Ca2+-activated non-selective cation current in rabbit ventricular myocytes

Uncoupling cytosolic calcium from membrane voltage by transient receptor potential melastatin 4 channel (TRPM4) modulation: A novel strategy to treat ventricular arrhythmias

The current antiarrhythmic paradigm is mainly centered around modulating membrane voltage. However, abnormal cytosolic calcium (Ca2+) signaling, which plays an important role in driving membrane voltage, has not been targeted for therapeutic purposes in arrhythmogenesis. There is clear evidence for bidirectional coupling between membrane voltage and intracellular Ca2+. Cytosolic Ca2+ regulates membrane voltage through Ca2+-sensitive membrane currents. As a component of Ca2+-sensitive currents, Ca2+-activated nonspecific cationic current through the TRPM4 (transient receptor potential melastatin 4) channel plays a significant role in Ca2+-driven changes in membrane electrophysiology. In myopathic and ischemic ventricles, upregulation and/or enhanced activity of this current is associated with the generation of afterdepolarization (both early and delayed), reduction of repolarization reserve, and increased propensity to ventricular arrhythmias. In this review, we describe a novel concept for the management of ventricular arrhythmias in the remodeled ventricle based on mechanistic concepts from experimental studies, by uncoupling the Ca2+-induced changes in membrane voltage by inhibition of this TRPM4-mediated current.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Apamin-sensitive calcium-activated potassium currents (SK) are activated by persistent calcium currents in rat motoneurons

Low voltage-activated persistent inward calcium currents (Ca PICs) occur in rat motoneurons and are mediated by Cav1.3 L-type calcium channels (L-Ca current). The objectives of this paper were to determine whether this L-Ca current activates a sustained calcium-activated potassium current (SK current) and examine how such SK currents change with spinal injury. For comparison, the SK current that produces the postspike afterhyperpolarization (mAHP) was also quantified. Intracellular recordings were made from motoneurons of adult acute and chronic spinal rats while the whole sacrocaudal spinal cord was maintained in vitro. Spikes/AHPs were evoked with current injection or ventral root stimulation. Application of the SK channel blocker apamin completely eliminated the mAHP, which was not significantly different in chronic and acute spinal rats. The Ca PICs were measured with slow voltage ramps (or steps) with TTX to block sodium currents. In chronic spinal rats, the PICs were activated at -58.6 +/- 6.0 mV and were 2.2 +/- 1.2 nA in amplitude, significantly larger than in acute spinal rats. Apamin significantly increased the PIC, indicating that there was an SK current activated by L-Ca currents (SK(L) current), which ultimately reduced the net PIC. This SK(L) current was not different in acute and chronic spinal rats. The SK(AHP) and the SK(L) currents were activated by different calcium currents because the mAHP/SK(AHP) was blocked by the N, P-type calcium channel blocker omega-conotoxin MVIIC and was resistant to the L-type calcium channel blocker nimodipine, whereas the L-Ca and SK(L) currents were blocked by nimodipine. Furthermore, the SK(AHP) current activated within 10 ms of the spike, whereas the SK(L) current was delayed approximately 100 ms after the onset of the L-Ca current, suggesting that the SK(L) currents were not as spatially close to the L-Ca currents. Finally, the SK(L) and the L-Ca currents were poorly space clamped, with oscillations at their onset and hysteresis in their activation and deactivation voltages, consistent with currents of dendritic origin. The impact of these dendritic currents was especially pronounced in 15% of motoneurons, where apamin led to uncontrollable L-Ca currents that could not be deactivated, even with large hyperpolarizations of the soma. Thus, although the SK(L) currents are fairly small, they play a critical role in terminating the dendritic L-Ca currents.

Would modulation of intracellular Ca2+ be antiarrhythmic?

Under several types of conditions, reversal of steps of excitation-contraction coupling (RECC) can give rise to nondriven electrical activity. In this review we explore those conditions for several cardiac cell types (SA, atrial, Purkinje, ventricular cells). We find that abnormal spontaneous Ca2+ release from intracellular Ca2+ stores, aberrant Ca2+ influx from sarcolemmal channels or abnormal Ca2+ surges in nonuniform muscle can be the initiators of the RECC. Often, with such increases in Ca2+, spontaneous Ca2+ waves occur and lead to membrane depolarizations. Because the change in membrane voltage is produced by Ca2+-dependent changes in ion channel function, we also review here what is known about the molecular interaction of Ca2+ and several Ca2+-dependent processes, including the intracellular Ca2+ release channels implicated in the genetic basis of some forms of human arrhythmias. Finally, we review what is known about the effectiveness of several agents in modifying such Ca2+-dependent arrhythmias.

