L-type Ca2+ current as the predominant pathway of Ca2+ entry during I(Na) activation in beta-stimulated cardiac myocytes

SLMAP-3 is downregulated in human dilated ventricles and its overexpression promotes cardiomyocyte response to adrenergic stimuli by increasing intracellular calcium

Structural dilation of cardiomyocytes (CMs) imposes a decline in cardiac performance that precipitates cardiac failure and sudden death. Since membrane proteins are implicated in dilated cardiomyopathy and heart failure, we evaluated the expression of the sarcolemmal membrane-associated protein (SLMAP) in dilated cardiomyopathy and its effect on CM contraction. We found that all 3 SLMAP isoforms (SLMAP-1, -2, and -3) are expressed in CMs and are downregulated in human dilated ventricles. Knockdown of SLMAPs in cultured CMs transduced with recombinant adeno-associated viral particles releasing SLMAP-shRNA precipitated reduced spontaneous contractile rate that was not fully recovered in SLMAP-depleted CMs challenged with isoproterenol (ISO), thus phenotypically mimicking heart failure performance. Interestingly, the overexpression of the SLMAP-3 full-length isoform induced a positive chronotropic effect in CMs that was more pronounced in response to ISO insult (vs. ISO-treated naïve CMs). Confocal live imaging showed that H9c2 cardiac myoblasts overexpressing SLMAP-3 exhibit a higher intracellular calcium transient peak when treated with ISO (vs. ISO-treated cells carrying a control adeno-associated viral particle). Proteomics revealed that SLMAP-3 interacts with the regulator of CM contraction, striatin. Collectively, our data demonstrate that SLMAP-3 is a novel regulator of CM contraction rate and their response to adrenergic stimuli. Loss of SLMAPs phenotypically mimics cardiac failure and crystallizes SLMAPs as predictive of dilated cardiomyopathy and heart failure.

Reduced Na(+) and higher K(+) channel expression and function contribute to right ventricular origin of arrhythmias in Scn5a+/- mice

Brugada syndrome (BrS) is associated with ventricular tachycardia originating particularly in the right ventricle (RV). We explore electrophysiological features predisposing to such arrhythmic tendency and their possible RV localization in a heterozygotic Scn5a+/- murine model. Na(v)1.5 mRNA and protein expression were lower in Scn5a+/- than wild-type (WT), with a further reduction in the RV compared with the left ventricle (LV). RVs showed higher expression levels of K(v)4.2, K(v)4.3 and KChIP2 in both Scn5a+/- and WT. Action potential upstroke velocity and maximum Na(+) current (I(Na)) density were correspondingly decreased in Scn5a+/-, with a further reduction in the RV. The voltage dependence of inactivation was shifted to more negative values in Scn5a+/-. These findings are predictive of a localized depolarization abnormality leading to slowed conduction. Persistent Na(+) current (I(pNa)) density was decreased in a similar pattern to I(Na). RV transient outward current (I(to)) density was greater than LV in both WT and Scn5a+/-, and had larger time constants of inactivation. These findings were also consistent with the observation that AP durations were smallest in the RV of Scn5a+/-, fulfilling predictions of an increased heterogeneity of repolarization as an additional possible electrophysiological mechanism for arrhythmogenesis in BrS.

The inotropic effect of cardioactive glycosides in ventricular myocytes requires Na+-Ca2+ exchanger function

Glycoside-induced cardiac inotropy has traditionally been attributed to direct Na(+)-K(+)-ATPase inhibition, causing increased intracellular [Na(+)] and consequent Ca(2+) gain via the Na(+)-Ca(2+) exchanger (NCX). However, recent studies suggested alternative mechanisms of glycoside-induced inotropy: (1) direct activation of sarcoplasmic reticulum Ca(2+) release channels (ryanodine receptors; RyRs); (2) increased Ca(2+) selectivity of Na(+) channels (slip-mode conductance); and (3) other signal transduction pathways. None of these proposed mechanisms requires NCX or an altered [Na(+)] gradient. Here we tested the ability of ouabain (OUA, 3 microm), digoxin (DIG, 20 microm) or acetylstrophanthidin (ACS, 4 microm) to alter Ca(2+) transients in completely Na(+)-free conditions in intact ferret and cat ventricular myocytes. We also tested whether OUA directly activates RyRs in permeabilized cat myocytes (measuring Ca(2+) sparks by confocal microscopy). In intact ferret myocytes (stimulated at 0.2 Hz), DIG and ACS enhanced Ca(2+) transients and cell shortening during twitches, as expected. However, prior depletion of [Na(+)](i) (in Na(+)-free, Ca(2+)-free solution) and in Na(+)-free solution (replaced by Li(+)) the inotropic effects of DIG and ACS were completely prevented. In voltage-clamped cat myocytes, OUA increased Ca(2+) transients by 48 +/- 4% but OUA had no effect in Na(+)-depleted cells (replaced by N-methyl-d-glucamine). In permeabilized cat myocytes, OUA did not change Ca(2+) spark frequency, amplitude or spatial spread (although spark duration was slightly prolonged). We conclude that the acute inotropic effects of DIG, ACS and OUA (and the effects on RyRs) depend on the presence of Na(+) and a functional NCX in ferret and cat myocytes (rather than alternate Na(+)-independent mechanisms).

