Removal of Ca current inactivation in dialysed guinea-pig atrial cardioballs by Ca chelators

Physiological modulation of inactivation in L-type Ca2+ channels: one switch

The relative contributions of voltage- and Ca(2+)-dependent mechanisms of inactivation to the decay of L-type Ca(2+) channel currents (I(CaL)) is an old story to which recent results have given an unexpected twist. In cardiac myocytes voltage-dependent inactivation (VDI) was thought to be slow and Ca(2+)-dependent inactivation (CDI) resulting from Ca(2+) influx and Ca(2+)-induced Ca(2+)-release (CICR) from the sarcoplasmic reticulum provided an automatic negative feedback mechanism to limit Ca(2+) entry and the contribution of I(CaL) to the cardiac action potential. Physiological modulation of I(CaL) by Beta-adrenergic and muscarinic agonists then involved essentially more or less of the same by enhancing or reducing Ca(2+) channel activity, Ca(2+) influx, sarcoplasmic reticulum load and thus CDI. Recent results on the other hand place VDI at the centre of the regulation of I(CaL). Under basal conditions it has been found that depolarization increases the probability that an ion channel will show rapid VDI. This is prevented by Beta-adrenergic stimulation. Evidence also suggests that a channel which shows rapid VDI inactivates before CDI can become effective. Therefore the contributions of VDI and CDI to the decay of I(CaL) are determined by the turning on, by depolarization, and the turning off, by phosphorylation, of the mechanism of rapid VDI. The physiological implications of these ideas are that under basal conditions the contribution of I(CaL) to the action potential will be determined largely by voltage and by Ca(2+) following Beta-adrenergic stimulation.

Voltage-dependent inactivation of L-type Ca2+ currents in guinea-pig ventricular myocytes

The objective of this study was to describe the kinetics of voltage-dependent inactivation of native cardiac L-type Ca(2+) currents. Whole-cell currents were recorded from guinea-pig isolated ventricular myocytes. Voltage-dependent inactivation was separated from Ca(2+)-dependent inactivation by replacing extracellular Ca(2+) with Mg(2+) and recording outward currents through Ca(2+) channels. Voltage-dependent inactivation accelerated from slow monophasic decay at -30 mV to maximal rapid biphasic decay at +20 mV. Maximal voltage-dependent inactivation occurred with tau(f) approximately equal 30 ms and tau(s) approximately equal 300 ms, the fast component of decay accounted for 70 % of the current amplitude. In basal conditions Ca(2+) current availability was sigmoid. Isoproterenol (isoprenaline) evoked a large increase in a time-independent component of the Ca(2+) current which also increased with depolarisation. This was responsible for the apparent recovery of Ca(2+) channel current availability at positive membrane potentials and thus a U-shaped availability-voltage (A-V) relationship. It is concluded that beta-adrenergic stimulation altered the reaction of native cardiac L-type Ca(2+) channels to membrane voltage. In basal conditions, voltage accelerated inactivation. In isoproterenol, voltage could also reduce inactivation.

beta-Adrenergic stimulation modulates Ca2+- and voltage-dependent inactivation of L-type Ca2+ channel currents in guinea-pig ventricular myocytes

The objective of this study was to examine the effect of beta-adrenergic stimulation upon voltage- and Ca2+-induced inactivation of native cardiac L-type Ca2+ channels. Whole-cell currents were recorded from guinea-pig isolated ventricular myocytes. Total and voltage-dependent inactivation was separated by replacing extracellular Ca2+ with Mg2+. L-type Ca2+ channel behaviour was monitored with outward Ca2+ channel currents. First, the voltage dependence of inactivation was studied at fixed times (50 and 1000 ms) after activation. This showed that under control conditions Ca2+ contributed little to inactivation. In isoproterenol (isoprenaline), voltage-dependent inactivation was markedly reduced and Ca2+ contributed largely to total inactivation. Second, the time dependence of inactivation was studied at a fixed voltage (+10 mV). In control conditions the fast phase of inactivation (tau(f) approximately 15 ms) was reduced to the same extent by ryanodine (tau(f) approximately 30 ms) and the absence of Ca2+ (tau(f) approximately 30 ms) while the slow phase of inactivation (tau(s) approximately 70 ms) was reduced by ryanodine (tau(s) approximately 160 ms) and further reduced in the absence of Ca2+ (tau(s) approximately 300 ms). In isoproterenol, biphasic inactivation of Ca2+ currents (tau(f) approximately 4 ms, tau(s) approximately 60 ms) was replaced by a single slow (tau approximately 450 ms) phase of inactivation in the absence of Ca2+. It is concluded that, under control conditions Ca2+ channel current decay is largely dominated by rapid voltage-dependent inactivation, while in isoproterenol this is replaced by Ca2+-induced inactivation.

Voltage- and cation-dependent inactivation of L-type Ca2+ channel currents in guinea-pig ventricular myocytes

L-type Ca2+ channel currents in native ventricular myocytes inactivate according to voltage- and Ca2+-dependent processes. This study sought to examine the effect of beta-adrenergic stimulation on the contributions of voltage and Ca2+ to Ca2+ current decay. Ventricular myocytes were enzymatically isolated from guinea-pig hearts. Inward whole-cell Cd2+-sensitive L-type Ca2+ channel currents were recorded with the patch clamp technique and comparison was made between inward currents carried by Ca2+ and either Ba2+, Sr2+ or Na+. In control conditions the decay of Ca2+ currents was faster than Ba2+, Sr2+ or Na+ currents at negative voltages while at positive voltages there was no difference. The relationship between voltage and inactivation for Ca2+ currents was bell-shaped, while that for Ba2+, Sr2+, and Na+ currents was sigmoid. Thus depolarisation progressively replaced Ca2+-dependent inactivation in the fast phase of decay of Ca2+ channel currents with rapid voltage-dependent inactivation. In the presence of isoproterenol (isoprenaline) the decay of Ca2+ currents was faster than Ba2+, Sr2+ or Na+ currents at all measured voltages (-40 to +30 mV). The relationship between voltage and inactivation for Ca2+, Ba2+ and Sr2+ currents was bell-shaped, while that for Na+ currents was sigmoid with less inactivation than under control conditions. Therefore the fast phase of decay of Ca2+ channel currents was now almost entirely due to Ca2+. It is concluded that the relative contributions of Ca2+- and voltage-dependent mechanisms of inactivation of L-type Ca2+ channels in native cardiac myocytes are modulated by beta-adrenergic stimulation influencing the amount of rapid voltage-dependent inactivation.

Ca2+-dependent regulation of cardiac L-type Ca2+ channels: is a unifying mechanism at hand?

Ca2+ entry (I(Ca)) through cardiac L-type Ca2+ channels (LTCC) drives critical cellular processes ranging from contraction to gene expression, and, when disordered, is implicated in arrhythmias and hypertrophy. LTCC activation occurs by cell membrane depolarization, but LTCCs are also regulated by auxiliary proteins, phosphorylation, and intracellular CA2+([Ca2+]i). LTCC regulation by [Ca2+]i is especially intriguing because increased [Ca2+]i signals dual and conflicting commands for I(Ca)inactivation and facilitation. A recent explosion of work has shed new light on the mechanisms and molecular identity of domains necessary for [Ca2+]i-dependent regulation of LTCC.

Control of L-type calcium current during the action potential of guinea-pig ventricular myocytes

1. During an action potential the L-type Ca2+ current (ICa,L) activates rapidly, then partially declines leading to a sustained inward current during the plateau phase. The reason for the sustained part of ICa,L has been investigated here. 2. In the present study the mechanisms controlling the ICa,L during an action potential were investigated quantitatively in isolated guinea-pig ventricular myocytes by whole-cell patch clamp. To measure the actual time courses of ICa,L and the corresponding L-type channel inactivation (fAP) during an action potential, action potential-clamp protocols combined with square pulses were applied. 3. Within the first 10 ms of the action potential the ICa,L rapidly inactivated by about 50 %; during the plateau phase inactivation proceeded to 95 %. Later, during repolarization, the L-type channels recovered up to 25 %. 4. The voltage-dependent component of inactivation during an action potential was determined from measurements of L-type current carried by monovalent cations. This component of inactivation proceeded rather slowly and contributed only a little to fAP. ICa,L during an action potential is thus mainly controlled by Ca2+-dependent inactivation. 5. In order to investigate the source of the Ca2+ controlling fAP, internal Ca2+ homeostasis was manipulated by the use of Ca2+ buffers (EGTA, BAPTA), by blocking Na+-Ca2+ exchange, or by blocking Ca2+ release from the sarcoplasmic reticulum (SR). Internal BAPTA markedly reduced the L-type channel inactivation during the entire action potential, whereas EGTA affected fAP only during the middle and late plateau phases. Inhibition of Na+-Ca2+ exchange markedly increased inactivation of L-type channels. Although blocking SR Ca2+ release decreased the fura-2-measured cytoplasmic Ca2+ concentration ([Ca2+]i) transient by about 90 %, it reduced L-type channel inactivation only during the initial 50 ms of the action potential. Thus, it is Ca2+ entering the cell through the L-type channels that controls the inactivation process for the majority of the action potential. Nevertheless, SR Ca2+-release contributes 40-50 % to L-type channel inactivation during the initial period of the action potential. However, the maximum extent of inactivation reached during the plateau is independent of Ca2+ released from the SR. 6. For the first time, the actual time course of L-type channel inactivation has been directly determined during an action potential under various defined [Ca2+]i conditions. Thereby, the relative contribution to ICa,L inactivation of voltage, Ca2+ entering through L-type channels, and Ca2+ being released from the SR could be directly demonstrated.

