its time integral, respectively. This gave a maximal NCX rate (V(NCX)) of 151 amol pF(-1) s(-1) or 2.3 mM s(-1) and a half-maximal V(NCX) (K0.5) at a total cellular [Ca2+] ([Ca2+]tot) of 15.4 amol pF(-1). Using the same approach for the tail current induced by repolarization to -80 mV gave a K0.5 of 7.0 amol pF(-1) corresponding to 108 microM total or 2-4 microM free Ca2+. The relationship between [Ca2+]tot and V(NCX) was linear in the physiological range. Inhibition of the SR function with cyclopiazonic acid plus ryanodine reduced the slope significantly from 23.2+/-1.4 to 17.6+/-1.6 s(-1), while ryanodine alone had no effect. The relationship between [Ca2+]tot and V(NCX) was steeper at more negative membrane potentials, and with identical SR Ca2+ loads the maximal VNCX at -10 mV was reduced to 39.7+/-2.7% of the value at -90 mV. Long depolarizations caused SR Ca2+ loading through reverse-mode NCX. Between -30 and +10 mV reverse mode V(NCX)=Vm.0.047 amol pF(-1) s(-1) mV(-1)+2.51 amol pF(-1) s(-1), giving a reversal potential of -54 mV. In conclusion, the relationship between V(NCX) and [Ca2+]tot shows that the NCX is capable of removing a total Ca2+ transient of 60 microM at physiological heart rates, while reverse-mode NCX reloads the sarcoplasmic reticulum (SR) during depolarization. Furthermore, small alterations in the action potential configuration are predicted to change significantly the relative importance of the NCX in the regulation of cytosolic [Ca2+] and SR Ca2+ loading.

Na(+)/Ca(2+)-exchange activity regulates contraction and SR Ca(2+) content in rainbow trout atrial myocytes

We have used the whole cell configuration of the patch-clamp technique to measure sarcolemmal Ca(2+) transport by the Na(+)/Ca(2+) exchanger (NCX) and its contribution to the activation and relaxation of contraction in trout atrial myocytes. In contrast to mammals, cell shortening continued, increasing at membrane potentials above 0 mV in trout atrial myocytes. Furthermore, 5 microM nifedipine abolished L-type Ca(2+) current (I(Ca)) but only reduced cell shortening and the Ca(2+) carried by the tail current to 66 +/- 5 and 67 +/- 6% of the control value. Lowering of the pipette Na(+) concentration from 16 to 10 or 0 mM reduced Ca(2+) extrusion from the cell from 2.5 +/- 0.2 to 1.0 +/- 0.2 and 0.5 +/- 0.06 amol/pF. With 20 microM exchanger inhibitory peptide (XIP) in the patch pipette Ca(2+) extrusion 20 min after patch break was 39 +/- 8% of its initial value. With 16, 10, and 0 mM Na(+) in the pipette, the sarcoplasmic reticulum (SR) Ca(2+) content was 47 +/- 4, 29 +/- 6, and 10 +/- 3 amol/pF, respectively. Removal of Na(+) from or inclusion of 20 microM XIP in the pipette gradually eliminated the SR Ca(2+) content. Whereas I(Ca) was the same at -10 or +10 mV, Ca(2+) extrusion from the cell and the SR Ca(2+) content at -10 mV were 65 +/- 7 and 80 +/- 4% of that at +10 mV. The relative amount of Ca(2+) extruded by the NCX (about 55%) and taken up by the SR (about 45%) was, however, similar with depolarizations to -10 and +10 mV. We conclude that modulation of the NCX activity critically determines Ca(2+) entry and cell shortening in trout atrial myocytes. This is due to both an alteration of the transsarcolemmal Ca(2+) transport and a modulation of the SR Ca(2+) content.

