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J Arreola

Publications and source records attributed to J Arreola.

17 recordsLinked to original sources
PubMed2004

Permeant anions control gating of calcium-dependent chloride channels.

The effects of external anions (SCN(-), NO3-, I(-), Br(-), F(-), glutamate, and aspartate) on gating of Ca(2+)-dependent Cl(-) channels from rat parotid acinar cells were studied using the whole-cell configuration of the patch-clamp technique. Shifts in the reversal potential of the current induced by replacement of external Cl(-) with foreign anions, gave the following selectivity sequence based on permeability ratios ( P(x)/ P(Cl)): SCN(-)>I(-)>NO3->Br(-)>Cl(-)>F(-)>aspartate>glutamate. Using a continuum electrostatic model we calculated that this lyotropic sequence resulted from the interaction between anions and a polarizable tunnel with an effective dielectric constant of approximately 23. Our data revealed that anions with P(x)/P(Cl) > 1 accelerated activation kinetics in a voltage-independent manner and slowed deactivation kinetics. Moreover, permeant anions enhanced whole-cell conductance ( g, an index of the apparent open probability) in a voltage-dependent manner, and shifted leftward the membrane potential- g curves. All of these effects were produced by the anions with an effectiveness that followed the selectivity sequence. To explain the effects of permeant anions on activation kinetics and g(Cl) we propose that there are 2 different anion-binding sites in the channel. One site is located outside the electrical field and controls channel activation kinetics, while a second site is located within the pore and controls whole-cell conductance. Thus, interactions of permeant anions with these two sites hinder the closing mechanism and stabilize the channel in the open state.

Animals↗
PubMed1998

Comparison of voltage-activated Cl- channels in rat parotid acinar cells with ClC-2 in a mammalian expression system.

Rat parotid acinar cells express Cl- currents that are activated in a time-dependent manner by hyperpolarized potentials. ClC-2, a member of the ClC gene family, codes for a voltage-gated, inward rectifying anion channel when expressed in Xenopus oocytes. In the present study, we found that cDNA derived from individual parotid acinar cells contained sequence identical to that reported for ClC-2 in rat brain and heart. A polyclonal antibody generated against the N-terminal cytoplasmic domain of ClC-2 recognized an approximately 100 kD protein on western blots of both brain and parotid gland. ClC-2 expressed in oocytes has different kinetics from the currents found in parotid acinar cells. Since the ClC-2 channel was cloned from and its transcripts are expressed in mammalian tissue, we compared the channel properties of acinar cells to a mammalian expression system. We expressed ClC-2 channels in human embryonic kidney cells, HEK 293, using recombinant ClC-2 DNA and ClC-2 DNA fused with DNA coding for jellyfish green fluorescent protein (GFP). Confocal microscopy revealed that the expressed ClC-2-GFP chimera protein localized to the plasma membrane. Whole cell Cl- currents from HEK 293 cells expressing ClC-2-GFP were similar, if not identical, to the Cl- currents recorded from cells transfected with ClC-2 cDNA (no GFP). The voltage-dependence and kinetics of ClC-2 channels expressed in HEK 293 cells were quite similar to those in acinar cells. Channels in parotid acinar and HEK 293 cells activated at more positive membrane potentials and with a faster time course than the channels expressed in Xenopus oocytes. In summary, we found that ClC-2 message and protein are expressed in salivary cells and that the properties of voltage-activated, inward rectifying Cl- channels in acinar cells are similar to those generated by the ClC-2-GFP construct expressed in HEK 293 cells. The properties of the ClC-2 anion channel seem to be dependent on the type of cell background in which it is expressed.

Animals↗
PubMed1998

Interaction of Ba2+ with the pores of the cloned inward rectifier K+ channels Kir2.1 expressed in Xenopus oocytes.