Preferential closed-state inactivation of neuronal calcium channels

We have investigated the inactivation mechanism of neuronal N-, P/Q-, and R-type calcium channels. Although channels inactivate slowly during square-pulse depolarization, as observed previously, we now find that they inactivate profoundly during a train of action potential (AP) waveforms. The apparent paradox arises from a voltage-dependent mechanism in which channels inactivate preferentially from intermediate closed states along the activation pathway. Inactivation can therefore extend beyond the brief duration of AP waveforms to continue between spikes, as the channel undergoes repetitive cycles of activation and deactivation. The extent of inactivation during a train is strongly affected by the subunit composition of channels. Preferential closed-state inactivation of neuronal calcium channels could produce widely variable depression of Ca2+ entry during a train of APs.

Ca2+- and voltage-dependent inactivation of Ca2+ channels in nerve terminals of the neurohypophysis

Ca2+ channel inactivation was investigated in neurohypophysial nerve terminals by using patch-clamp techniques. The contribution of intracellular Ca2+ to inactivation was evaluated by replacing Ca2+ with Ba2+ or by including BAPTA in the internal recording solution. Ca2+ channel inactivation during depolarizing pulses was primarily voltage-dependent. A contribution of intracellular Ca2+ was revealed by comparing steady-state inactivation of Ca2+ channels with Ca2+ current and with intracellular [Ca2+]. However, this contribution was small compared to that of voltage. In contrast to voltage-gated Ca2+ channels in other preparations, in the neurohypophysis Ba2+ substitution or intracellular BAPTA increased the speed of inactivation while reducing the steady-state level of inactivation. Ca2+ channel recovery from inactivation was studied by using a paired-pulse protocol. The rate of Ca2+ channel recovery from inactivation at negative potentials was increased dramatically by Ba2+ substitution or intracellular BAPTA, indicating that intracellular Ca2+ inhibits recovery. Stimulation with trains of brief pulses designed to mimic physiological bursts of electrical activity showed that Ca2+ channel inactivation was much greater with 20 Hz trains than with 14 Hz trains. Inactivation induced by 20 Hz trains was reduced by intracellular BAPTA, suggesting an important role for Ca2+-dependent inactivation during physiologically relevant forms of electrical activity. Inhibitors of calmodulin and calcineurin had no effect on Ca2+ channel inactivation, arguing against a mechanism of inactivation involving these Ca2+-dependent proteins. The inactivation behavior described here, in which voltage effects on Ca2+ channel inactivation predominate at positive potentials and Ca2+ effects predominate at negative potentials, may be relevant to the regulation of neuropeptide release.

Multiple modulations of action potential duration by different calcium channel blocking agents in guinea pig ventricular myocytes

Effects of extracellular applications of different types of Ca2+ channel blocking agents (Mn2+, verapamil, and nisoldipine) on action-potential duration and membrane currents were studied by the whole-cell patch-clamp technique in guinea pig ventricular myocytes. Low concentrations of Mn2+ (1 mM) and verapamil (1 microM) prolonged action-potential duration at 90% repolarization (APD90) with a suppressed plateau phase. Increases in Mn2+ (5 mM) and verapamil (5 microM) shortened APD90 with a further depression of the plateau. Nisoldipine (0.2-1 microM) shortened APD90 without lengthening it. Applications of Mn2+ and verapamil suppressed amplitudes of the L-type Ca2+ current (ICa), the delayed outward K+ current (IK), and the inward rectifier K+ current (IK1). Furthermore, the ratios of ICa:IK inhibition were similar by low and high concentrations of Mn2+ and verapamil. Nisoldipine selectively suppressed ICa without effect on IK and IK1. A low concentration (1 mM) of Mn2+ not only decreased the peak amplitude of ICa but also delayed its decay time course, which caused an increase in late ICa amplitude at the end of a 200-ms depolarizing pulse. Both verapamil and nisoldipine suppressed peak ICa without affecting its decay. Whereas Mn2+ suppressed IBa without changing its decay time course, verapamil and nisoldipine speeded up the IBa decay with suppressed amplitude of IBa. We conclude that different types of Ca2+ channel blocking agents (Mn2+, verapamil, and nisoldipine) diversely modulate APD because of their multiple modes of actions on ICa and IK.

Subcellular properties of triggered Ca2+ waves in isolated citrate-loaded guinea-pig atrial myocytes characterized by ratiometric confocal microscopy

1. Spatiotemporal aspects of subcellular Ca2+ signalling were studied in cultured adult guinea-pig atrial myocytes. A mixture of the Ca2+ indicators fluo-3 and Fura Red in combination with laser-scanning confocal microscopy was used for [Ca2+]i measurements while membrane currents were recorded simultaneously. 2. In citrate-loaded atrial myocytes not every Ca2+ current (ICa) could trigger Ca2+ release from the sarcoplasmic reticulum (SR). Two types of Ca2+ signals could be observed: Ca2+ transients resulting from (i) Ca2+ influx alone and (ii) additional Ca2+ release. 3. Ca2+ release elicited by voltage steps of 100-150 ms duration was either apparently homogeneous or propagated as Ca2+ waves through the entire cell. With brief ICa (50-75 ms), Ca2+ waves with limited subcellular propagation were observed frequently. These waves always originated from either end of the myocyte. 4. The time course of changes in Na(+)-Ca2+ exchange current (INaCa) depended on the subcellular properties of the underlying Ca2+ transient and on the particular cell geometry. Apparently homogeneous Ca2+ release was accompanied by an inward change of INaCa the onset phase of which was fused with ICa. Changes in INaCa caused by a Ca2+ wave propagating through the entire cell showed a W shape, which could be attributed to differences of the fractional surface-to-volume ratio in different cell segments during propagation of the Ca2+ wavefront. Those waves with limited spreading only activated a small component of INaCa. 5. The different subcellular patterns of Ca2+ release signals can be explained by spatial inhomogeneities in the positive feedback of the SR. This depends on the local SR Ca2+ loading state under the control of the local Ca2+ influx during activation of ICa. Due to the higher surface-to-volume ratio at the two ends of the myocyte, SR loading and therefore the positive feedback in Ca(2+)-induced Ca2+ release may be higher at the ends, locations where Ca2+ waves are preferentially triggered. 6. We conclude that the individual cell geometry may be an important determinant of subcellular Ca2+ signalling not only in cardiac muscle cells but presumably also in other types of cells that depend on Ca2+ signalling. In addition, the cell geometry in combination with varying subcellular Ca2+ release patterns can greatly affect the time course of Ca(2+)-activated membrane currents.

Activation of a high affinity Gi protein-coupled plasma membrane receptor by sphingosine-1-phosphate

Sphingosine-1-phosphate (SPP) has attracted much attention as a possible second messenger controlling cell proliferation and motility and as an intracellular Ca(2+)-releasing agent. Here, we present evidence that SPP activates a G protein-coupled receptor in the plasma membrane of various cells, leading to increase in cytoplasmic Ca2+ concentration ([Ca2+]i), inhibition of adenylyl cyclase, and opening of G protein-regulated potassium channels. In human enbryonic kidney (HEK) cells, SPP potently (EC50, 2 nM) and rapidly increased [Ca2+]i in a pertussis toxin-sensitive manner. Pertussis toxin-sensitive increase in [Ca2+]i was also observed with sphingosylphosphorylcholine (EC50, 460 nM), whereas other sphingolipids, including ceramide-1-phosphate, N-palmitoyl-sphingosine, psychosine, and D-erythro-sphingosine at micromolar concentrations did not or only marginally increased [Ca2+]i. Furthermore, SPP inhibited forskolin-stimulated cAMP accumulation in HEK cells and increased binding of guanosine 5'3-O-(thio) triphosphate to HEK cell membranes. Rapid [Ca2+]i responses were also observed in human transitional bladder carcinoma (J82) cells, monkey COS-1 cells, mouse NIH 3T3 cells, Chinese hamster ovary (CHO-K1) cells, and rat C6 glioma cells, whereas human HL-60 leukemia cells and human erythroleukemia cells failed to respond to SPP. In guinea pig atrial myocytes, SPP activated Gi protein-regulated inwardly rectifying potassium channels. Activation of these channels occurred strictly when SPP was applied at the extracellular face of atrial myocyte plasma membrane as measured in cell-attached and inside-out patch clamp current recordings. We conclude that SPP, in addition to its proposed direct action on intracellular Ca2+ stores, interacts with a high affinity Gi protein-coupled receptor in the plasma membrane of apparently many different cell types.