Ca(2+) uptake in the sarcoplasmic reticulum from the systemic heart of octopod cephalopods

We have measured Ca(2+) uptake in crude homogenates of heart tissue, as well as cell shortening and ionic currents in isolated myocytes exposed to caffeine, to characterize Ca(2+) uptake in the sarcoplasmic reticulum (SR) of the systemic heart of octopus. The maximal rate of SR Ca(2+) uptake in crude homogenates of octopus heart was 43+/−4 (mean +/− s.e.m., N=7), compared with 28+/−2 nmol min(−)(1)mg(−)(1) protein (N=4) in homogenates of rat heart. The Ca(2+)-dependency of SR Ca(2+) uptake was similar for the two species, with a Ca(2+) activity at half-maximal uptake rate (pCa(50)) of 6.04+/−0.02 for octopus and 6.02+/−0.05 for rat. Exposure of isolated myocytes to 10 mmol l(−)(1) caffeine resulted in cell shortening to 53+/−2 % of the resting cell length and an inward trans-sarcolemmal ionic current. The charge carried by this current was 3.28+/−0.70 pC pF(−)(1) (mean +/− s.e.m., N=5) corresponding to extrusion of 34.0+/−0.7 amol Ca(2+)pF(−)(1) from the cell by Na(+)/Ca(2+) exchange. This is approximately 50 times more than the Ca(2+) carried by the Ca(2+) current elicited by a 200 ms depolarization from −80 to 0 mV and corresponds to an increase in the total intracellular [Ca(2+)] of 404+/−86 (μ)mol l(−)(1) non-mitochondrial volume due to Ca(2+) release from the SR. Thus, we find that at 20 degrees C in the SR both Ca(2+) content and Ca(2+) uptake rate in the systemic heart of octopus are comparable with or larger than the corresponding values obtained in the rat heart. These results support the argument that the SR may play an important role in the regulation of contraction in the systemic heart of cephalopods.

Quantification of calcium release from the sarcoplasmic reticulum in rainbow trout atrial myocytes

Using the whole-cell configuration of the patch-clamp technique, we quantified calcium release from the sarcoplasmic reticulum (SR) elicited by short depolarization pulses before and after clearance of the SR Ca2+ content with 10 mM caffeine (CAF). With a loaded SR, the first detectable contraction occurred with a pulse duration of 5 ms. CAF exposure increased this pulse duration to 85 ms and slowed the inactivation of the Ca2+ current (ICa). Repolarization of the cell to -80 mV after a short depolarization elicited a tail current that was attenuated markedly after CAF exposure. The difference between the charge carried by the tail currents obtained before and after CAF exposure was taken as a measure of the Ca2+ released from the SR. SR Ca2+ release increased with increasing SR Ca2+ load over the range of loads examined. In contrast, SR Ca2+ release reached a maximum when the duration of the preceding depolarization exceeded 10 ms. Maximal Ca2+ release was 1.64 amol/pF or 62 microM and elicited a contraction that was 40 +/- 6% of the amplitude of a normal contraction. This release could account for 48 +/- 10% of the total Ca2+ required to activate contraction but only a few percent of the CAF-releasable Ca2+. Thus, contrary to the general view of excitation-contraction coupling in lower vertebrates, SR Ca2+ release in trout atrial myocytes may account for up to 50% of the total Ca2+ transient at room temperature.

Quantification of Ca2+ uptake in the sarcoplasmic reticulum of trout ventricular myocytes