Interactions of Ba2+ with K+ and molecules contributing to inward rectification were studied in the cloned inward rectifier K+ channels, Kir2.1. Extracellular Ba2+ blocked Kir2.1 channels with first-order kinetics in a Vm-dependent manner. At Vm more negative than -120 mV, the Kd-Vm relationship became less steep and the dissociation rate constants were larger, suggesting Ba2+ dissociation into the extracellular space. Both depolarization and increasing [K+]i accelerated the recovery from extracellular Ba2+ blockade. Intracellular K+ appears to relieve Ba2+ blockade by competitively slowing the Ba2+ entrance rate, instead of increasing its exit rate by knocking off action. Intracellular spermine (100 microM) reduced, whereas 1 mM [Mg2+]i only slightly reduced, the ability of intracellular K+ to repulse Ba2+ from the channel pore. Intracellular Ba2+ also blocked outward IKir2.1 in a voltage-dependent fashion. At Vm >/= +40 mV, where intrinsic inactivation is prominent, intracellular Ba2+ accelerated the inactivation rate of the outward IKir2.1 in a Vm-independent manner, suggesting interaction of Ba2+ with the intrinsic gate of Kir2.1 channels.

Animals↗
PubMed1998

Nonindependent K+ movement through the pore in IRK1 potassium channels.

We measured unidirectional K+ in- and efflux through an inward rectifier K channel (IRK1) expressed in Xenopus oocytes. The ratio of these unidirectional fluxes differed significantly from expectations based on independent ion movement. In an extracellular solution with a K+ concentration of 25 mM, the data were described by a Ussing flux-ratio exponent, n', of approximately 2.2 and was constant over a voltage range from -50 to -25 mV. This result indicates that the pore of IRK1 channels may be simultaneously occupied by at least three ions. The IRK1 n' value of 2.2 is significantly smaller than the value of 3.5 obtained for Shaker K channels under identical conditions. To determine if other permeation properties that reflect multi-ion behavior differed between these two channel types, we measured the conductance (at 0 mV) of single IRK1 channels as a function of symmetrical K+ concentration. The conductance could be fit by a saturating hyperbola with a half-saturation K+ activity of 40 mM, substantially less than the reported value of 300 mM for Shaker K channels. We investigated the ability of simple permeation models based on absolute reaction rate theory to simulate IRK1 current-voltage, conductance, and flux-ratio data. Certain classes of four-barrier, three-site permeation models are inconsistent with the data, but models with high lateral barriers and a deep central well were able to account for the flux-ratio and single channel data. We conclude that while the pore in IRK1 and Shaker channels share important similarities, including K+ selectivity and multi-ion occupancy, they differ in other properties, including the sensitivity of pore conductance to K+ concentration, and may differ in the number of K+ ions that can simultaneously occupy the pore: IRK1 channels may contain three ions, but the pore in Shaker channels can accommodate four or more ions.

Animals↗
PubMed1998

Differences in regulation of Ca(2+)-activated Cl- channels in colonic and parotid secretory cells.

We investigated the regulation of Ca(2+)-activated Cl- channels in cells from the human colonic cell line T84 and acinar cells from rat parotid glands. The participation of multifunctional Ca(2+)- and calmodulin-dependent protein kinase (CaM kinase) II in the activation of these channels was studied using selective inhibitors of calmodulin and CaM kinase II. Ca(2+)-dependent Cl- currents were recorded using the whole cell patch-clamp technique. Direct inhibition of CaM kinase II by 40 microM peptide 281-302 or by 10 microM KN-62, another CaM kinase inhibitor, did not block the Cl- current in parotid acinar cells, whereas in T84 cells KN-62 markedly inhibited the Ca(2+)-dependent Cl- current. We also used the calmodulin-binding domain peptide 290-309 (0.5 microM), which competitively inhibits the activation of CaM kinase II. This peptide reduced the Cl- current in T84 cells by approximately 70% but was without effect on the channels in parotid acinar cells. We conclude that the Ca(2+)-dependent Cl- channels in T84 cells are activated by CaM kinase II but that the channels in parotid acinar cells must be regulated by a fundamentally different Ca(2+)-dependent mechanism that does not utilize CaM kinase II or any calmodulin-dependent process.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1996

Three distinct chloride channels control anion movements in rat parotid acinar cells.