Comparison of functional properties of the Ca2+-activated cation channels TRPM4 and TRPM5 from mice

Non-selective cation (NSC) channels activated by intracellular Ca2+ ([Ca2+]i) play an important role in Ca2+ signaling and membrane excitability in many cell types. TRPM4 and TRPM5, two Ca2+-activated cation channels of the TRP superfamily, are potential molecular correlates of NSC channels. We compared the functional properties of mouse TRPM4 and TRPM5 heterologously expressed in HEK 293 cells. Dialyzing cells with different Ca2+ concentrations revealed a difference in Ca2+ sensitivity between TRPM4 and TRPM5, with EC50 values of 20.2+/-4.0 microM and 0.70+/-0.1 microM, respectively. Similarly, TRPM5 activated at lower Ca2+ concentration than TRPM4 when [Ca2+]i was raised by UV uncaging of the Ca2+-cage DMNP-EDTA. Current amplitudes of TRPM4 and TRPM5 were not correlated to the rate of changes in [Ca2+]i. The Ca2+ sensitivity of both channels was strongly reduced in inside-out patches, resulting in approximately 10-30 times higher EC50 values than under whole-cell conditions. Currents through TRPM4 and TRPM5 deactivated at negative and activated at positive potentials with similar kinetics. Both channels were equally sensitive to block by intracellular spermine. TRPM4 displayed a 10-fold higher affinity for block by flufenamic acid. Importantly, ATP4- blocked TRPM4 with high affinity (IC50 of 0.8+/-0.1 microM), whereas TRPM5 is insensitive to ATP4- at concentrations up to 1 mM.

Mechanisms of postinhibitory rebound and its modulation by serotonin in excitatory swim motor neurons of the medicinal leech

Postinhibitory rebound (PIR) is defined as membrane depolarization occurring at the offset of a hyperpolarizing stimulus and is one of several intrinsic properties that may promote rhythmic electrical activity. PIR can be produced by several mechanisms including hyperpolarization-activated cation current (I(h)) or de-inactivation of depolarization-activated inward currents. Excitatory swim motor neurons in the leech exhibit PIR in response to injected current pulses or inhibitory synaptic input. Serotonin, a potent modulator of leech swimming behavior, increases the peak amplitude of PIR and decreases its duration, effects consistent with supporting rhythmic activity. In this study, we performed current clamp experiments on dorsal excitatory cell 3 (DE-3) and ventral excitatory cell 4 (VE-4). We found a significant difference in the shape of PIR responses expressed by these two cell types in normal saline, with DE-3 exhibiting a larger prolonged component. Exposing motor neurons to serotonin eliminated this difference. Cs+ had no effect on PIR, suggesting that I(h) plays no role. PIR was suppressed completely when low Na+ solution was combined with Ca2+-channel blockers. Our data support the hypothesis that PIR in swim motor neurons is produced by a combination of low-threshold Na+ and Ca2+ currents that begin to activate near -60 mV.

Calmodulin kinase is functionally targeted to the action potential plateau for regulation of L-type Ca2+ current in rabbit cardiomyocytes

L-type Ca2+ current (ICa-L) triggers Ca2+ release from the sarcoplasmic reticulum (SR) and both SR and ICa-L are potential sources of intracellular Ca2+ (Ca2+i) for feedback regulation of ICa-L. Ca2+i bound to calmodulin (Ca2+-CaM) can inhibit ICa-L, while Ca2+-CaM can also activate Ca2+-CaM-dependent protein kinase II (CaMK) to increase ICa. However, it is not known whether ICa-L or the SR is the primary source of Ca2+ for ICa-L regulation. The L-type Ca2+ channel C terminus is implicated as a critical transduction element for ICa-L responses to Ca2+-CaM and CaMK, and the C terminus undergoes voltage-dependent steric changes, suggesting that Ca2+i control of ICa-L may also be regulated by cell membrane potential. We developed conditions to separately test the relationship of Ca2+-CaM and CaMK to ICa-L and SR Ca2+i release during voltage clamp conditions modelled upon time and voltage domains relevant to the cardiac action potential. Here we show that CaMK increases ICa-L after brief positive conditioning pulses, whereas Ca2+-CaM reduces ICa-L over a broad range of positive and negative conditioning potentials. SR Ca2+ release was required for both Ca2+-CaM and CaMK ICa-L responses after strongly positive conditioning pulses (+10 and +40 mV), while Ca2+i from ICa-L was sufficient for Ca2+-CaM during weaker depolarizations. These findings show that ICa-L responses to CaMK are voltage dependent and suggest a new model of L-type Ca2+ channel regulation where voltage-dependent changes control ICa-L responses to Ca2+-CaM and CaMK signalling.