We measured Ca2+ uptake by the sarcoplasmic reticulum (SR) in trout ventricular myocytes, measuring indo 1 fluorescence in permeabilized cells or ionic currents in single myocytes subjected to voltage clamp. Titration of the SR Ca2+ pumps with thapsigargin gave a pump site density of 454 pmol/mg cell protein. Lowering the temperature from 20 degreesC to 10 or 5 degreesC reduced the SR Ca2+ uptake rate in permeabilized myocytes by 50 and 63%, respectively. Surprisingly, Ca2+ leak from the SR also decreased with decreasing temperatures. Exposure of single myocytes to 10 mM caffeine (Caf) induced a cell contracture and an inward ionic current. Neither contracture nor current decreased significantly after rest periods of 120 and 320 s. The inward current was due to Ca2+ extrusion by the Na+/Ca2+ exchanger (NCX), and the time integral of the exchange current (INCX) was used to calculate the SR Ca2+ content. This gave a steady-state SR Ca2+ content of 22.5 +/- 2.8 amol Ca2+/pF or 750 microM. When the SR was loaded by depolarizing the cell to +50 mV, the Ca2+ content increased with increasing length of the depolarization, reaching a maximum of 52.0 +/- 5.9 amol Ca2+/pF. When the cell was depolarized to different voltages for 3 s, a subsequent Caf-induced INCX increased with increasing voltage. At +100 mV, the Ca2+ content was 36.6 +/- 3.8 amol/pF, giving a maximal SR Ca2+ uptake rate of 12.2 +/- 1.2 amol Ca2+. pF-1. s-1 or 417 microM/s. We conclude that maximal SR Ca2+ content and Ca2+ uptake rates can be estimated using specific SR Ca2+ loading protocols. Contrary to the general assumption that contraction in lower vertebrates depends largely on transsarcolemmal Ca2+ fluxes, we found that although the L-type Ca2+ current is insufficient to fully activate contraction, the SR is capable of participating in the regulation of the cytosolic Ca2+ during the excitation-contraction coupling in trout ventricular myocytes.

L-type Ca2+ current and excitation-contraction coupling in single atrial myocytes from rainbow trout

We have examined the contribution of L-type Ca2+ current (ICa) to the activation of contraction in trout atrial myocytes under basal and phosphorylating conditions. The average myocyte length was 197 +/- 14 micrometer, width was 5.5 +/- 0.2 micrometer, and cell capacitance was 36.2 +/- 2.2 pF. With 25 microM EGTA in the patch pipette and a stimulation frequency of 0.125 Hz, ICa was 2.6 +/- 0.4 pA/pF and it carried a total charge of 0.10 +/- 0.01 pC/pF, giving rise to a contraction of 15.2 +/- 2.8% of the resting cell length. With a cell volume of 2.4 +/- 0.3 pl, the charge carried by ICa corresponded to 14.7 +/- 2.2 micromol Ca2+/l nonmitochondrial cell volume (microM). This can account for only 30-40% of the Ca2+ binding to the myofilaments during a contraction. Increasing the stimulation frequency from 0.25 to 2 Hz decreased ICa amplitude and charge by 66 +/- 5 and 80 +/- 3%, respectively. Elevating the pipette EGTA concentration from 25 microM to 5 mM increased ICa amplitude and charge by approximately 290%. Both isoproterenol and cAMP increased ICa by approximately 230%. The total charge carried by the isoproterenol- or cAMP-stimulated current was increased by 170%. We conclude that the use of high-EGTA concentration may overestimate the total Ca2+ carried by ICa under physiological conditions. Furthermore, the results suggest that, in contrast to previous reports from other lower vertebrates, Ca2+ flux through L-type Ca2+ channels alone is not sufficient to fully activate contraction in trout atrial myocytes at room temperature.

Methylene blue is a muscarinic antagonist in cardiac myocytes

We studied the mechanism of action of methylene blue (Mblue), a putative guanylyl cyclase inhibitor, on the L-type calcium current (ICa) and the muscarinic activated K+ current (IK,ACh) in rat ventricular and atrial myocytes, respectively, and on the binding of [3H]quinuclidinyl benzylate in rat ventricular membranes. Superfusion, but not internal dialysis, with 30 microM Mblue antagonized the inhibitory effect of acetylcholine (ACh, 1 microM) on beta-adrenergic stimulation of ICa with isoprenaline (Iso, 10 nM or 1 microM). However, Mblue had no effect on the basal ICa or on the stimulation of ICa by Iso in the absence of ACh. The activation of IK,ACh by 3 microM ACh was also antagonized by Mblue in a dose-dependent manner. In contrast, Mblue had no effect on the activation of IK,ACh by either guanosine-5'-O-(3-thio)triphosphate or guanosine-5'-(beta,gamma-imido)triphosphate. Chlorpromazine (CPZ), a piperazine derivative like Mblue, also inhibited the muscarinic activation of IK,ACh in a dose-dependent manner. The specific binding of [3H]QNB, a muscarinic ligand, to rat ventricular membranes was displaced in a dose-dependent manner by Mblue and CPZ. The piperazine derivatives behaved like competitive antagonists of [3H]QNB binding, exhibiting equilibrium dissociation constant (Ki) values of 187 nM for Mblue and 366 nM for CPZ. In conclusion, Mblue exerts antimuscarinic effects on ICa and IK,ACh in rat cardiac myocytes that are best explained by the binding of Mblue to the M2 subtype of muscarinic receptors. This property probably contributes to the antimuscarinic effect of the putative guanylyl cyclase inhibitor reported in previous studies.

