Ouabain Enhances Basal and Stimulus-Induced Cytoplasmic Calcium Concentrations in Platelets

Incubation of platelet-rich plasma (PRP) with ouabain, an inhibitor of sodium/potassium ATPase (Na+/K+ ATPase), induced a significant rise in basal platelet intracellular calcium concentration ([Ca2+]i) when measured using fura 2. Ouabain induced an enhanced aggregation response to low doses of collagen in both PRP and washed platelets loaded with aequorin. In aequorin loaded platelets this enhanced aggregation response was associated with an enhanced rise in [Ca2+]j such that the relationship between [Ca2+]i and aggregation was unchanged. As inhibition of plasma membrane Na+/K+ ATPase would lead to a raised intracellular sodium ion concentration ([Na+]i) the results suggest that in the platelet, [Na+]i can modulate [Ca2+]i and hence influence the response of platelets to stimuli such as collagen.

Do t-tubules play a role in arrhythmogenesis in cardiac ventricular myocytes?

The transverse (t-) tubules of mammalian ventricular myocytes are invaginations of the surface membrane. The function of many of the key proteins involved in excitation-contraction coupling is located predominantly at the t-tubules, which thus form a Ca(2+)-handling micro-environment that is central to the normal rapid activation and relaxation of the ventricular myocyte. Although cellular arrhythmogenesis shares many ion flux pathways with normal excitation-contraction coupling, the role of the t-tubules in such arrhythmogenesis has not previously been considered. In this brief review we consider how the location and co-location of proteins at the t-tubules may contribute to the generation of arrhythmogenic delayed and early afterdepolarisations, and how the loss of t-tubules that occurs during heart failure may alter the generation of such arrhythmias, as well as contributing to other types of arrhythmia as a result of changes of electrical heterogeneity within the whole heart.

Spontaneous frequency of rabbit atrioventricular node myocytes depends on SR function

Spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) appears to play an important role in cardiac sinoatrial node pacemaking. However, comparatively little is known about the role of intracellular Ca(2+) in the atrioventricular node (AVN). Intracellular Ca(2+) was therefore monitored in cells isolated from the rabbit AVN, using fluo-3 in conjunction with confocal microscopy. These cells displayed spontaneous Ca(2+) transients and action potentials. Ca(2+) transients were normally preceded by a small, slow increase (ramp) of intracellular Ca(2+) which was sometimes, but not always, accompanied by Ca(2+) sparks. During the Ca(2+) transient, intracellular [Ca(2+)] increased initially at the cell periphery and propagated inhomogeneously to the cell centre. The rate of spontaneous activity was decreased by ryanodine (1muM) and increased by isoprenaline (500nM); these changes were accompanied by a decrease and increase, respectively, in the slope of the preceding Ca(2+) ramp, with no significant change in Ca(2+) spark characteristics. Rapidly reducing bathing [Na(+)] inhibited spontaneous activity. These findings provide the first information on Ca(2+) handling at the sub-cellular level and link cellular Ca(2+) cycling to the genesis of spontaneous activity in the AVN.

Quantification of t-tubule area and protein distribution in rat cardiac ventricular myocytes

The transverse (t-) tubules of cardiac ventricular myocytes are invaginations of the surface membrane that form a complex network within the cell. Many of the key proteins involved in excitation-contraction coupling appear to be located predominantly at the t-tubule membrane. Despite their importance, the fraction of cell membrane within the t-tubules remains unclear: measurement of cell capacitance following detubulation suggests approximately 32%, whereas optical measurements suggest up to approximately 65%. We have, therefore, investigated the factors that may account for this discrepancy. Calculation of the combinations of t-tubule radius, length and density that produce t-tubular membrane fractions of 32% or 56% suggest that the true fraction is at the upper end of this range. Assessment of detubulation using confocal and electron microscopy suggests that incomplete detubulation can account for some, but not all of the difference. High cholesterol, and a consequent decrease in specific capacitance, in the t-tubule membrane, may also cause the t-tubule fraction calculated from the loss of capacitance following detubulation to be underestimated. Correcting for both of these factors results in an estimate that is still lower than that obtained from optical measurements suggesting either that optical methods overestimate the fraction of membrane in the t-tubules, or that other, unknown, factors, reduce the apparent fraction obtained by detubulation. A biophysically realistic computer model of a rat ventricular myocyte, incorporating a t-tubule network, is used to assess the effect of the altered estimates of t-tubular membrane fraction on the calculated distribution of ion flux pathways.