Ca(2+) elevation evoked by membrane depolarization regulates G protein cycle via RGS proteins in the heart

Regulators of G protein signaling (RGS), which act as GTPase activators, are a family of cytosolic proteins emerging rapidly as an important means of controlling G protein-mediated cell signals. The importance of RGS action has been verified in vitro for various kinds of cell function. Their in situ modes of action in intact cells are, however, poorly understood. Here we show that an increase in intracellular Ca(2+) evoked by membrane depolarization controls the RGS action on G protein activation of muscarinic K(+) (K(G)) channel in the heart. Acetylcholine-induced K(G) current exhibits a slow time-dependent increase during hyperpolarizing voltage steps, referred to as "relaxation." This reflects the relief from the decrease in available K(G) channel number induced by cell depolarization. This phenomenon is abolished when an increase in intracellular Ca(2+) is prevented. It is also abolished when a calmodulin inhibitor or a mutant RGS4 is applied that can bind to calmodulin but that does not accelerate GTPase activity. Therefore, an increase in intracellular Ca(2+) and the resultant formation of Ca(2+)/calmodulin facilitate GTPase activity of RGS and thus decrease the available channel number on depolarization. These results indicate a novel and probably general pathway that Ca(2+)-dependent signaling regulates the G protein cycle via RGS proteins.

Calmodulin kinase and a calmodulin-binding 'IQ' domain facilitate L-type Ca2+ current in rabbit ventricular myocytes by a common mechanism

1. Ca2+-calmodulin-dependent protein kinase II (CaMK) and a calmodulin (CaM)-binding 'IQ' domain (IQ) are both implicated in Ca2+-dependent regulation of L-type Ca2+ current (I(Ca)). We used an IQ-mimetic peptide (IQmp), under conditions in which CaMK activity was controlled, to test the relationship between these CaM-activated signalling elements in the regulation of L-type Ca2+ channels (LTCCs) and I(Ca) in rabbit ventricular myocytes. 2. A specific CaMK inhibitory peptide nearly abolished I(Ca) facilitation, but the facilitation was 'rescued' by cell dialysis with IQmp. 3. IQmp significantly enhanced I(Ca) facilitation and slowed the fast component of I(Ca) inactivation, compared with an inactive control peptide. Neither effect could be elicited by a more avid CaM-binding peptide, suggesting that generalized CaM buffering did not account for the effects of IQmp. 4. I(Ca) facilitation was abolished and the fast component of inactivation eliminated by ryanodine, caffeine or thapsigargin, suggesting that the sarcoplasmic reticulum (SR) is an important source of Ca2+ for I(Ca) facilitation and inactivation. IQmp did not restore I(Ca) facilitation under these conditions. 5. Engineered Ca2+-independent CaMK and IQmp each markedly increased LTCC open probability (P(o)) in excised cell membrane patches. The LTCC P(o) increases with CaMK and IQmp were non-additive, suggesting that CaMK and IQmp are components of a shared signalling pathway. 6. Both CaMK and IQmp induced a modal gating shift in LTCCs that favoured prolonged openings, indicating that CaMK and IQmp affect LTCCs through a common biophysical mechanism. 7. These findings support the hypothesis that CaMK is required for physiological I(Ca) facilitation in cardiac myocytes. Both CaMK and IQmp were able to induce a modal gating shift in LTCCs, suggesting that each of these signalling elements is important for Ca2+-CaM-dependent LTCC facilitation in cardiac myocytes.

Sarcoplasmic reticulum Ca(2+) release causes myocyte depolarization. Underlying mechanism and threshold for triggered action potentials

Spontaneous sarcoplasmic reticulum (SR) Ca(2+) release causes delayed afterdepolarizations (DADs) via Ca(2+)-induced transient inward currents (I:(ti)). However, no quantitative data exists regarding (1) Ca(2+) dependence of DADs, (2) Ca(2+) required to depolarize the cell to threshold and trigger an action potential (AP), or (3) relative contributions of Ca(2+)-activated currents to DADs. To address these points, we evoked SR Ca(2+) release by rapid application of caffeine in indo 1-AM-loaded rabbit ventricular myocytes and measured caffeine-induced DADs (cDADs) with whole-cell current clamp. The SR Ca(2+) load of the myocyte was varied by different AP frequencies. The cDAD amplitude doubled for every 88+/-8 nmol/L of Delta[Ca(2+)](i) (simple exponential), and the Delta[Ca(2+)](i) threshold of 424+/-58 nmol/L was sufficient to trigger an AP. Blocking Na(+)-Ca(2+) exchange current (I(Na/Ca)) by removal of [Na](o) and [Ca(2+)](o) (or with 5 mmol/L Ni(2+)) reduced cDADs by >90%, for the same Delta[Ca(2+)](i). In contrast, blockade of Ca(2+)-activated Cl(-) current (I(Cl(Ca))) with 50 micromol/L niflumate did not significantly alter cDADs. We conclude that DADs are almost entirely due to I(Na/Ca), not I(Cl(Ca)) or Ca(2+)-activated nonselective cation current. To trigger an AP requires 30 to 40 micromol/L cytosolic Ca(2+) or a [Ca(2+)](i) transient of 424 nmol/L. Current injection, simulating I(ti)s with different time courses, revealed that faster I:(ti)s require less charge for AP triggering. Given that spontaneous SR Ca(2+) release occurs in waves, which are slower than cDADs or fast I(ti)s, the true Delta[Ca(2+)](i) threshold for AP activation may be approximately 3-fold higher in normal myocytes. This provides a safety margin against arrhythmia in normal ventricular myocytes.