A comparative study of the effects of three guanylyl cyclase inhibitors on the L-type Ca2+ and muscarinic K+ currents in frog cardiac myocytes

1. To investigate the participation of guanylyl cyclase in the muscarinic regulation of the cardiac L-type calcium current (ICa), we examined the effects of three guanylyl cyclase inhibitors, 1H-[1,2,4]oxidiazo-lo[4,3-a]quinoxaline-1-one (ODQ), 6-anilino-5,8-quinolinedione (LY 83583), and methylene blue (MBlue), on the beta-adrenoceptor; muscarinic receptor and nitric oxide (NO) regulation of ICa and on the muscarinic activated potassium current I(K,ACh), in frog atrial and ventricular myocytes. 2. ODQ (10 microM) and LY 83583 (30 microM) antagonized the inhibitory effect of an NO-donor (S-nitroso-N-acetylpenicillamine, SNAP, 1 microM) on the isoprenaline (Iso)-stimulated ICa which was consistent with their inhibitory action on guanylyl cyclase. However, MBlue (30 microM) had no effect under similar conditions. 3. In the absence of SNAP, LY 83583 (30 microM) potentiated the stimulations of ICa by either Iso (20 nM), forskolin (0.2 microM) or intracellular cyclic AMP (5-10 microM). ODQ (10 microM) had no effect under these conditions, while MBlue (30 microM) inhibited the Iso-stimulated ICa. 4. LY 83583 and MBlue, but not ODQ, reduced the inhibitory effect of up to 10 microM acetylcholine (ACh) on ICa. 5. MBlue, but not LY 83583 and ODQ, antagonized the activation of I(K,ACh) by ACh in the presence of intracellular GTP, and this inhibition was weakened when I(K,ACh) was activated by intracellular GTPgammaS. 6. The potentiating effect of LY 83583 on Iso-stimulated ICa was absent in the presence of either DL-dithiothreitol (DTT, 100 microM) or a combination of superoxide dismutase (150 u ml(-1)) and catalase (100 u ml(-1)). 7. All together, our data demonstrate that, among the three compounds tested, only ODQ acts in a manner which is consistent with its inhibitory action on the NO-sensitive guanylyl cyclase. The two other compounds produced severe side effects which may involve superoxide anion generation in the case of LY 83583 and alteration of beta-adrenoceptor and muscarinic receptor-coupling mechanisms in the case of M Blue.

Sympathetic modulation of the effect of nifedipine on myocardial contraction and Ca current in the rat