Intracellular citrate induces regenerative calcium release from sarcoplasmic reticulum in guinea-pig atrial myocytes

Ca2+ release from the sarcoplasmic reticulum was studied in voltage-clamped guinea-pig atrial myocytes. Cells were dialysed with a pipette solution containing the Ca2+ indicator 1- [2-amino-5-(6-carboxyindol-2-yl) phenoxy]-2-(2'-amino-5'-methylphenoxy) ethane-N,N,N',N'-tetraacetic acid] (Indo-1, 100 microM) and as main anion either chloride or the low-affinity Ca2+ buffer citrate. Intracellular Ca2+ transients (Cai transients) were elicited by depolarizations from a holding potential of -50 mV. In chloride-dialysed cells, Cai transients showed a bell-shaped dependence on the amplitude of the depolarizing pulse. In citrate-dialysed cells, membrane depolarizations were associated with a small rise in [Ca2+]i. These small changes in [Ca2+]i were either followed by a large Cai transient or failed to induce large changes in [Ca2+]i. The peak amplitude of the large Cai transient did not vary with the amplitude of the depolarizing pulse. These results demonstrate that in the presence of intracellular chloride, Ca2+ release in atrial cells is a graded process triggered by Ca2+ influx. Using citrate as the main intracellular anoin, Ca2+ release triggered by Ca2+ entry was no longer graded but occurred in a regenerative manner. The results are discussed in terms of two models in which citrate, affects the spatial distribution of [Ca2+]i or the loading state of the sarcoplasmic reticulum.

Myocardial action potential prolongation by calcium channel activation under calcium free-EGTA condition in rats: developmental and regional variations

1. Prolongation of action potentials upon the addition of isoproterenol, forskolin, isobutylmethyl-xanthine (IBMX) and dibutyril cAMP (dbcAMP) under Ca-free EGTA condition was examined in isolated myocardial preparations from neonatal and adult rats, whose action potential configuration greatly differ. 2. The prolongation of the action potential was previously suggested to be produced by persistent sodium influx through calcium channel due to the lack of calcium-mediated inactivation of calcium channels under such experimental condition. 3. Preparations used were papillary muscles and free walls of the right and left ventricles from neonatal and adult rats. 4. In adult preparations, the prolongation produced by isoproterenol, forskolin and IBMX in the right free wall was smaller than those in the other three regions, while no regional difference was observed with dbcAMP. 5. The degree of prolongation by all of the four drugs were smaller in the neonate than in the adult. No regional difference was observed with any of the drugs in the neonate. 6. Our present results suggest that contribution of calcium-mediated inactivation of calcium channels to the repolarization of rat myocardium may increase postnatally to produce the developmental shortening of its action potential. Also, regional difference in the cAMP related mechanisms may appear postnatally.

Down-regulation of A1 adenosine receptors coupled to muscarinic K+ current in cultured guinea-pig atrial myocytes

1. Muscarinic K+ current (IK(ACh)) was measured in cultured atrial myocytes from hearts of adult guinea-pigs using whole-cell voltage clamp. IK(ACh) was activated by superfusion with solutions containing either acetylcholine (ACh) or adenosine (Ado), in saturating concentrations of 2 microM (ACh) and 1 mM (Ado), respectively. 2. In freshly isolated cells the amplitude of the current activated by Ado (IK(Ado)) was 58% (mean) of the current that was induced by ACh. In serum-free culture this relation, but also the absolute density of IK(ACh), remained fairly constant for up to 8 days. 3. If the culture medium was supplemented with fetal calf serum (FCS, 5%) the relation IK(Ado)/IK(ACh) gradually decayed, reaching a value of less than 0.1 on days 7-8, whereas the response to ACh remained stable over this period of time. 4. After treatment of cells with FCS-containing medium, no recovery was observed upon FCS withdrawal for up to 4 days. 5. The effect of FCS on responsiveness to Ado was half-maximal at about 1% (v/v). The active principle can be dialysed (mol. mass exclusion: 10 kDa). It is not identical with an albumin-associated factor that has been shown to be a potent activator of atrial IK(ACh) upon acute superfusion. Loss of responsiveness to Ado was paralleled by a reduction of binding sites to the A1 adenosine receptor-specific radioligand 8-cyclopentyl-1,3-dipropylxanthine ([3H]CPX). 6. It is concluded that FCS contains a factor that causes down-regulation of A1 Ado receptors. The signalling pathway that leads to an increased opening activity of IK(ACh) channels and other receptors, such as the M2 muscarinic receptor, linked to this signalling pathway are not affected by this factor.

Evolution of the Ca2+ current during dialysis of isolated bovine chromaffin cells: effect of internal calcium

We have examined the internal Ca(2+)-dependence of the long-term evolution of whole cell high voltage activated Ca current in chromaffin cells. The evolution of the peak Ca current was characterized by 2 distinct phases: after an initial facilitation, there followed a rundown, which represented a reduction by 70% within some 10 min. The rundown process was shown not to depend on Ca2+ entry nor on membrane depolarization. It resulted from cell dialysis with a saline solution and, once initiated, it proceeded at a rate of 0.28 min-1 at 4 different Ca2+ concentrations (pCa 5-9). The facilitation is also initiated by cell dialysis but this process developed faster at higher internal Ca2+ concentrations. Thus, globally, high-voltage activated Ca2+ current runs down faster when using a recording pipette solution with a higher internal Ca2+ concentration (pCa 5 or 6). Some leupeptin-sensitive proteases may be involved in the initiation of facilitation and rundown processes.

Myocardial action potential prolongation by calcium channel activation under calcium-free EGTA condition in guinea pigs and rats

1. Prolongation of action potentials upon the addition of isoproterenol or dbcAMP under Ca-free EGTA condition were observed in isolated myocardial preparations from both the guinea pig and the rat, whose action potential configuration greatly differ. The degree of prolongation was greater in the rat than in the guinea pig. 2. The prolongation of the action potential was rapidly reversed upon the addition of calcium ion and was dose-dependently suppressed by the addition of calcium antagonists. The sensitivity to nicardipine of this action potential was tenfold higher than of the so-called slow response action potentials. The duration of the prolonged action potential was dependent on the external sodium concentration, but was not affected by tetrodotoxin. 3. Thus, it was demonstrated in intact myocardia that sodium ion may persistently pass through the calcium channel to prolong the action potential when it is activated under the condition where the calcium-mediated inactivation of calcium channels is removed. 4. Contribution of calcium-mediated inactivation of calcium channels to the repolarization of normal myocardium may be larger in the rat than in the guinea pig.

Calcium-dependent inactivation of L-type calcium channels in planar lipid bilayers

Intracellular Ca2+ can inhibit the activity of voltage-gated Ca channels by modulating the rate of channel inactivation. Ca(2+)-dependent inactivation of these channels may be a common negative feedback process important for regulating Ca2+ entry under physiological and pathological conditions. This article demonstrates that the inactivation of cardiac L-type Ca channels, reconstituted into planar lipid bilayers and studied in the presence of a dihydropyridine agonist, is sensitive to Ca2+. The rates and extents of inactivation, determined from ensemble averages of unitary Ba2+ currents, decreased when the calcium concentration facing the intracellular surface of the channel ([Ca2+]i) was lowered from approximately 10 microM to 20 nM by the addition of Ca2+ chelators. The rates and extents of Ba2+ current inactivation could also be increased by subsequent addition of Ca2+ raising the [Ca2+]i to 15 microM, thus demonstrating that the Ca2+ dependence of inactivation could be reversibly regulated by changes in [Ca2+]i. In addition, reconstituted Ca channels inactivated more quickly when the inward current was carried by Ca2+ than when it was carried by Ba2+, suggesting that local increases in [Ca2+]i could activate Ca(2+)-dependent inactivation. These data support models in which Ca2+ binds to the channel itself or to closely associated regulatory proteins to control the rate of channel inactivation, and are inconsistent with purely enzymatic models for channel inactivation.