1. We used the whole-cell configuration of the patch clamp technique to examine the different macroscopic Cl- currents present in single rat parotid acinar cells. 2. Cell swelling produced by negative osmotic pressure (hypotonic bath solutions) induced a large outwardly rectifying Cl- current with little or no time and voltage dependence. In contrast, an increase in intracellular [Ca2+] induced by ionomycin activated Cl- currents with very different properties. Ca(2+)-activated Cl- currents showed outward rectification, relatively slow activation kinetics and marked voltage dependence. These results are consistent with the existence of two different outwardly rectifying Cl- channels in rat parotid cells. 3. In conditions designed to eliminate the activation of these two Cl- currents, a third type of current was observed. This third current was activated in a time-dependent manner by hyperpolarized potentials and was about equally permeant to Cl-, I- and Br-. 4. The properties of the hyperpolarization-activated current were similar to those of the cloned ClC-2 channel. Polymerase chain reaction-based methods and ribonuclease protection analyses indicated the presence in parotid gland of mRNA homologous to ClC-2. 5. Individual parotid acinar cells expressed all three types of Cl- channels. Each type of channel may contribute to Cl- efflux in distinct stages of the secretion process depending on the intracellular [Ca2+], cell volume and membrane potential.

Animals↗
PubMed1996

Activation of calcium-dependent chloride channels in rat parotid acinar cells.

The Ca2+ and voltage dependence of Ca(2+)-activated Cl- currents in rat parotid acinar cells was examined with the whole-cell patch clamp technique. Acinar cells were dialyzed with buffered free Ca2+ concentrations ([Ca2+]i) from < 1 nM to 5 microM. Increasing [Ca2+]i induced an increase in Cl- current at all membrane potentials. In cells dialyzed with [Ca2+]i > 25 nM, depolarizing test pulses activated a Cl- current that was composed of an instantaneous and a slow monoexponential component. The steady-state current-voltage relationship showed outward rectification at low [Ca2+]i but became more linear as the [Ca2+]i increased because of a shift in Cl- channel activation toward more negative voltages. The Ca2+ dependence of steady-state channel activation at various membrane voltages was fit by the Hill equation. The apparent Kd and Hill coefficient obtained from this analysis were both functions of membrane potential. The Kd decreased from 417 to 63 nM between -106 and +94 mV, whereas the Hill coefficient was always > 1 and increased to values as large as 2.5 at large positive potentials. We found that a relatively simple mechanistic model can account for the channel steady-state and kinetic behavior. In this model, channel activation involves two identical, independent, sequential Ca2+ binding steps before a final Ca(2+)-independent transition to the conducting conformation. Channel activation proceeds sequentially through three closed states before reaching the open state. The Ca2+ binding steps of this model have a voltage dependence similar to that of the Kd from the Hill analysis. The simplest interpretation of our findings is that these channels are directly activated by Ca2+ ions that bind to sites approximately 13% into the membrane electric field from the cytoplasmic surface.

Animals↗
PubMed1995

Volume-activated chloride channels in rat parotid acinar cells.

1. Rat parotid acinar cells undergo a regulatory volume decrease in response to hypotonically induced cell swelling that is sensitive to K+ and Cl- gradients. To investigate the potential mechanisms involved, the whole-cell patch-clamp technique was used to characterize a volume-sensitive Cl- channel in rat parotid acinar cells. 2. Exposure of cells to a hyposmotic gradient induced large Cl- currents that exhibited outward rectification and were not affected by membrane potential or the absence of intracellular Ca2+. Low external pH increased the currents at all potentials without affecting current kinetics. These currents were nearly abolished when the cells were in hypertonic conditions. This decrease in the current amplitude was correlated with a decrease in the cell size. 3. The volume-sensitive currents displayed little or no time dependence, whereas Ca(2+)-activated Cl- channels, present in the same cells, displayed slow activation kinetics and large, time-dependent tail currents upon repolarization to the holding potential. 4. The reversal potential of the osmotically activated channels was close to the predicted chloride equilibrium potential and was sensitive to the physiological extracellular Cl- concentration ([Cl-]o). The relationship between reversal potential and [Cl-]o was fitted to a modified Nernst equation with a slope of 51 mV per decade, consistent with a Cl- selective conductance. 5. The anion permeability sequence of the channel, obtained from the shifts of the reversal potentials of the volume-sensitive Cl- current, was: SCN- > I- > NO-3 > Br- > Cl- > formate > propionate = methanesulphonate = acetate > or = F- > or = butyrate > valerate > gluconate = glucuronate = glutamate. 6. The current through the volume-sensitive channels was inhibited by the Cl- channel blocker SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) in a voltage-dependent manner. 7. We conclude that rat parotid acinar cells express an outwardly rectifying Cl- current that can be activated by swelling under hypotonic conditions. This Cl- conductance may be an element of the cellular mechanisms of volume regulation in exocrine glands.