The regulation of cardiac L-type Ca2- current (Ica) and contraction by dihydropyridine antagonists and beta-adrenergic receptor agonists has been the subject of numerous studies over the last decade. However, little is known on the crosstalk between these two regulatory pathways. For instance, a fundamental question that remains unanswered is: does activation of the beta-adrenergic receptors modify the sensitivity of the myocardium to dihydropyridine agonists? To answer this question, we examined in the present study how activation of the beta-adrenergic receptors modifies the effects of nifedipine on the mechanical and energetic parameters of the isolated perfused rat heart. Activation of the beta-adrenergic receptors was achieved by perfusing the hearts with isoprenaline, a non-selective beta-adrenergic receptor agonist, and could be reduced by atenolol, a beta-adrenergic receptor antagonist. To examine possible alterations during hypertension in the sensitivity of the hearts to the drug, tested, the study was performed in both normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive animals (SHR). While 0.1 microM nifedipine reduced left ventricular pressure (LVP) by 36% and 34% in WKY and SHR rats, respectively, under basal conditions, its effects became negligible in both groups of rats after stimulation of the hearts with 0.1 microM isoprenaline. Addition of 1 microM atenolol in the presence of isoprenaline restored the inhibitory effect of nifedipine to control values in both WKY and SHR rats. Additional experiments were performed in isolated ventricular myocytes from WKY rats using the whole-cell patch-clamp technique. The inhibitory effects of 0.1 to 1 microM nifedipine were significantly larger on basal Ica than after the current had been previously elevated by 0.1 microM isoprenaline. Addition of 1 microM atenolol in the presence of isoprenaline partially restored the inhibitory effect of nifedipine on Ica. Our results demonstrate a reduced sensitivity of the heart muscle to nifedipine during activation of beta 1-adrenergic receptors. This effect is partly explained by a reduced inhibitory effect of nifedipine on Ic during activation of cAMP-dependent phosphorylation.

Binding constants determined from Ca2+ current responses to rapid applications and washouts of nifedipine in frog cardiac myocytes

1. A fast perfusion system was used to analyse the kinetics of the response of L-type calcium current (ICa) to rapid applications and washouts of the dihydropyridine antagonist nifedipine in whole-cell patch-clamped frog ventricular myocytes. 2. Both the inhibition of ICa induced by nifedipine and the recovery from inhibition upon washout of the drug behaved as mono-exponential functions of time. 3. During application or washout of 100 nM nifedipine, only the peak amplitude of ICa varied but not its time course of activation or inactivation. 4. The rate constant of the onset of ICa inhibition increased with the concentration of nifedipine. However, the time course of the recovery from inhibition was independent of drug concentration. 5. Both rate constants were strongly sensitive to the holding potential but insensitive to the test potential. 6. Using simple rate equations and a one-binding-site analysis it was possible to determine the rate constants for association (k1) and dissociation (k-1) and the equilibrium dissociation constant (KD) of the reaction between nifedipine and Ca2+ channels. KD values for nifedipine were identical to IC50 values obtained from classical steady-state experiments. 7. With depolarized holding potentials, KD decreased strongly due to a large reduction in k-1 and a modest increase in k1. Assuming that these changes result from the distribution of Ca2+ channels between resting and inactivated states, a low-affinity binding to the resting state (R) and a high-affinity binding to the inactivated state (I) were obtained with the binding constants: k1R = 1.0 x 10(6) M-1 S-1, k-1R = 0.077 S-1, and KDR = 77 nM for the resting state; k1I = 4.47 x 10(6) M-1 S-1, k-1I = 7.7 x 10(-4) S-1, and KDI = 0.17 nM for the inactivated state. 8. Rapid application/washout experiments provide a unique way to determine, in an intact cell and in a relatively short period (2-4 min), the binding rate constants and the KD value of the reaction between a dihydropyridine antagonist and the Ca2+ channels.

Nitric oxide synthase does not participate in negative inotropic effect of acetylcholine in frog heart