A novel steroid-like compound F90927 exerting positive-inotropic effects in cardiac muscle

Here we report a novel steroid-like compound F90363, exhibiting positive inotropy in vivo and in vitro in various cardiac muscle preparations. F90363 is a racemic mixture composed of the stereoisomers (-)-F90926 and (+)-F90927. Only F90927 exerted positive inotropy, while F90926 induced a weak negative inotropy, but only at concentrations 10(3) times higher than F90927 and most likely resulting from an unspecific interaction. The rapid time course of the action of F90927 suggested a direct interaction with a cellular target rather than a genomic alteration. We could identify the L-type Ca2+ current I(Ca(L)) as a main target of F90927, while excluding other components of cardiac Ca2+ signalling as potential contributors. In addition, several other signaling pathways known to lead to positive inotropy (e.g. alpha- and beta-adrenergic stimulation, cAMP pathways) could be excluded as targets of F90927. However, vessel contraction and stiffening of the cardiac muscle at high doses (>30 microM, 0.36 mg kg(-1), respectively) prevent the use of F90927 as a candidate for drug development. Since the compound may still find valuable applications in research, the aim of the present study was to identify the cellular target and the mechanism of inotropy of F90927.

Synergistic interactions between Ca2+ entries through L-type Ca2+ channels and Na+-Ca2+ exchanger in normal and failing rat heart

We used confocal Ca2+ imaging and the patch-clamp technique to investigate the interplay between Ca2+ entries through L-type Ca2+ channels (LCCs) and reverse-mode Na+-Ca2+ exchange (NCX) in activating Ca2+-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR) in cardiac myocytes from normal and failing rat hearts. In normal myocytes exposed to N(6),2'-O-dibutyryl adenosine-3',5'-cyclic monophosphate (db-cAMP, membrane-permeable form of cAMP), the bell-shaped voltage dependence of cytosolic Ca2+ transients was dramatically broadened due to activation of SR Ca2+ release at high membrane potentials (30-120 mV). This broadening of Ca2+-transient voltage dependence could be prevented by KB-R7943, an inhibitor of the reverse-mode NCX. Trans-sarcolemmal Ca2+ entries were measured fluorometrically in myocytes during depolarizing steps to high membrane potentials. The total Ca2+ entry (deltaF(Tot)) was separated into two Ca2+ entry components, LCC-mediated (deltaF(LCC)) and NCX-mediated (deltaF(NCX)), by exposing the cells to the specific inhibitors of LCCs and reverse-mode NCX, nifedipine and KB-R7943, respectively. In the absence of protein kinase A (PKA) stimulation the amplitude of the Ca2+-inflow signal (deltaF(Tot)) corresponded to the arithmetic sum of the amplitudes of the KB-R7943- and nifedipine-resistant components (deltaF(Tot)=deltaF(LCC)+deltaF(NCX)). PKA activation resulted in significant increases in deltaF(Tot) and deltaF(LCC). Paradoxically, deltaF(Tot) became approximately threefold larger than the sum of the deltaF(NCX) and deltaF(LCC) components. In myocytes from failing hearts, stimulation of PKA failed to induce a shift in Ca2+ release voltage dependence toward more positive membrane potentials. Although the total and NCX-mediated Ca2+ entries were increased again, deltaF(Tot) did not significantly exceed the sum of deltaF(LCC) and deltaF(NCX). We conclude that the LCC and NCX Ca2+-entry pathways interact synergistically to trigger SR Ca2+ release on depolarization to positive membrane potentials in PKA-stimulated cardiac muscle. In heart failure, this new form of Ca2+ release is diminished and may potentially account for the compromised contractile performance and reduced functional reserve in failing hearts.