Modelling the cardiac transverse-axial tubular system

The transverse-axial tubular system (TATS) of cardiac ventricular myocytes is a complex network of tubules that arises as invaginations of the surface membrane; it appears to form a specialised region of cell membrane that is particularly important for excitation-contraction coupling. However, much remains unknown about the structure and role of the TATS. In this brief review we use experimental data and computer modelling to address the following key questions: (i) What fraction of the cell membrane is within the TATS? (ii) Is the composition of the TATS membrane the same as the surface membrane? (iii) How good is electrical coupling between the surface and TATS membranes? (iv) What fraction of each current is within the TATS? (v) How important is the complex structure of the TATS network? (vi) What is the effect of current inhomogeneity on lumenal ion concentrations? (vii) Does the TATS contribute to the functional changes observed in heart failure? Although there are many areas in which experimental evidence is lacking, computer models provide a method to assess and predict the possible function of the TATS; such models suggest that although the surface and TATS membranes are electrically well coupled, concentration of ion flux pathways within the TATS, coupled to restricted diffusion, may result in the ionic composition in the TATS lumen being different from that in the bulk extracellular space, and varying with activity and in pathological conditions.

Effect of cytoskeleton disruptors on L-type Ca channel distribution in rat ventricular myocytes

The cytoskeleton plays an important role in many aspects of cardiac cell function, including protein trafficking. However, the role of the cytoskeleton in determining Ca channel location in cardiac myocytes is unknown. In the present study we therefore investigated the effect of the cytoskeletal disruptors cytochalasin D, latrunculin, nocadazole and colchicine on the distribution of Ca channels in rat ventricular myocytes during culture for up to 96 h. During culture in the absence of these agents, cell edges became rounded, t-tubule density decreased, and the normal transverse distribution of the alpha1 (pore-forming) subunit of the L-type Ca channel became more punctate and peri-nuclear; these changes were associated with loss of synchronous Ca release in response to electrical stimulation. Disruption of tubulin using nocadazole or colchicine or sequestration of monomeric actin by latrunculin had no effect on these changes. In contrast, cytochalasin D inhibited these changes: cell shape, t-tubule density, transverse Ca channel staining and synchronous Ca release were maintained during culture. The protein synthesis inhibitor cycloheximide had similar effects to cytochalasin. These data suggest that cytochalasin stabilizes actin in adult ventricular myocytes in culture, thus stabilizing cell structure and function, and that actin is important in trafficking L-type Ca channels from the peri-nuclear region to the t-tubules, where they are normally located and provide the trigger for Ca release.

Quantification of calcium entry at the T-tubules and surface membrane in rat ventricular myocytes

The action potential of cardiac ventricular myocytes is characterized by its long duration, mainly due to Ca flux through L-type Ca channels. Ca entry also serves to trigger the release of Ca from the sarcoplasmic reticulum. The aim of this study was to investigate the role of cell membrane invaginations called transverse (T)-tubules in determining Ca influx and action potential duration in cardiac ventricular myocytes. We used the whole cell patch clamp technique to record electrophysiological activity in intact rat ventricular myocytes (i.e., from the T-tubules and surface sarcolemma) and in detubulated myocytes (i.e., from the surface sarcolemma only). Action potentials were significantly shorter in detubulated cells than in control cells. In contrast, resting membrane potential and action potential amplitude were similar in control and detubulated myocytes. Experiments under voltage clamp using action potential waveforms were used to quantify Ca entry via the Ca current. Ca entry after detubulation was reduced by approximately 60%, a value similar to the decrease in action potential duration. We calculated that Ca influx at the T-tubules is 1.3 times that at the cell surface (4.9 vs. 3.8 micromol/L cytosol, respectively) during a square voltage clamp pulse. In contrast, during a cardiac action potential, Ca entry at the T-tubules is 2.2 times that at the cell surface (3.0 vs. 1.4 micromol/L cytosol, respectively). However, more Ca entry occurs per microm(2) of junctional membrane at the cell surface than in the T-tubules (in nM/microm(2): 1.43 vs. 1.06 during a cardiac action potential). This difference is unlikely to be due to a difference in the number of Ca channels/junction at each site because we estimate that the same number of Ca channels is present at cell surface and T-tubule junctions ( approximately 35). This study provides the first evidence that the T-tubules are a key site for the regulation of action potential duration in ventricular cardiac myocytes. Our data also provide the first direct measurements of T-tubular Ca influx, which are consistent with the idea that cardiac excitation-contraction coupling largely occurs at the T-tubule dyadic clefts.