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗
PubMed1995

Inhibition of Ca(2+)-dependent Cl- channels from secretory epithelial cells by low internal pH.

The actions of intracellular pH (pHi) on Ca(2+)-dependent Cl- channels were studied in secretory epithelial cells derived from human colon carcinoma (T84) and in isolated rat parotid acinar cells. Channel currents were measured with the whole cell voltage clamp technique with pipette solutions of different pH. Ca(2+)-dependent Cl- channels were activated by superfusing ionomycin to increase the intracellular calcium concentration ([Ca2+]i) or by using pipette solutions with buffered Ca2+ levels. Large currents were activated in T84 and parotid cells by both methods with pHi levels of 7.3 or 8.3. Little or no Cl- channel current was activated with pHi at 6.4. We used on-cell patch clamp methods to investigate the actions of low pHi on single Cl- channel current amplitude in T84 cells. Lowering the pHi had little or no effect on the current amplitude of a 8 pS Cl- channel, but did reduce channel activity. These results suggest that cytosolic acidification may be able to modulate stimulus-secretion coupling in fluid-secreting epithelia by inhibiting the activation of Ca(2+)-activated Cl- channels.

Animals↗
PubMed1995

Inhibition by thiocyanate of muscarinic-induced cytosolic acidification and Ca2+ entry in rat sublingual acini.

Thiocyanate (SCN-) plays a critical part in an oral antimicrobial system by acting as a substrate for peroxidases. Salivary glands concentrate SCN- from blood up to 5 mM in saliva; however, the influence of SCN- on salivary acinar-cell function is unknown. The present study examined the effects of SCN- on the regulation of cytosolic pH (pHi) and free Ca2+ concentration ([Ca2+]i) in rat sublingual mucous acini using the pH- and Ca(2+)-sensitive fluorescent indicators, 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein and fura-2, respectively. SCN- induced a concentration-dependent inhibition of the carbachol-stimulated cytosolic acidification (K1/2, approx. 1.4 mM SCN-). Cytosolic pH recovery from an acid load was not changed by substitution of Cl- by SCN-, suggesting that Na+/H+ exchange activity was not affected by SCN-. SCN- did not alter the initial carbachol-stimulated increase in [Ca2+]i; however, the sustained [Ca2+]i increase was inhibited by > 65% (K1/2, approx. 1.0 mM SCN-). Furthermore, SCN- prevented the carbachol-stimulated Mn2+ influx, indicating that it inhibits the divalent-cation entry pathway. Consistent with decreased Ca2+ mobilization being involved in the blockade of the agonist-induced acidification by SCN-, only total replacement of Cl- with SCN- significantly inhibited the acidification induced by the Ca2+ ionophore ionomycin. The permeability to SCN- through the Ca(2+)-dependent Cl- channels was 5.2-fold higher than the permeability to Cl-. These results suggest that inhibition of the agonist-induced cytosolic acidification by high-concentration SCN- may be mediated by both competitive inhibition of HCO3- efflux and by blockade of Ca2+ influx.

Acids↗
PubMed1995

Volume-activated chloride channels in HL-60 cells: potent inhibition by an oxonol dye.