Atrial G protein-activated K+ channel: expression cloning and molecular properties

Activity of several ion channels is controlled by heterotrimeric GTP-binding proteins (G proteins) via a membrane-delimited pathway that does not involve cytoplasmic intermediates. The best studied example is the K+ channel activated by muscarinic agonists in the atrium, which plays a crucial role in regulating the heartbeat. To enable studies of the molecular mechanisms of activation, this channel, denoted KGA, was cloned from a rat atrium cDNA library by functional coupling to coexpressed serotonin type 1A receptors in Xenopus oocytes. KGA displays regions of sequence homology to other inwardly rectifying channels as well as unique regions that may govern G-protein interaction. The expressed KGA channel is activated by serotonin 1A, muscarinic m2, and delta-opioid receptors via G proteins. KGA is activated by guanosine 5'-[gamma-thio]triphosphate in excised patches, confirming activation by a membrane-delimited pathway, and displays a conductance equal to that of the endogenous channel in atrial cells. The hypothesis that similar channels play a role in neuronal inhibition is supported by the cloning of a nearly identical channel (KGB1) from a rat brain cDNA library.

The molecular mode of action of the Ca agonist (-) BAY K 8644 on the cardiac Ca channel

The primary drug action of (-) BAY K 8644 on whole-cell Ca current in atrial myocytes was measured under conditions where secondary Ca-mediated changes of Ca channel activity were minimized. The most direct action of (-) BAY K 8644 is the change of gating kinetics which results in a strictly voltage-dependent increase of the peak current in the voltage range between -40 and 0 mV. Peak currents were increased dose dependently in the concentration range from 1 to 30 nM. Analysis of peak current/voltage relations revealed a linear shift of the current activation by approximately 23 mV to more negative membrane potentials, without any change in its voltage dependence and in the current reversal potential or the maximum whole-cell conductance. Measurement of Ca current activation and deactivation time constants suggests that (-) BAY K 8644 prolongs the single-channel open time without affecting the closed time. From the shift of the open time function to more negative voltages by about 50 mV the energy transferred to the gating process is calculated to be 5.4 kJ/mol (1.3 kcal/mol). The drug-induced slow component of tail current has been used to estimate the true dose/response relation for (-) BAY K 8644. A KD value of 4.3 nM and a Hill coefficient of 1.25 were determined. Flash-induced competition experiments with the Ca antagonist nifedipine allowed the measurement of binding kinetics of (-) BAY K 8644. The association rate constant is estimated to about 5 x 10(6) mol-1.s-1 and dissociation time constant is approximately 50-70 s; both are in close agreement with receptor binding studies. Results are discussed in relation to models for drug action of dihydropyridine-type compounds and to implications for the structure of the Ca channel protein.

Serum contains a potent factor that decreases beta-adrenergic receptor-stimulated L-type Ca2+ current in cardiac myocytes

L-type Ca2+ current (ICa) was measured in cultured atrial myocytes from hearts of adult guinea-pigs using whole-cell voltage clamp. Potentiation of ICa induced by beta-adrenergic stimulation (isoprenaline 2.10(-7) M) could be completely antagonized by diluted sera (1:100 v/v). Half-maximal inhibition of beta-receptor-stimulated ICa occurred at about 1:1000. Basal ICa was not affected by serum. Atropine in a concentration (10(-6) M) that completely antagonized the anti-adrenergic effect of acetylcholine (ACh, 2.10(-6) M) did not interfere with the effect of serum. In cells dialysed with cyclic adenosine monophosphate (cAMP)-containing (10(-4) M) pipette solution, potentiated ICa was insensitive to both ACh and serum. Preincubation of the myocytes with pertussis toxin almost completely abolished the anti-adrenergic effects of both ACh and serum. The potency of serum was not reduced by dialysis. It is concluded that serum contains a factor which, like ACh, inhibits beta-receptor-stimulated adenylyl cyclase via Gi-protein.

Macroscopic and unitary properties of physiological ion flux through L-type Ca2+ channels in guinea-pig heart cells

1. We investigated the currents through L-type Ca2+ channels when Ca2+ (1-10 mM) was the charge carrier, as is the case physiologically. 2. Na+ was removed from both the external and internal solutions to eliminate currents through Na+ channels and Na(+)-Ca2+ exchange. 3. From a holding potential of -50 mV only L-type channels were available to open with depolarization. Macroscopic L-type currents were maximal during depolarizing pulses to +10 mV (peak current density of 4.7 +/- 0.3 nA nF-1). 4. During depolarizing steps as long as 180 ms, the decay of current through L-type channels was incomplete, in contrast to that of T-type current. 5. Unitary currents recorded with comparable ionic conditions and voltage protocols exhibited a single-channel conductance of 6.9 pS in 10 mM Ca2+. Ensemble average currents reproduced accurately the features of whole-cell L-type current, including the maintained component. 6. Convolution analysis was employed to clarify the single-channel basis of the complex current waveform of L-type channels. First openings underlie the peak, while the maintained pedestal is generated by multiple re-openings. As with T-type channels, single openings are brief and contribute little to the time course of the average current. 7. The prominent maintained component of macroscopic and ensemble average L-type current cannot be explained by simple Markov models in which current decay reflects the progressive entry of channels into an absorbing inactivated state. 8. We considered the possibility that the maintained component of current arises from the existence of multiple distinct gating patterns, one of which lacks inactivation. Individual sweeps were sorted among three patterns of gating (no openings, active-early and active-late). Patterns of activity are not randomly distributed; instead, they tend to cluster over time. 9. Most of the maintained current is attributable to the 'active-late' pattern of gating. Considered separately, this pattern can be well described by a simple Markov chain lacking an inactivated state. The 'active-early' gating pattern accounts entirely for the initial current transient, and for about one-third of the maintained component; thus, inactivation, even when present, must be reversible rather than absorbing. 10. The unitary current amplitudes and peak open probabilities measured for single L-type channels, when compared to the average macroscopic L-type current density, predict 170 functional channels per picofarad, or 28,000 L-type channels per typical ventricular myocyte.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of oxygen free radicals on isolated cardiac myocytes from guinea-pig ventricle: electrophysiological studies

Free oxygen radicals are formed during early reperfusion and are thought to contribute to some types of reperfusion abnormalities, including arrhythmias and myocardial stunning. The purpose of this study was to investigate electrophysiological effects of oxygen free radicals using voltage clamped single ventricular myocytes from guinea-pig hearts. Oxygen free radicals were produced enzymatically by the direct addition of xanthine oxidase (XOD, 0.04 U/ml) in the experimental chamber to a solution containing hypoxanthine (0.96 mM). The generation of oxygen radicals was confirmed by the formation of adrenochrome from adrenaline. Oxygen radicals caused automaticity of isolated myocytes within 20-30 min, followed by later hypercontracture. The percentage of rod-shaped cells declined sigmoidally as a function of time, with a half maximal value at 40.9 +/- 1.6 min, and a Hill slope of -0.10 +/- 0.01 (n = 26). These effects were prevented by a combination of superoxide dismutase (10(5) U/L) plus catalase (10(6) U/L). The rate at which cells underwent morphological shape changes was unchanged by ryanodine (0.5 microM) which is thought to act on the sarcoplasmic reticulum or by the Ca2+ channel blockers nisoldipine (1 microM) or Cd2+ (30 microM). Cellular automaticity and hypercontracture were delayed by variable degrees, and sometimes completely prevented, by zero (1 mM EGTA) extracellular Ca2+, MnCl2 (2 mM) and LaCl3 (50 microM), and amiloride (1 mM). On the other hand, in the presence of a low extracellular Na+ (30 mM) or caffeine (10 mM), hypercontracture occurred at a faster time scale. Whole cell voltage clamping revealed a decrease of the inward rectifying K+ current (IK1), and a decrease of the peak of the L-type Ca2+ current (ICa,L). The total ICa,L during the clamp step was increased, mainly because of an increased time constant of inactivation (47.6 +/- 4.7 ms to 72.7 +/- 15.5 ms after 30 min, n = 4, P less than 0.05). We conclude that oxygen radicals cause automaticity and hypercontracture of isolated myocytes, that these effects may be due to an increased intracellular Ca2+ concentration ([Ca2+]i), and despite an increased ICa,L, that the enhanced Ca2+ influx may occur predominantly via the Na/Ca exchange.