Ca2+ currents in cardiac myocytes: Old story, new insights

Calcium is a ubiquitous second messenger which plays key roles in numerous physiological functions. In cardiac myocytes, Ca2+ crosses the plasma membrane via specialized voltage-gated Ca2+ channels which have two main functions: (i) carrying depolarizing current by allowing positively charged Ca2+ ions to move into the cell; (ii) triggering Ca2+ release from the sarcoplasmic reticulum. Recently, it has been suggested than Ca2+ channels also participate in excitation-transcription coupling. The purpose of this review is to discuss the physiological roles of Ca2+ currents in cardiac myocytes. Next, we describe local regulation of Ca2+ channels by cyclic nucleotides. We also provide an overview of recent studies investigating the structure-function relationship of Ca2+ channels in cardiac myocytes using heterologous system expression and transgenic mice, with descriptions of the recently discovered Ca2+ channels alpha(1D) and alpha(1E). We finally discuss the potential involvement of Ca2+ currents in cardiac pathologies, such as diseases with autoimmune components, and cardiac remodeling.

Cellular mechanism of calcium-mediated triggered activity in the heart

Calcium overload due to enhanced calcium entry is a mechanism for spontaneous calcium release (SCR) from the sarcoplasmic reticulum, delayed-afterdepolarizations (DAD), and triggered activity. However, the exact mechanistic relationship between elevated intracellular calcium levels and triggered activity originating from a specific location remains unclear. We hypothesize that under conditions of enhanced calcium entry, elevation of intracellular calcium will result in multiple calcium release events of which only one is more likely to initiate a triggered beat. We used optical mapping of action potentials and ratiometric calcium transients in an electromechanically-uncoupled canine wedge model of enhanced calcium entry, using I(Ks) blockade with beta-adrenergic stimulation. Under conditions of enhanced calcium entry, the rate of calcium uptake was faster compared with control conditions; however, during rapid pacing, cytoplasmic calcium elevation at the endocardium was significantly increased (15+/-4%) compared with control (10+/-3, P<0.04). Rapid pacing induced multiple simultaneous SCR events with largest amplitude and earliest onset near the endocardium compared with the epicardium. Furthermore, SCR events with largest amplitude and earliest onset served as a focus for DAD-mediated triggered activity. Interestingly, polymorphic VT occurred in some experiments when multiple SCR events occurred. In conclusion, multiple, simultaneous SCR events occur over a broad region of relatively slower calcium uptake and elevated diastolic calcium levels. However, SCR events closer to the endocardium have the largest amplitude and earliest onset and are, thereby, more likely to initiate DAD-mediated triggered activity. Finally, multiple SCR events may be a mechanism of polymorphic VT under calcium overload conditions.

Spatial characteristics of sarcoplasmic reticulum Ca2+ release events triggered by L-type Ca2+ current and Na+ current in guinea-pig cardiac myocytes

Ca2+ signals in cardiac muscle cells are composed of spatially limited elementary events termed Ca2+ sparks. Several studies have also indicated that Ca2+ signals smaller than Ca2+ sparks can be elicited. These signals have been termed Ca2+ quarks and were proposed to result from the opening of a single Ca2+ release channel of the sarcoplasmic reticulum. We used laser-scanning confocal microscopy to examine the subcellular properties of Na+ current (I(Na))- and L-type Ca2+ current (I(Ca,L))-induced Ca2+ transients in voltage-clamped ventricular myocytes isolated from guinea-pigs. Both currents, I(Na) and I(Ca,L), evoked substantial, global Ca2+ transients. To examine the spatiotemporal properties of such Ca2+ signals, we performed power spectral analysis of these Ca2+ transients and found that both lacked spatial frequency components characteristic for Ca2+ sparks. The application of 10 microM verapamil to partially block L-type Ca2+ current reduced the corresponding Ca2+ transients down to individual Ca2+ sparks. In contrast, I(Na)-induced Ca2+ responses were still spatially homogeneous and lacked Ca2+ sparks even for small current amplitudes. By using high resistance patch pipettes (> 4 MOmega) to exaggerate the loss of voltage control during I(Na), Ca2+ sparks appeared superimposed on a homogeneous Ca2+ release component and were exclusively triggered during the flow of I(Na). In the presence of 10 microM ryanodine both I(Ca,L) and I(Na) elicited small, residual Ca2+ transients that were spatially homogeneous but displayed distinctively different temporal profiles. We conclude that I(Na) is indeed able to cause Ca2+ release in guinea-pig ventricular myocytes. In contrast to I(Ca,L)-induced Ca2+ transients, which are built up from the recruitment of individual Ca2+ sparks, the I(Na)-evoked cellular responses were always homogeneous, indicating that their underlying elementary Ca2+ release event is distinct from the Ca2+ spark. Thus, I(Na)-induced Ca2+ transients are composed of smaller Ca2+ signals, most likely Ca2+ quarks.