When swollen in hypotonic media, HL-60 cells exhibit a regulatory volume decrease (RVD) response as a result of net losses of K+ and Cl-. This is primarily caused by a dramatic increase in Cl- permeability, which may reflect the opening of volume-sensitive channels (11). To test this hypothesis, we measured volume-activated Cl- currents in HL-60 cells using the patch-clamp technique. The whole cell Cl- conductance (in nS/pF at 100 mV) increased from 0.09 +/- 0.06 to 1.15 +/- 0.19 to 1.64 +/- 0.40 as the tonicity (in mosmol/kgH2O) of the external medium was decreased from 334 to 263 to 164, respectively. Cl- currents showed no significant inactivation during 800-ms pulses. Current-voltage curves exhibited outward rectification and were identical at holding potentials of 0 or -50 mV, suggesting that the gating of the channels is voltage independent. The selectivity sequence, based on permeability ratios (PX/PCl) calculated from the shifts of the reversal potentials, was SCN- > I- approximately NO3- > Br- > Cl- >> gluconate. 4-Acetamido-4'- isothiocyanostilbene-2,2'-disulfonic acid (SITS; 0.5 mM) inhibits HL-60 Cl- channels in a voltage-dependent manner, with approximately 10-fold increased affinity at potentials greater than +40 mV. Voltage-dependent blockade by SITS indicates that the binding site is located near the outside, where it senses 20% of the membrane potential. These Cl- channels were also inhibited in a voltage-independent manner by the oxonol dye bis-(1,3-dibutylbarbituric acid)pentamethine oxonol [diBA-(5)-C4] with a concentration that gives half inhibition (IC50) of 1.8 microM at room temperature. A similar apparent IC50 value (1.2 microM) was observed for net 36Cl- efflux into a Cl(-)-free hypotonic medium at 21 degrees C. It seems likely, therefore, that the volume-activated Cl- channels are responsible for the net Cl- efflux during RVD. These Cl- channels have properties similar to the "mini-Cl-" channels described in lymphocytes and neutrophils and are strongly inhibited by low concentrations of diBA-(5)-C4.

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗
PubMed1994

Autonomic modulation of action potential and tension in guinea pig papillary muscles.

The effects of alpha 1-adrenoceptor and muscarinic acetylcholine receptor stimulation on action potential and tension were studied in guinea pig papillary muscles obtained from both right and left ventricles. Stimulation of muscarinic acetylcholine receptors with carbachol produced a reduction of the action potential duration and a positive inotropic effect in papillary muscles from both ventricles. Both effects were concentration dependent and atropine sensitive. However, differential responsiveness was found upon alpha 1-adrenoceptor activation in muscles obtained from left and right ventricles. In right side papillary muscles, the alpha 1-adrenoceptor agonist, methoxamine, decreased the action potential duration and produced a positive inotropic effect. In contrast, methoxamine decreased the action potential duration but failed to produce a positive inotropic effect in left side papillary muscles. All methoxamine effects were antagonized by prazosin. Responses to maximum concentration of carbachol and methoxamine on the action potential duration and contractility were additive in right side papillary muscles. Phorbol 12,13-dibutyrate (PDB), a direct protein kinase C activator, also decreased the action potential duration in a manner that was additive to both carbachol and methoxamine. However, PDB reversed the positive inotropic effect of carbachol and methoxamine. The methoxamine-induced shortening of the action potential duration was prevented by pretreatment with indomethacin and nordihydroguaiaretic acid, blockers of arachidonic acid metabolism, but not by the protein kinase C antagonist, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7).(ABSTRACT TRUNCATED AT 250 WORDS)

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1993

Na+ channels in cardiac and neuronal cells derived from a mouse embryonal carcinoma cell line.

1. Cells from a pluripotent murine embryonal carcinoma cell line (P19) were differentiated in vitro into cells with neurone- and cardiac-like phenotypes. Cells treated with 0.5 microM retinoic acid developed into neurone-like cells possessing extensive neurites. Dimethyl sulphoxide treatment (0.5%) produced large, spontaneously contracting cell aggregates with many properties of cardiac cells. 2. The neurone- and cardiac-like cells contained voltage-sensitive Na+ channels with properties similar to those of native neuronal and cardiac cells. 3. We used whole-cell patch clamp techniques to measure inward currents from the neurone- and cardiac-like cells. Undifferentiated (untreated) cells had only small inward currents (peak of -0.15 nA in 150 mM external Na+). The peak inward current in the neurone-like and cardiac-like cells was -1.2 nA (in 154 mM external Na+) and -2.8 nA (in only 46 mM Na+), respectively. These large currents were absent when the external solution contained no Na+. 4. Tetrodotoxin (TTX) blocked the Na+ currents in the neurone- and cardiac-like cells in a dose-dependent manner. The Kd for TTX block of the Na+ current in the neurone-like cells was 6.7 nM. The Na+ current in the cardiac-like cells was much more resistant to TTX; the half-blocking concentration was two orders of magnitude higher, 710 nM. 5. The kinetic properties of the Na+ channel currents in the neurone- and cardiac-like cells were similar but developed over somewhat different voltage ranges. The voltage sensitivity of activation was similar in both cell types but the activation mid-point voltage was different: -12 mV in the neuronal cells and -34 mV for cardiac cells. Inactivation of the neuronal Na+ channels had a mid-point near -47 mV and was more sensitive to the membrane voltage than inactivation of the cardiac channels. The mid-point of inactivation for the cardiac Na+ channels was -80 mV.