beta-adrenergic stimulation restores the Ca transient of ventricular myocytes lacking t-tubules

beta-adrenergic stimulation helps to synchronize Ca release in myocytes from failing hearts. Transverse (t-) tubules, which synchronize Ca release in normal cells and contain many of the elements of the beta-adrenergic pathway, may be depleted in such cells. The objective of the present study was to determine whether beta-adrenergic stimulation could reverse the desynchronization of Ca release observed in detubulated ventricular myocytes. The effect of isoprenaline (0.5 microM) on control and detubulated rat ventricular myocytes was investigated. Ca transients were monitored using whole-cell fluorescence and confocal microscopy, and Ca current recorded using the patch-clamp technique. Immunocytochemistry was used to investigate phospholamban (PLB) phosphorylation. Detubulation reduces and slows the Ca transient; these effects were reversed by isoprenaline. This restoration was associated with partial reversal of the desynchronization of Ca release that occurs in detubulated cells. Sarcoplasmic reticulum Ca load increased by the same amount in normal and detubulated cells, but Ca current increased less in detubulated cells (64%) than in control cells (124%) in response to isoprenaline. The pattern and extent of cAMP-dependent protein kinase and CaMKII-induced phosphorylation of PLB in response to isoprenaline was the same in both cell types. Thus, the beta-adrenergic pathway is functional in the absence of t-tubules; such stimulation appears to increase the speed of propagation of Ca via Ca-induced Ca release between adjacent clusters of ryanodine receptors, which may be relevant in pathological conditions, such as heart failure, in which t-tubules are depleted. The data also suggest that the Ca current responds to local signaling pathways, which are better coupled to the channel in the t-tubules than at the surface membrane, whereas PLB responds to whole-cell signaling.

Na/Ca exchange and Na/K-ATPase function are equally concentrated in transverse tubules of rat ventricular myocytes

Formamide-induced detubulation of rat ventricular myocytes was used to investigate the functional distribution of the Na/Ca exchanger (NCX) and Na/K-ATPase between the t-tubules and external sarcolemma. Detubulation resulted in a 32% decrease in cell capacitance, whereas cell volume was unchanged. Thus, the surface-to-volume ratio was used to assess the success of detubulation. NCX current (I(NCX)) and Na/K pump current (I(pump)) were recorded using whole-cell patch clamp, as Cd-sensitive and K-activated currents, respectively. Both inward and outward I(NCX) density was significantly reduced by approximately 40% in detubulated cells. I(NCX) density at 0 mV decreased from 0.19 +/- 0.03 to 0.10 +/- 0.03 pA/pF upon detubulation. I(pump) density was also lower in detubulated myocytes over the range of voltages (-50 to +100 mV) and internal [Na] ([Na](i)) investigated (7-22 mM). At [Na](i) = 10 mM and -20 mV, I(pump) density was reduced by 39% in detubulated myocytes (0.28 +/- 0.02 vs. 0.17 +/- 0.03 pA/pF), but the apparent K(m) for [Na](i) was unchanged (16.9 +/- 0.4 vs. 17.0 +/- 0.3 mM). These results indicate that although thet-tubules represent only approximately 32% of the total sarcolemma, they contribute approximately 60% to the total I(NCX) and I(pump). Thus, the functional density of NCX and Na/K pump in the t-tubules is 3-3.5-fold higher than in the external sarcolemma.

Two pore residues mediate acidosis-induced enhancement of C-type inactivation of the Kv1.4 K(+) channel

Acidosis inhibits current through the Kv1.4 K(+) channel, perhaps as a result of enhancement of C-type inactivation. The mechanism of action of acidosis on C-type inactivation has been studied. A mutant Kv1.4 channel that lacks N-type inactivation (fKv1.4 Delta2-146) was expressed in Xenopus oocytes, and currents were recorded using two-microelectrode voltage clamp. Acidosis increased fKv1.4 Delta2-146 C-type inactivation. Replacement of a pore histidine with cysteine (H508C) abolished the increase. Application of positively charged thiol-specific methanethiosulfonate to fKv1.4 Delta2-146 H508C increased C-type inactivation, mimicking the effect of acidosis. Replacement of a pore lysine with cysteine (K532C) abolished the acidosis-induced increase of C-type inactivation. A model of the Kv1.4 pore, based on the crystal structure of KcsA, shows that H508 and K532 lie close together. It is suggested that the acidosis-induced increase of C-type inactivation involves the charge on H508 and K532.