Effects of intracellular Ca2+ chelating compounds on inward currents caused by Ca2+ release from sarcoplasmic reticulum in guinea-pig atrial myocytes

Ca2+ release from the sarcoplasmic reticulum (SR) of mammalian cardiac myocytes occurring either due to activation by a depolarization or the resulting transmembrane Ca2+ current (ICa), or spontaneously due to Ca2+ overload has been shown to cause inward current(s) at negative membrane potentials. In this study, the effects of different intracellular Ca2+ chelating compounds on ICa-evoked or spontaneous Ca(2+)-release-dependent inward currents were examined in dialysed atrial myocytes from hearts of adult guinea-pigs by means of whole-cell voltage-clamp. As compared to dialysis with solutions containing only a low concentration of a high affinity ethylene glycol-bis(beta-aminoethylether) N,N,N',N'-tetraacetic acid (EGTA) like chelator (50-200 microM), inward membrane currents (at -50 mV) due to evoked Ca2+ release, spontaneous Ca2+ release or Ca2+ overload following long-lasting depolarizations to very positive membrane potentials are prolonged if tne dialysing fluid contains a high concentration of a low affinity Ca2+ chelating compound such as citrate or free adenosine 5'-triphosphate (ATP). Without such a non-saturable Ca2+ chelator in the dialysing fluid, Ca(2+)-release-dependent inward currents are often oscillatory and show an irregular amplitude. With a low affinity chelator in a non-saturable concentration, discrete inward currents with constant properties can be recorded. We conclude that the variability in Ca(2+)-release-dependent inward current seen in single cells arises from spatial inhomogeneities of intracellular Ca2+ concentration ([Ca2+]i) due to localized saturation of endogenous and exogenous high affinity Ca2+ buffers (e.g.). This can be avoided experimentally by addition of a non-saturable buffer to the intracellular solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Citrate decreases contraction and Ca current in cardiac muscle independent of its buffering action

Extracellular Ca (Cao) depletions that occur during cardiac muscle contractions are indicative of net Ca entry. Buffering Cao concentration ([Ca]o) with citrate can limit the magnitude of these Cao depletions [e.g., Shattock and Bers. Am. J. Physiol. 256 (Cell Physiol. 25): C813-C822, 1989] which theoretically would allow more Ca entry and consequently greater force at the same free [Ca]o. However, Shimoni and Ginsburg [Am. J. Physiol. 252 (Cell Physiol. 21): C248-C252, 1987] have shown that citrate inhibits cardiac contractions and suggested that this was due to its Ca-buffering action (i.e., dissipating a local elevation of [Ca] at the outer sarcolemmal surface and thereby decreasing Ca influx). To examine the effects of Ca buffering per se, we compared the effects of four low-affinity Ca buffers [citrate, nitrilotriacetic acid (NTA), dipicolinic acid (DPA), and acetamidoiminodiacetic acid (ADA)] on several cardiac preparations. In Mg-free medium with 2 mM free Ca (measured using murexide), citrate, DPA, and ADA (10 mM) decreased the force of twitch contractions in rabbit ventricle to 76 +/- 2, 60 +/- 2, and 85 +/- 2%, respectively, but 10 mM NTA increased force slightly to 105 +/- 2%. No simple correlation was observed between the Ca affinity of the buffer and its effect on tension. These effects were not due to changes in sarcoplasmic reticulum (SR) Ca loading because rapid cooling contractures were not affected and similar results were observed in the presence of caffeine or ryanodine. The depressant effects of citrate and ADA on tension were greater at pH 5.5-6 and ADA had no effect at pH 8.5. Thus the depressant effect is stronger with more protonated forms of citrate and ADA, which are also poorer Ca buffers. Citrate (but not NTA) decreased Ca current in whole cell voltage clamp and shifted the current-voltage relationship and reversal potential to more negative potentials. Citrate decreased Ca current more effectively at higher citrate and lower Ca concentrations. We conclude that citrate (and some other weak Ca buffers) may directly decrease Ca current and contraction in a manner independent of Ca buffering ability.

Calpastatin and nucleotides stabilize cardiac calcium channel activity in excised patches

The activity of single L-type Ca2+ channels is rapidly lost (run-down) when contact between the membrane and cytosol is interrupted. We have now achieved the stabilization of cardiac Ca2+ channel activity of guinea-pig ventricular myocytes by using either cytosol or defined components added to excised patches. The endogenous protease inhibitor, calpastatin, together with nucleotides, ATP + GTP, was found to prevent run-down as effectively as cardiac cytosolic solution. These results suggest the involvement of proteolysis by calpain in run-down of channel activity and enable the study of cardiac Ca2+ channel regulation with free access to both sides of the membrane.

Measurement of Ca2(+)-release-dependent inward current reveals two distinct components of Ca2+ release from sarcoplasmic reticulum in guinea-pig atrial myocytes

Ca2+ current (L-type) and inward current caused by Ca2+ release from the sarcoplasmic reticulum and carried by electrogenic Na+/Ca2+ exchange have been measured in cultured atrial myocytes from hearts of adult guinea-pigs using whole-cell voltage clamp techniques. The pipette solution, used for internal dialysis of the cells, contained a high concentration, 60 mM or 25 mM, of citrate as a non-saturable low-affinity Ca2(+)-chelating compound. It has been shown previously that Ca2(+)-release-dependent inward current under these conditions is carried by electrogenic Na+/Ca2+ exchange. Furthermore, Ca2(+)-release-dependent inward current (the release signal) can be completely separated from triggering Ca2+ current if brief depolarizations for activating ICa are used. In the majority of cells that did not produce spontaneous Ca2+ release, conditions could be found that caused the release signal to be split into two components: an early component of variable amplitude and a late component of rather constant amplitude. The delay of the late component with regard to triggering ICa was inversely related to the amplitude of the first one. Below a certain amplitude of the first component, the second one failed to be elicited. This suggests the second component to be triggered by the first one. Weakly Ca2(+)-buffered cells produced spontaneous Ca2+ release, resulting in irregular "transient inward currents" at constant membrane-holding potential. Synchronization by trains of step depolarizations unmasked two components also in the spontaneous release signals. In none of the cells studied was any indication of more than two components of the release signal detected. The results are discussed in terms of two distinct compartments of sarcoplasmic reticulum with different properties of Ca2+ release.

Evidence for two types of calcium currents in frog cardiac sinus venosus cells

Two types of calcium currents were recorded in single sinus venosus cells of the frog heart, using the whole-cell patch-clamp technique. The threshold potentials were approximately -65 mV for T-type current and -40 mV for L-type current. The amplitude and time course of T-type current were unaffected by exchanging calcium for barium, while the amplitude of L-type current was increased and its decay slowed. T-type current was neither modified by 10(-6) M nifedipine nor by 10(-7) M isoprenaline in contrast with the effects of these agents on L-type current. T-type current began to inactivate at -90 mV and was fully inactivated at -45 mV. Its steady-state inactivation curve was approximately 35 mV negative to that of L-type current. Overlap of activation and inactivation relationships was present for both T- and L-type currents and was maximal at -57 and -30 mV, respectively. It was concluded that T- and L-type currents can easily be separated by their voltage, kinetic and pharmacological differences. The presence of a high density of T-type current may be correlated to its contribution to the pacemaking function of the sinus venosus cells.

Calcium-sensitive inactivation in the gating of single calcium channels

Voltage-activated calcium channels open and close, or gate, according to molecular transition rates that are regulated by transmembrane voltage and neurotransmitters. Here evidence for the control of gating by calcium was found in electrophysiological records of single, L-type calcium channels in heart cells. Conditional open probability analysis revealed that calcium entry during the opening of a single channel produces alterations in gating transition rates that evolve over the course of hundreds of milliseconds. Such alteration of calcium-channel gating by entry of a favored permeant ion provides a mechanism for the short-term modulation of single-ion channels.

Mechanism of extracellular ATP-induced depolarization in rat isolated ventricular cardiomyocytes

Adenosine triphosphate (ATP) is released during neural stimulation and cardiac hypoxia and several mechanisms of its action have been reported in different tissues. ATP stimulates P1 and P2 purinergic receptors; it also activates receptor-operated channels and increases membrane permeability to small ions. In single rat ventricular cells under whole-cell patch-clamp, a stepwise application of ATP in the micromolar range affects the resting potential and membrane currents through an entirely novel mechanism of action which involves several steps. Extracellular ATP induces an inward current and depolarization of the cell, leading to automaticity. The inward current is non-specific for cations, its reversal potential is around -5 mV. The conductance change evoked by ATP is suppressed by 4,4-diisothiocyanostilbene 2,2-disulphonic acid (DIDS) and low-chloride media and is prolonged by adding intracellular bicarbonate. These effects are specific for ATP in the presence of magnesium and are not evoked by a non-hydrolysable analogue of ATP or in the presence of vanadate. Other nucleotides are ineffective. We propose that ATP hydrolysis activates the chloride/bicarbonate (Cl-/HCO3-) exchanger. The induced local acidification could then increase intracellular free calcium and as a consequence, increases the sarcolemmal conductance. Thus, a sudden release of ATP in pathological conditions would induce a depolarization which could generate ventricular arrhythmias.