In the heart, the parasympathetic neurotransmitter acetylcholine (ACh) reduces the force of contraction. Although the effect of ACh can be partly explained by an inhibition of adenylyl cyclase, some of the effects of ACh may also be mediated via stimulation of nitric oxide synthase (NOS) and production of guanosine 3', 5'-cycle monophosphate (cGMP). NOS inhibitors can prevent the negative chronotropic effect of ACh on spontaneously beating cardiomyocytes and suppress the inhibition of the L-type calcium current (ICa) by ACh in sinoatrial myocytes. This pathway may be relevant not only to the chronotropic effect of ACh but also to its inotropic effect, because ACh, NO, and cGMP regulate the force of contraction and ICa in the cardiac ventricle. Here we report the effects of L-arginine (L-Arg), the substrate of NOS, and NG-monomethyl-L-arginine (L-NMMA) and NG-nitro-L-arginine (L-NNA), two NOS inhibitors, on muscarinic effects in the cardiac ventricle. We found that L-Arg, L-NMMA, and L-NNA have no effect on the muscarinic inhibition of ICa in isolated frog myocytes. In addition, these compounds have no significant effects on basal ICa or beta-adrenergic stimulation of ICa. L-Arg and its analogues did not change the negative inotropic effect of ACh in frog ventricular fibers. Basal active tension and the positive inotropic effect of isoproterenol, a beta-adrenergic agonist, also were unaffected. We conclude that NOS in not involved in muscarinic inhibition of ICa in isolated from ventricular myocytes or the negative inotropic effect of ACh in the frog ventricle.

Regulation of myocardial calcium channels by cyclic AMP metabolism

Hormonal regulation of cardiac inotropism is often correlated with modification of the L-type Ca-channel current. Among several regulatory pathways that control Ca-channel activity, the best described one is the cAMP cascade. Cyclic AMP-dependent phosphorylation of the Ca-channel results in an increase of the mean open probability of the individual Ca-channels and, thus, of the macroscopic Ca current. Modulation of cAMP concentration can take place at the level of adenylyl cyclases or cAMP phosphodiesterases. Of major interest is the fact that the activity of two different forms of phosphodiesterases is controlled by the level of intracellular cGMP. Thus, cAMP metabolism is intimately associated with cGMP metabolism, and both determine the degree of cAMP-dependent phosphorylation of cardiac Ca-channels. This brief discussion will focus on these two levels of control and their relative importance in the cAMP-dependent regulation of myocardial Ca-channels.

Protein kinase inhibitors reduce SR Ca transport in permeabilized cardiac myocytes

Phosphorylation of the sarcoplasmic reticulum (SR) protein phospholamban by adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA) and Ca-calmodulin-dependent protein kinase (CaM-KII) stimulates Ca-adenosinetriphosphatase (ATPase) activity and SR Ca transport, but the role of CaM-KII-dependent phosphorylation is not well defined. We studied the PKA- and CaM-KII-dependent regulation of SR Ca transport in digitonin-permeabilized rabbit ventricular myocytes. SR Ca uptake and free Ca concentration were measured on line with indo 1 and Ca electrodes in the presence of 20 microM ruthenium red and 10 mM oxalate. neither N5,2'-w-dibutyryl-cAMP (up to 500 microM) nor the nonhydrolyzable cAMP agonist adenosine 3'5'-cyclic monophosphorothioate sodium salt (Sp-cAMP[S]; up to 275 microM) affected the maximum uptake rate (Vmax) or the dissociation constant (Kd) for Ca uptake. However, the PKA inhibitor H-89 significantly increased Kd (e.g., from 307 +/- 67 to 826 +/- 62 nM Ca at 40-65 microM H-89) without significantly affecting Vmax. Both CaM-KII inhibitors, KN-62 (60 microM) and a CaM-KII inhibitory peptide (10 microM), significantly decreased Vmax from 11.95 +/- 0.5 to 9.48 +/- 0.6 nmol.mg-1.min-1 and from 10.95 +/- 1.72 to 7.37 +/- 0.94 nmol.mg-1.min-1, respectively, without consistently changing Kd. The effects of H-89 on Kd and of KN-62 on Vmax were prevented by a monoclonal antibody to phospholamban 2D12 (consistent with the antibody removing the inhibitory effect of phospholamban on the SR Ca-ATPase).(ABSTRACT TRUNCATED AT 250 WORDS)

Sarcoplasmic reticulum Ca2+ uptake and thapsigargin sensitivity in permeabilized rabbit and rat ventricular myocytes