Autonomic nervous system influences on QT interval in normal subjects

OBJECTIVES

We sought to determine whether the relationship between heart rate (HR) and QT interval (QT) differs as HR increases in response to exercise, atropine and isoproterenol.

BACKGROUND

Autonomic nervous system influences on repolarization are poorly understood and may complicate the interpretation of QT measurements.

METHODS

Twenty-five normal subjects sequentially underwent graded-intensity bicycle exercise, atropine injection and isoproterenol infusion. Serial 12-lead electrocardiograms were recorded at steady state during each condition and analyzed using interactive computer software. The HR-QT data were modeled linearly and the slopes (quantifying QT adaptation to HR) as well as the QT intervals at 100 beats/min for each intervention were compared by repeated-measures analysis of variance.

RESULTS

As HR increased, QT was longer for isoproterenol in comparison to exercise or atropine, which were similar. The HR-QT slope (ms/beats/min) was less steep for isoproterenol (-0.83 +/- 0.53) than for atropine (-1.45 +/- 0.21) or exercise (-1.37 +/- 0.23) (p < 0.0001). In comparison to men, women had more negative HR-QT slopes during all interventions. At 100 beats/min, the QT was 364 ms during isoproterenol, which was significantly longer than that during exercise (330 ms) or atropine (339 ms) (p < 0.0001). Isoproterenol produced a dose-dependent increase in U-wave amplitude that was not observed during exercise or atropine.

CONCLUSIONS

In comparison to exercise and atropine, isoproterenol is associated with much less QT shortening for a given increase in HR and, therefore, greater absolute QT intervals. Our findings demonstrate that autonomic conditions directly affect the ventricular myocardium of healthy subjects, causing differences in QT that are independent of HR.

L-type Ca(2+) currents overlapping threshold Na(+) currents: could they be responsible for the "slip-mode" phenomenon in cardiac myocytes?

Phosphorylation of Na channels has been suggested to increase their Ca permeability. Termed "slip-mode conductance" (SMC), this hypothesis predicts that Ca influx via protein kinase A (PKA)-modified Na channels can induce sarcoplasmic reticulum (SR) Ca release. We tested this hypothesis by determining if SR Ca release is graded with I(Na) in the presence of activated PKA (with Isoproterenol, ISO). V(m), I(m), and [Ca](i) were measured in feline (n=26) and failing human (n=19) ventricular myocytes. Voltage steps from -70 through -40 mV were used to grade I(Na). Na channel antagonists (tetrodotoxin), L-type Ca channel (I(Ca,L)) antagonists (nifedipine, cadmium, verapamil), and agonists (Bay K 8644, FPL 64176) were used to separate SMC from I(Ca,L). In the absence of ISO, I(Na) was associated with SR Ca release in human but not feline myocytes. After ISO, graded I(Na) was associated with small amounts of SR Ca release in feline myocytes and the magnitude of release increased in human myocytes. I(Na)-related SR Ca release was insensitive to tetrodotoxin (n=10) but was blocked by nifedipine (n=10) and cadmium (n=3). SR Ca release was induced over the same voltage range in the absence of ISO with Bay K 8644 and FPL 64176 (n=9). Positive voltage steps (to 0 mV) to fully activate Na channels (SMC) in the presence of ISO and Verapamil only caused SR Ca release when block of I(Ca,L) was incomplete. We conclude that PKA-mediated increases in I(Ca,L) and SR Ca loading can reproduce many of the experimental features of SMC.

Cardiac excitation-contraction coupling

Of the ions involved in the intricate workings of the heart, calcium is considered perhaps the most important. It is crucial to the very process that enables the chambers of the heart to contract and relax, a process called excitation-contraction coupling. It is important to understand in quantitative detail exactly how calcium is moved around the various organelles of the myocyte in order to bring about excitation-contraction coupling if we are to understand the basic physiology of heart function. Furthermore, spatial microdomains within the cell are important in localizing the molecular players that orchestrate cardiac function.