Large-conductance ion channel measured by whole-cell voltage clamp in single cardiac cells: modulation by beta-adrenergic stimulation and inhibition by octanol

Membrane currents in single cardiac myocytes from adult guinea pigs were studied by means of the patch-clamp technique (whole-cell mode). During spontaneous or caffeine-induced Ca2+ release from the sarcoplasmic reticulum openings of a novel ion channel with large unitary conductance (280 pS) can be recorded. The density of these channels and/or its open-state probability are unusually low. On average in the whole-cell mode simultaneous maximum superposition of only four channels is observed. Opening events of this channel require an intracellular Ca2+ transient. Activation by [Ca2+]i, however, seems to be indirect; maximum opening activity occurs with a delay of several hundred milliseconds after peak [Ca2+]i. Single-channel activity can be enhanced by a cyclic AMP dependent process via beta-adrenergic stimulation of a cell. This can also be mimicked by caffeine, most likely via inhibition of phosphodiesterase. Octanol, an inhibitor of gap-junctional coupling in a variety of tissues, causes a concentration-dependent and reversible decrease in single-channel activity. Unitary conductance is not affected by octanol. The low density of these channels in cardiac membranes and their poor selectivity render any role in normal cardiac electrical activity unlikely. A possible relation of the channel to cardiac gap junctions is discussed.

Quantification of [Ca2+]i in perfused hearts. Critical evaluation of the 5F-BAPTA and nuclear magnetic resonance method as applied to the study of ischemia and reperfusion

Calcium has been implicated as a mediator of cell injury in ischemia and reperfusion, but direct measurements of Ca2+ are required to refine this idea. We used nuclear magnetic resonance spectroscopy and the Ca2+ indicator 5F-BAPTA to measure [Ca2+]i in perfused ferret hearts. Several lines of evidence are presented to show that loading with the acetoxymethyl ester of 5F-BAPTA is not significantly complicated by accumulation of partially de-esterified metabolites, compartmentalization into mitochondria, or disproportionate uptake into endothelial cells. During 20 minutes of total global ischemia at 30 degrees C, time-averaged [Ca2+]i increased significantly, reaching peak values roughly three times control at 15-20 minutes. Reperfusion resulted in a persistent elevation of [Ca2+]i during the first 5 minutes, but not afterward. Although the nonlinear response of 5F-BAPTA to [Ca2+] leads to underestimation of the true time-averaged [Ca2+]i, the measured alterations of intracellular Ca2+ homeostasis during ischemia are large compared with the likely errors in quantification. Phosphorus nuclear magnetic resonance spectroscopy of 5F-BAPTA-loaded hearts reveals changes during ischemia similar to those recorded previously in hearts not containing a Ca2+ indicator. Developed pressure recovers to only 50% of control values during reflow, indicating that the presence of 5F-BAPTA in the cytosol does not protect against stunning, at least when the extracellular calcium concentration has been raised to 8 mM. We conclude that 5F-BAPTA provides useful measurements that reveal that time-averaged [Ca2+]i rises during ischemia and returns to control levels soon after reperfusion.

Ca-agonists: a new class of inotropic drugs

The basic pharmacology of dihydropyridine Ca-agonists published so far (BAY k8644, CGP 28-392, H 160/51, YC 170, and 202-791) is described. The importance of the potency of the enantiomeres for the effect of a racemic compound is underlined. The Ca agonist prototype BAY k8644 leads to an increase of the maximal rate of rise of left ventricular pressure (LV(dP/dt)) and an increase of left ventricular stroke work in conscious dogs. When the vascular effects of BAY k8644 are counterbalanced by intravenous injection of sodium-nitroprusside, the left ventricular functions curves show markedly increased stroke work against the same mean arterial blood pressure at the same filling pressure. BAY k8644 stimulates the heart economically: the net efficiency in isolated working guinea-pig hearts is about 20%, identical to a stimulation by calcium or ouabain. Cardiotonic drugs acting via cAMP-dependent mechanisms like isoprenaline, amrinone, or pimobendane however, stimulate the heart about 1/3 less economically. The mechanism of action of Ca-agonists is explained from electrophysiological findings: Ca-agonistic dihydropyridines increase the open probability of the Ca-channels by a shift of the open-probability curve to more negative membrane potentials. As a consequence, the steady-state inactivation curve of the Ca-channel is also shifted in the same direction. While the effect on open-probability is the underlying mechanism for Ca-agonism, the latter effect results in Ca-antagonism. Therefore, depending on drug concentration and on membrane resting potential, a single chemical compound can act either as a Ca-agonist or a Ca-antagonist. A kinetic model of dihydropyridine action on the Ca-channel is described.

Beta-adrenergic modulation of transient inward current in guinea-pig cardiac myocytes. Evidence for regulation of Ca2(+)-release from sarcoplasmic reticulum by a cyclic AMP dependent mechanism

Transient inward current (Iti) indicating Ca2(+)-release from the sarcoplasmic reticulum and L-type Ca2(+)-current (ICa) were studied in atrial and ventricular myocytes from hearts of adult guinea-pigs by means of whole-cell voltage-clamp. The increase of ICa caused by beta-adrenergic stimulation using isoprenaline (ISO) or related experimental manoeuvres such as superfusion with forskolin (FORSK) was used as a qualitative monitor of an increase of intracellular cAMP. Changes of Iti were used to manifest changes of sarcoplasmic Ca2(+)-release. In myocytes dialysed with citrate-based (60 mM) pipette filling solution containing 100 microM EGTA spontaneous transient inward currents were recorded at a constant holding potential of -50 mV in the majority of myocytes. Superfusion with a solution containing ISO (greater than or equal to 5 x 10(-8) M) increased the amplitude of spontaneous Iti and reduced its time-to-peak. The effects of ISO on Iti developed in parallel to stimulation of ICa. In myocytes which did not show spontaneous cyclic Ca2(+)-release in the above condition, this could be evoked de novo by ISO. Spontaneous Iti was suppressed in the majority of cells by increasing the concentration of EGTA in the dialysing solution to 200 microM. Brief (50 ms) activation of ICa by voltage steps from -50 to +10 mV usually failed to trigger Ca2(+)-release from the SR. The increase of ICa-amplitude upon administration of ISO went ahead with the induction of Ca2(+)-release by brief activation of ICa. The effects of ISO could be mimicked by FORSK or intracellular dialysis with 3'5'-cyclic adenosine monophosphate. The effects on ICa and SR Ca2(+)-release were dependent o the concentration of the stimulating substance. In a given cell changing superfusion from a low to a high concentration of ISO or FORSK resulted in an increase of the number of Ca2(+)-release events per number of Ca2(+)-currents elicited and a shortening of time-to-peak of Iti's. The stimulating effects of ISO or FORSK on Ca2(+)-release were only partially due to an increase of the triggering ICa. Ca2(+)-currents too small to trigger Ca2(+)-release before beta-adrenergic stimulation could evoke Ca2(+)-release after augmentation of intracellular cAMP. Whereas the effects of ISO and FORSK on ICa were reversible, the stimulatory effects on Ca2(+)-release persisted after washing out the substances. The results give support to the hypothesis that beta-adrenoceptor-mediated positive inotropic and arrhythmogenic effects are, at least partly, due to a cyclic AMP-dependent regulatory mechanism modulating sarcoplasmic Ca2(+)-release.

Effects of magnesium on inactivation of the voltage-gated calcium current in cardiac myocytes

The effects of changes in intracellular and extracellular free ionized [Mg2+] on inactivation of ICa and IBa in isolated ventricular myocytes of the frog were investigated using the whole-cell configuration of the patch-clamp technique. Intracellular [Mg2+] was varied by internal perfusion with solutions having different calculated free [Mg2+]. Increasing [Mg2+]i from 0.3 mM to 3.0 mM caused a 16% reduction in peak ICa amplitude and a 36% reduction in peak IBa amplitude, shifted the current-voltage relationship and the inactivation curve approximately 10 mV to the left, decreased relief from inactivation, and caused a dramatic increase in the rate of inactivation of IBa. The shifts in the current-voltage and inactivation curves were attributed to screening of internal surface charge by Mg2+. The increased rate of inactivation of IBa was due to an increase in both the steady-state level of inactivation as well as an increase in the rate of inactivation, as measured by two-pulse inactivation protocols. Increasing external [Mg2+] decreased IBa amplitude and shifted the current-voltage and inactivation curves to the right, but, in contrast to the effect of internal Mg2+, had little effect on the inactivation kinetics or the steady-state inactivation of IBa at potentials positive to 0 mV. These observations suggest that the Ca channel can be blocked quite rapidly by external Mg2+, whereas the block by [Mg2+]i is time and voltage dependent. We propose that inactivation of Ca channels can occur by both calcium-dependent and purely voltage-dependent mechanisms, and that a component of voltage-dependent inactivation can be modulated by changes in cytoplasmic Mg2+.