Ca2+ uptake by the sarcoplasmic reticulum (SR) and free [Ca2+] were measured simultaneously with indo 1 and a Ca(2+)-selective minielectrode in suspensions of permeabilized rabbit or rat ventricular myocytes (approximately 10 mg/mL protein). In the presence of 25 mumol/L ruthenium red and 10 mmol/L oxalate, the Km for Ca2+ uptake by the SR was approximately 250 nmol/L in rabbit and rat ventricular myocytes. The maximal Ca2+ uptake rate was 2.4 times higher in rat than in rabbit. Addition of 5 nmol thapsigargin (TG) per milligram cell protein abolished Ca2+ uptake completely in both species. The [TG] necessary for a half-maximal reduction of the uptake rate (K1/2) was 55 pmol/mg cell protein for rabbit and 390 pmol/mg cell protein for rat. Assuming that the number of pump sites is two times the concentration of TG necessary to inhibit half of the Ca2+ pump activity (ie, the TG affinity is very high), the density of pump sites is 7.7 mumol/kg wet wt for rabbit and 54.6 mumol/kg wet wt for rat. Despite a fivefold decrease of the Ca2+ uptake rate by a submaximal [TG], the permeabilized myocytes were still able to lower the free [Ca2+] to < 150 nmol/L from a peak value > 10 mumol/L. The relative inhibition of Ca2+ uptake by TG did not depend on the free [Ca2+]. Addition of more than 5 nmol TG per milligram cell protein abolished Ca2+ uptake by the SR completely in < 15 seconds and reduced the uptake rate by 95% in 5 seconds.(ABSTRACT TRUNCATED AT 250 WORDS)

Passive Ca buffering and SR Ca uptake in permeabilized rabbit ventricular myocytes

Passive Ca binding was measured with a Ca-selective minielectrode in suspensions of permeabilized rabbit ventricular myocytes equilibrated with 5 microM thapsigargin and 30 microM ruthenium red to prevent sarcoplasmic reticulum (SR) or mitochondrial Ca uptake. Passive Ca binding was obtained by titration of the myocytes with Ca and subtraction of Ca binding in a blank titration without myocytes. Passive Ca binding could be described by a Michaelis binding curve with two sites: K1 = 0.42 microM n1 = 1.27 nmol/mg cell protein and K2 = 79 microM, n2 = 4.13 nmol/mg cell protein. The passive Ca buffering over the physiological Ca concentration was approximately twice the value expected from the values compiled by Fabiato [A. Fabiato. Am. J. Physiol. 245 (Cell Physiol. 14): C1-C14, 1983]. The maximal SR Ca uptake in the presence of 30 microM ruthenium red was fit by an uptake curve with a maximal uptake of 5.16 nmol/mg cell protein and a K 1/2 of 1.0 microM. In the presence of 5 microM thapsigargin and no ruthenium red, a significant Ca uptake attributed to mitochondria was measured between 10 and 100 microM free Ca. Rapid changes in free Ca concentration ([Ca]) measured with a Ca electrode were slower than simultaneous measurements of free [Ca] with indo-1 in permeabilized myocytes. However, oxalate, which buffers Ca and maximizes SR Ca uptake, increased the uptake rate and eliminated the difference in free [Ca] measured with Ca electrode and indo-1. This suggests that spatial gradients of [Ca] exist in permeabilized myocytes without Ca buffering. The new estimates of the buffering of intracellular Ca in cardiac myocytes should be valuable in developing quantitative insights into cardiac Ca regulation.

Indo-1 binding to protein in permeabilized ventricular myocytes alters its spectral and Ca binding properties