Animals↗
PubMed1991

Ca2+ current and Ca2+ transients under action potential clamp in guinea pig ventricular myocytes.

Precise characterization of the magnitude and kinetics of transsarcolemmal Ca2+ influx during an action potential (AP) is essential for a complete understanding of excitation-contraction coupling in heart. Using a voltage-clamp protocol that simulated a physiological AP (AP clamp), we characterized the properties of the Ca2+ current (ICa) in guinea pig ventricular myocytes. The AP-generated ICa showed a complex time course that was different from ICa generated by a square pulse. ICa activated rapidly during the upstroke of the AP and then partially inactivated during the plateau. The fast component of ICa reached a peak value of -7.6 +/- 1.0 pA/pF at 2.40 +/- 0.30 ms after depolarization, followed by a slow component with a peak value of -2.9 +/- 0.4 pA/pF during the plateau. ICa generated by an AP was composed of both L- and T-type Ca2+ channels. T-type Ca2+ current contributed to the fast component of ICa and L-type Ca2+ current contributed to both fast and slow components of ICa. Activation of beta-adrenoceptors enhanced ICa with a maximal effect lasting throughout the entire plateau of the AP. Measurements of cytosolic Ca2+ transients using fura-2 indicated that the ICa was responsible for triggering Ca2+ release from the sarcoplasmic reticulum. The AP clamp provides a new approach for investigation of the relationship between ICa and Ca2+ transients under more physiological conditions.

Action Potentials↗
PubMed1987

Modulation of calcium channels of twitch skeletal muscle fibres of the frog by adrenaline and cyclic adenosine monophosphate.

1. Modulation of fast and slow Ca2+ channels of frog skeletal muscle by adrenaline (10(-6) to 10(-5) M) and cyclic AMP was investigated using intracellular voltage recordings in intact fibres and a voltage-clamp technique in cut fibres. 2. In tetraethylammonium (TEA), Cl(-)-free Ringer solution, adrenaline increased the maximum rate of rise of Ca2+ spikes by 85% and in a similar solution, peak slow Ca2+ current (ICa,s) by 51%. 3. Application of cyclic AMP to the cut ends of fibres, produced a relative increase of ICa,s of ca. 24%. The effect was maintained for ca. 2 h. 4. Changes in the time course of ICa,s were produced by adrenaline and cyclic AMP: the limiting values of time-to-peak current measured as a function of membrane potential were lower (ca. 41% in adrenaline and ca. 34% in cyclic AMP) than those found in control experiments. Also, ICa,s decayed faster in the presence of adrenaline or cyclic AMP. These changes can be explained by exhaustion of Ca2+ in the lumen of transverse tubular system and do not require the assumption of kinetic variations. 5. Fast Ca2+ currents (ICa,f) which could not be blocked by nifedipine were also recorded. Cyclic AMP greatly increased the amplitude of ICa,f but had no obvious effects on ICa,f kinetics. 6. Application of catalytic subunit of cyclic AMP-dependent protein kinase by diffusion or by pressure injection also increased the amplitude of ICa,s and ICa,f. Pressure injection brought about modifications in the time course of ICa,s that cannot be explained by depletion of Ca2+. 7. Mechanical experiments were performed on single fibres. Nominally Ca2+-free solutions prevented the development and the maintenance of positive inotropic effect of adrenaline on twitch tension. Development of twitch potentiation was dependent upon the frequency of stimulation. Adrenaline was practically ineffective if no stimulation was applied. 8. It is concluded that both populations of Ca2+ channels are modulated by adrenergic stimulation probably via cyclic AMP, and that twitch potentiation may be mediated by a Ca2+ entry through Ca2+ channels.

Action Potentials↗