Regulation of Ca2+ current in frog ventricular myocytes by the holding potential, c-AMP and frequency

The whole-cell patch-clamp technique was used to study the effects of holding potential and frequency on the Ca2+ current in frog ventricular myocytes. INa was blocked by TTX, and ica was activated with depolarizing clamps from different holding potentials. Variation of the holding potential revealed three new effects on ica: (1) At -40 mV iCa declined with a time constant of 15 min, while at -90 mV, this irreversible decline (run down) in iCa did not occur. (2) The decline of iCa at -40 mV was biphasic: run down was preceeded by a slow inactivation with a time constant of 40 s, which was reversible upon returning the holding potential to -90 mV. (3) Increasing the frequency of the clamp pulses from 0.1 to 1 Hz led to a rapid decline of iCa when the holding potential was positive to -60 mV, but at -90 mV had either no effect or increased iCa by 35%, if c-AMP was included in the dialyzing solution. On the other hand, c-AMP did not alter the time course of the run down and the slow inactivation. Replacement of extracellular Ca2+ by Ba2+ markedly slowed iCa kinetics, but did not change the very slow inactivation or the frequency-induced enhancement of iCa. Injection of c-AMP led to a transient increase of iCa. The phosphodiesterase inhibitor theophylline enhanced the amplitude of the transient and slowed its decay. This effect was mimicked by increased frequency. It is concluded that frequency-induced enhancement of iCa is highly dependent on the holding potential, independent of Ca2+, and may involve elevation of the intracellular level of c-AMP via inhibition of phosphodiesterase activity. The new type of very slow inactivation is probably under direct voltage control and independent of Ca2+ and c-AMP.

Augmentation of cardiac calcium current by flash photolysis of intracellular caged-Ca2+ molecules

The entry of calcium ions into cells through voltage-activated Ca2+ channels in the plasma membrane triggers many important cellular processes. The activity of these channels is regulated by several hormones and neurotransmitters, as well as intracellular messengers such as Ca2+ itself (for examples, see refs 1-9). In cardiac muscle, myoplasmic Ca2+ has been proposed to potentiate Ca2+ influx, although a direct effect of Ca2+ on these channels has not yet been demonstrated. Photosensitive 'caged-Ca2+' molecules such as nitr-5, however, provide powerful tools for investigating possible regulatory roles of Ca2+ on the functioning of Ca2+ channels. Because its affinity for Ca2+ is reduced by irradiation, nitr-5 can be loaded into cells and induced to release Ca2+ with a flash of light. By using this technique we found that the elevation of intracellular Ca2+ concentration directly augmented Ca2+-channel currents in isolated cardiac muscle cells from both frog and guinea pig. The time course of the current potentiation was similar to that seen with beta-adrenergic stimulation. Thus Ca2+ may work through a similar pathway, involving phosphorylation of a regulatory Ca2+-channel protein. This mechanism is probably important for the accumulation of Ca2+ and the amplification of the contractile response in cardiac muscle, and may have a role in other excitable cells.

Properties of calcium channels in guinea-pig gastric myocytes

1. The inward membrane current in enzymatically dispersed guinea-pig gastric myocytes was studied using whole-cell voltage clamp technique. 2. Only one inward membrane current was found in gastric myocytes which was identified as the Ca2+ current based on its inhibition by Ni2+, Cd2+ and Co2+, its dependence on [Ca2+]o, and its insensitivity to variations of [Na+]o. 3. Ca2+ current activated at -20 mV, peaked around +10 mV and was markedly enhanced when the holding potential was increased from -40 to -90 mV. The enhancement of ICa at negative holding potentials did not alter the activation threshold of ICa. When Ba2+ was substituted for Ca2+, IBa was similarly enhanced at more negative potentials. 4. In cells where internal Ca2+ was buffered with 10 mM-EGTA, the time course of inactivation was fitted with two exponentials, with time constants: tau f = 53.4 +/- 18.1 ms and tau s = 175.2 +/- 46.1 ms. When Ba2+ was the charge carrier through the channel, the time course of inactivation could be fitted often by only one exponential which approximated tau s for inactivation of ICa. The voltage dependence of steady-state inactivation of Ca2+ channels was not significantly altered when Ba2+ was the charge carrier. 5. Using different buffering systems (EGTA, EDTA and citrate), we found that citrate maintained the ICa and slowed inactivation more effectively than the other buffers tested. Because the calculated change in [Ca2+]i did not differ significantly between buffer systems, we speculate that suppression of inactivation by citrate is related to increased accessability of the buffer to cytoplasmic Ca2+ near the Ca2+ channel. Changes in [Mg2+]i affected peak ICa but not the kinetics of inactivation indicating that [Mg2+]i may regulate the steady-state inactivation or the availability of the Ca2+ channels. 6. The divalent selectivity of the Ca2+ channel had the following sequence: Ba2+ greater than Ca2+ greater than or equal to Sr2+ much greater than Mg2+. In very low extracellular Ca2+ (less than 10(-7) M), the Ca2+ channel conducted Na+. 7. Increasing [H+]o appeared to differentially affect peak and maintained components of ICa. At pH less than 6.5, the maintained component of ICa was suppressed more than the peak component indicating possible time- and voltage-dependent inhibition of ICa by protons. 8. Nifedipine, D600 and diltiazem inhibited ICa in a voltage-dependent manner. The order of potency for inhibition of peak ICa was nifedipine approximately D600 much greater than diltiazem.(ABSTRACT TRUNCATED AT 400 WORDS)

Ionic currents in single smooth muscle cells from the ureter of the guinea-pig

1. Ionic currents underlying the action potential were recorded from enzymatically isolated smooth muscle cells of guinea-pig ureter. 2. The action potential recorded from a single cell was similar to that from a multicellular preparation. It showed repetitive spikes on a plateau potential which followed the first spike. Treatment with 10 mM-tetraethylammonium (TEA) increased the amplitude and duration of the plateau phase and abolished the repetitive spikes. 3. Under voltage clamp mode, at least two (maybe three) kinds of outward currents were activated during depolarizing pulses. The main outward current was Ca2+-dependent K+ current (IK(Ca], which was mostly blocked in Ca2+-free solution, or by application of 1 mM-cadmium (Cd2+) or 2 mM-tetraethylammonium (TEA). IK(Ca) was greatly decreased by treatment with 5 mM-caffeine or an addition of 10 mM-EGTA in a pipette solution. 4. In the presence of 1 mM-Cd2+ and 2 mM-TEA, a small transient outward current remained. 4-Aminopyridine (1 mM) suppressed the transient outward current by about 40%. Time- and voltage-dependent delayed rectifier outward currents were small in ureter cells. An inwardly rectifying K+ current was not detected. 5. An application of 1 mM-Cd2+, 5 mM-cobalt (Co2+), 1 mM-lanthanum (La3+) or 0.1 microM-nifedipine completely blocked the action potential. Replacement of 80-90% of extracellular Na+ with Li+ or Tris almost abolished the plateau potential and repetitive spikes but did not change significantly the first spike. 6. In the presence of 30 mM-TEA, the inward current elicited by depolarization was monophasic and lasted for more than 1 s. Application of 1 mM-Cd2+, 1 mM-La3+, 0.1 microM-nifedipine, or 5 mM-Co2+ completely blocked inward current. The replacement (87%) of extracellular Na+ ions with Li+, Tris, sucrose or TEA speeded up the decay of inward current; the inward current decreased by 10-60% at the end of a 500 ms pulse. 7. Even in low-Na+ solution (120 mM-TEA), the inactivation of ICa had a quite slow component (tau = 1 s), in addition to another faster component (tau = 100 ms) at 0 mV. When short depolarizing clamp pulses (50 ms) were repetitively applied at short intervals (50 ms) and with interpulse voltage of -10 or -20 mV to mimic the repetitive spikes on the plateau of the action potential, the decline of peak Ca2+ current during the train of pulses was smaller than the decay of Ca2+ current during a long pulse.(ABSTRACT TRUNCATED AT 400 WORDS)

Modification of L-type calcium current by intracellularly applied trypsin in guinea-pig ventricular myocytes

1. The L-type Ca2+ current was recorded in guinea-pig ventricular myocytes by the patch clamp technique in the whole-cell configuration. The modification of the current by intracellular application of proteases was studied. 2. During the first phase of action, trypsin, an endopeptidase, increased the amplitude of Ca2+ current about 3-fold. 3. Thereafter, there was a drastic slowing of the inactivation time course of the enhanced Ca2+ current. The half-time of inactivation increased from a control value of about 25 ms to values larger than 200 ms. 4. Cell dialysis with carboxypeptidase A, an exopeptidase, also enlarged the amplitude of Ca2+ current, but did not affect the kinetics of Ca2+ current. Leuaminopeptidase did not modify the Ca2+ current. 5. The hypothesis that Ca2+ channels are affected by the protease is supported by the fact that alterations of the extracellular Na+ or K+ concentration did not influence the modification of the membrane current. Another argument for the involvement of Ca2+ channels is that the modified membrane current could be blocked by inorganic and organic Ca2+ channel blockers (e.g. 10 microM-Cd2+, 100 microM-La3+ or 1 microM-D600). 6. Although the actions of trypsin and maximal concentrations of isoprenaline on the amplitude of the Ca2+ current were not additive, the slowing of inactivation by trypsin occurred independently from beta-adrenergic stimulation. 7. The effect of trypsin on the Ca2+ current could not be blocked by intracellular 5'-adenylyl-imidodiphosphate (AMP-PNP) or Rp-adenosine 3'5'-monothionophosphate (Rp-cAMPS), both of which are known to suppress the cyclic AMP-dependent phosphorylation of the Ca2+ channel. 8. It was concluded that trypsin may directly modify the membrane protein which forms the Ca2+ channel. Since the increment in peak Ca2+ current resembled the action of cyclic AMP-dependent phosphorylation, it may be related to the removal of a 'chemical' inactivation gate which is normally controlled by phosphorylation. The slowing of the time course of Ca2+ current inactivation by trypsin could be due to a modification of the voltage-dependent inactivation gate. Alternatively, the endopeptidase might remove an internal Ca2+ binding site normally responsible for Ca2+-dependent inactivation.