We have examined the binding of the fluorescent Ca indicator indo-1 to cellular protein in permeabilized ventricular myocytes and also to soluble and particulate myocyte protein. Using either a filtration technique or equilibrium dialysis, and conditions similar to those in a cardiac myocyte patch clamped with 100 microM indo-1 in the patch pipette, we found that 72% of the total indo-1 was bound to myocyte protein at a protein concentration of 100 mg/ml. This corresponds to a binding of 3.8 +/- 0.5 nmol indo-1/mg protein. Separation of the myocyte protein into a soluble and a particulate fraction showed that 63% of the bound indo-1 was bound to soluble protein, corresponding to a binding of 3.22 +/- 0.99 nmol/mg, whereas 37% of the bound indo-1 was bound to particulate protein (0.85 +/- 0.14 nmol/mg) at a low [Ca] (pCa approximately 9). Binding of indo-1 in permeabilized myocytes was approximately 60% higher at a saturating Ca concentration (pCa = 3), than under Ca free conditions (1 mM EGTA). Simultaneous measurements of free [Ca] with a Ca selective electrode and indo-1 fluorescence showed that, the dissociation constant (Kd) for Ca was increased 4-5 fold in the presence of permeabilized myocytes as compared to the value obtained in vitro. In agreement with the binding experiments we estimate that the true Kd and the apparent Kd (using ratiometric measurements) for Ca binding to indo-1 are increased approximately four fold, at a myocyte protein concentration of 100 mg/ml.

The influence of temperature on ryanodine sensitivity and the force-frequency relationship in the myocardium of rainbow trout

The relationship between stimulation frequency and contraction was established for ventricular strips from rainbow trout heart at 5, 15 and 25 degrees C. Compared to mammalian species, changes in temperature had little impact on force development in trout ventricle at physiologically relevant stimulation frequencies. However, the force-frequency relationship was changed from a biphasic response with a minimum around 0.2 Hz at 5 and 15 degrees C to a monophasic decline in force with increasing frequency at 25 degrees C. Ryanodine reversed the negative force-frequency relationship at 25 degrees C. Potentiation of twitch force after a 5 min rest period was increased from 121 +/− 4% at 15 degrees C to 209 +/− 12% at 25 degrees C. A similar augmentation was seen for the maximal rate of force development. Rest potentiation of both force and maximal rate of force development (dF/dT) was abolished by ryanodine at both 15 and 25 degrees C. The ryanodine concentration causing a half-maximal reduction in rest potentiation of force was 51 nmol l-1 at 25 degrees C and 483 nmol l-1 at 15 degrees C. Rest potentiation was maximally reduced by 10 mumol l-1 ryanodine to 50 and 79% of the value in the absence of ryanodine at 25 and 15 degrees C, respectively. At 5 degrees C, rest potentiation was similar to that at 15 degrees C. At 5 degrees C, there was no rest potentiation of dF/dT and ryanodine did not reduce rest potentiation of force. Instead, rest potentiation was correlated with a potentiation of time to peak tension (TPT) at 5 degrees C. Thus, in trout ventricle, force correlates with TPT at 5 degrees C and seems to be regulated by a ryanodine-insensitive mechanism, while at 25 degrees C force is correlated with the maximal rate of force development and the sarcoplasmic reticulum appears to contribute significantly to excitation-contraction coupling.

Force frequency relation in the myocardium of rainbow trout. Effects of K+ and adrenaline

Isolated heart ventricular preparations from rainbow trout were electrically stimulated to contraction. Following a temporary change in stimulation rate from 0.2 Hz to a higher value, the force fell to a minimum after which it increased and levelled off. Upon the return to 0.2 Hz a further transient increase in force appeared. The latter two responses were stimulated by an increased extracellular K+, which is known to inactivate the Na+ channel. The initial negative inotropic effect, in contrast to the two subsequent positive effects, was associated with a parallel decrease in amplitude of the action potential measured in 15 mM K+, used as an index of the Ca2+ influx. One micromolar (1 microM) ryanodine did not affect either the negative or the positive responses due to an increase in stimulation rate, but depressed the force developed after prolonged periods of rest. Ten micromolar (10 microM) adrenaline strongly inhibited the positive effects of an elevation of frequency. An elevation of extracellular Na+ from 141 to 166 mM had a similar effect. In conclusion, the positive effects occurring in 15 mM K+ do not seem to depend on the initial Na+ current. They may nevertheless depend on changes of the cellular Na+ balance as suggested by the effects of adrenaline, K+ and Na+. The functional role of the sarcoplasmic reticulum is unclear.