Properties and calcium-dependent inactivation of calcium currents in cultured mouse pancreatic B-cells

1. Ca2+ currents were recorded using the whole-cell mode of the patch-clamp technique from mouse pancreatic B-cells kept in culture for 1-4 days. B-cells were identified in the cell-attached mode by their response to a change in the glucose concentration from 3 to 15 or 20 mM or by their inward currents. 2. Only one component of Ca2+ current was observed in these cells, which activated at potentials greater than -50 mV and was blocked by nitrendipine (5 microM), and increased in amplitude by CGP 28392 (5 microM). 3. During maintained depolarizations the Ca2+ current inactivated considerably but not completely. Inactivation was most marked at potentials where the Ca2+ currents were large, but in general was slower for currents at potentials greater than 0 mV than at more negative potentials. 4. Two-pulse experiments showed that the inactivation curve for the Ca2+ current was U-shaped, returning to unity at potentials approaching the Ca2+ equilibrium potential. Measurements of Ca2+ entry showed that inactivation was dependent on the amount of Ca2+ entering during the pre-pulse, independent of the pre-pulse potential. 5. Ca2+ currents were not appreciably slowed when BAPTA, a faster buffer of Ca2+, replaced EGTA in the pipette solution. 6. Replacement of Ca2+ in the external solution by Ba2+ increased the amplitude of the inward current and largely abolished inactivation. Large inward currents through Ca2+ channels were observed in the absence of divalent cations in the external solution (+EGTA), which were presumably carried by Na+. These currents did not inactivate during 150 ms depolarizations, but were increased in amplitude by CGP 28392 (5 microM) and blocked by D600 (30 microM). 7. The observations suggest that normal mouse pancreatic B-cells have only one type of Ca2+ channel which is dihydropyridine sensitive and inactivates by a mechanism which is almost purely Ca2+ dependent. Inactivation of the Ca2+ current will probably be important in the control of Ca2+ entry during glucose-induced electrical activity.

Voltage dependence of sodium-calcium exchange current in guinea-pig atrial myocytes determined by means of an inhibitor

1. Spontaneous transient inward currents (Iti) caused by cyclic release of Ca2+ ions from the sarcoplasmic reticulum were studied in cultured atrial myocytes from hearts of adult guinea-pigs. K+ channel currents were blocked by replacing K+ on both sides of the membrane by Cs+; the L-type Ca2+ current was inhibited by D600. 2. The voltage dependence of peak Iti and the background current displayed distinct outward-going rectification. The I-V curves for both currents approach each other at strongly positive membrane potentials but do not intersect. 3. 3'-4'Dichlorobenzamil (DCB) causes a concentration-dependent inhibition of peak Iti and a shift of the holding current (at -60 to -40 mV) in the inward direction. Inhibition of Iti is half-maximal at a concentration of 30 microM. 4. DCB reduces the outward-rectifying component of both peak Iti and the background current. The I-V curves of the control and DCB-inhibited currents intersect at ca. +10 mV (peak Iti) and negative to -75 mV (background current), suggesting the reversal potential of the DCB-inhibited current to be shifted by ca. 85 mV in the positive direction if Cai2+ rises following Ca2+ release. 5. The voltage dependence of the DCB-inhibited currents is highly compatible with the concept of Na+-Ca2+ exchange being the charge-carrying mechanism of the outward-rectifying background current. Ca2+ release from the SR alters the I-V curve of this current according to the shift of the thermodynamic driving force.

Inactivation, reactivation and pacing dependence of calcium current in frog cardiocytes: correlation with current density

1. Ca2+ currents were measured in single cells isolated from frog ventricle using the whole-cell patch clamp technique and a perfused pipette. K+ currents were blocked with intracellular (120 mM) and extracellular (20 mM) Cs+. 2. A single type of Ca2+ current (ICa) was found in these cells. The current activated at voltages positive to -30 mV, exhibited a symmetrical current-voltage relationship with a peak at 0 mV, and was slowly inactivating with Ba2+ as charge carrier. 3. Large variations in ICa amplitude were observed from cell to cell (ICa at 0 mV = 293.1 +/- 283.3 pA; N = 152). These variations were not due simply to differences in cell membrane area, which was estimated by cell membrane capacitance (Cm), because the density of Ca2+ current (dICa = ICa/Cm) also varied significantly from cell to cell (1.3-28 pA/pF at 0 mV; mean +/- S.D. = 4.49 +/- 3.96; N = 152). 4. The inactivation curve of ICa was a complex function of membrane potential. 200 ms pre-pulses to voltages between -60 and +20 mV progressively inactivated ICa elicited by a subsequent test pulse with half-maximal inactivation occurring for pre-pulses to approximately -40 mV. With pre-pulses positive to +20 mV, ICa elicited by the test pulse became progressively larger. The degree of inactivation induced by a 200 ms depolarization to potentials more positive than +20 mV varied significantly from cell to cell, while no such variations were observed in the negative range of membrane potentials. 5. The time course of reactivation (i.e. removal from inactivation) of ICa at -80 mV often exhibited an overshoot. The amplitude of the overshoot varied between 100% (i.e. no overshoot) and approximately 180% in eighty-one cells. 6. The degree of inactivation at positive potentials (+100 mV) and the amplitude of the overshoot were strongly correlated with the Ca2+ current density. The overshoot was more pronounced, the reactivation was faster, and the inactivation at positive potentials was less in cells with lower ICa density. 7. Increasing the stimulation frequency from 0.125 to 2 Hz induced a positive staircase of ICa in cells with ICa density less than 2 pA/pF and a negative staircase in cells with ICa density greater than 3 pA/pF. 8. Perfusing the patch pipette with 5 mM-BAPTA instead of EGTA reduced the amplitude of the overshoot and slightly slowed the inactivation kinetics. Replacing extracellular Ca2+ ions by Ba2+ ions completely suppressed the overshoot.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiac calcium channels in planar lipid bilayers. L-type channels and calcium-permeable channels open at negative membrane potentials

Planar lipid bilayer recordings were used to study Ca channels from bovine cardiac sarcolemmal membranes. Ca channel activity was recorded in the absence of nucleotides or soluble enzymes, over a range of membrane potentials and ionic conditions that cannot be achieved in intact cells. The dihydropyridine-sensitive L-type Ca channel, studied in the presence of Bay K 8644, was identified by a detailed comparison of its properties in artificial membranes and in intact cells. L-type Ca channels in bilayers showed voltage dependence of channel activation and inactivation, open and closed times, and single-channel conductances in Ba2+ and Ca2+ very similar to those found in cell-attached patch recordings. Open channels were blocked by micromolar concentrations of external Cd2+. In this cell-free system, channel activity tended to decrease during the course of an experiment, reminiscent of Ca2+ channel "rundown" in whole-cell and excised-patch recordings. A purely voltage-dependent component of inactivation was observed in the absence of Ca2+ stores or changes in intracellular Ca2+. Millimolar internal Ca2+ reduced unitary Ba2+ influx but did not greatly increase the rate or extent of inactivation or the rate of channel rundown. In symmetrical Ba2+ solutions, unitary conductance saturated as the Ba2+ concentration was increased up to 500 mM. The bilayer recordings also revealed activity of a novel Ca2+-permeable channel, termed "B-type" because it may contribute a steady background current at negative membrane potentials, which is distinct from L-type or T-type Ca channels previously reported. Unlike L-type channels, B-type channels have a small unitary Ba2+ conductance (7 pS), but do not discriminate between Ba2+ and Ca2+, show no obvious sensitivity to Bay K 8644, and do not run down. Unlike either L- or T-type channels, B-type channels did not require a depolarization for activation and displayed mean open times of greater than 100 ms.