Na+-Ca2+ exchange activity is localized in the T-tubules of rat ventricular myocytes

Detubulation of rat ventricular myocytes has been used to investigate the role of the t-tubules in Ca2+ cycling during excitation-contraction coupling in rat ventricular myocytes. Ca2+ was monitored using fluo-3 and confocal microscopy. In control myocytes, electrical stimulation caused a spatially uniform increase in intracellular [Ca2+] across the cell width. After detubulation, [Ca2+] rose initially at the cell periphery and then propagated into the center of the cell. Application of caffeine to control myocytes resulted in a rapid and uniform increase of intracellular [Ca2+]; the distribution and amplitude of this increase was the same in detubulated myocytes, although its decline was slower. On application of caffeine to control cells, there was a large, rapid, and transient rise in extracellular [Ca2+] as Ca2+ was extruded from the cell; this rise was significantly smaller in detubulated cells, and the remaining increase was blocked by the sarcolemmal Ca2+ ATPase inhibitor carboxyeosin. The treatment used to produce detubulation had no significant effect on Ca2+ efflux in atrial cells, which lack t-tubules. Detubulation of ventricular myocytes also resulted in loss of Na+-Ca2+ exchange current, although the density of the fast Na+ current was unaltered. It is concluded that Na+-Ca2+ exchange function, and hence Ca2+ efflux by this mechanism, is concentrated in the t-tubules, and that the concentration of Ca2+ flux pathways in the t-tubules is important in producing a uniform increase in intracellular Ca2+ on stimulation.

Compensatory role of CaMKII on ICa and SR function during acidosis in rat ventricular myocytes

It has been suggested that the activity of Ca2+/calmodulin-dependent protein kinase II (CaMKII) increases during acidosis in cardiac muscle. Thus we have investigated the role of CaMKII during acidosis by monitoring intracellular Ca2+ (using fura-2) and ICa (using the perforated patch clamp technique) during acidosis, in the absence and presence of the CaMKII inhibitor KN-93, in rat isolated ventricular myocytes. In the absence of KN-93, acidosis (pH 6.5) increased the amplitude of the fura-2 transient and prolonged its decay, but in the presence of KN-93 acidosis did not alter the amplitude and prolonged the decay more. In the absence of KN-93, acidosis increased the amplitude of the caffeine-induced fura-2 transient but did not alter its amplitude in the presence of KN-93. ICa did not change significantly during acidosis in the absence of KN-93 but decreased during acidosis in the presence of KN-93. These results suggest that activation of CaMKII during acidosis helps to compensate for the direct inhibitory effects of acidosis on sarcoplasmic reticular Ca2+ uptake and ICa.

The effect of acidosis on the ECG of the rat heart

We have investigated the effect of acidosis on the ECG in isolated rat heart to determine whether acidosis has marked effects on the ECG, and have used pharmacological agents to investigate possible mechanisms whereby acidosis alters the ECG. Acidosis produced a marked decrease in heart rate and an increase in P-R interval with little apparent effect on the duration of the QRS complex. The effects of acidosis did not appear to be due to acidosis-induced changes in transmitter release from severed autonomic nerve terminals within the heart. Experimental Physiology (2001) 86.1, 27-31.

The effect of acidosis on systolic Ca2+ and sarcoplasmic reticulum calcium content in isolated rat ventricular myocytes

We have investigated the mechanisms responsible for the changes of systolic Ca2+ that occur in voltage-clamped rat ventricular myocytes during acidosis produced by application of the weak acid butyrate (30 mM). Intracellular pH regulation was inhibited with dimethylamiloride (bicarbonate-free solution). The application of butyrate produced an intracellular acidification of 0.33 pH units. This was accompanied by a decrease in systolic Ca2+ to about 50% of control. However, within 2 min, systolic Ca2+ returned to control levels. The decrease in systolic Ca2+ was accompanied by a decrease in the Na+-Ca2+ exchange current observed on repolarisation so that the calculated Ca2+ efflux on Na+-Ca2+ exchange was less than the entry on the L-type Ca2+ current. The magnitude of the Na+-Ca2+ exchange current recovered along with systolic Ca2+ until it equalled the Ca2+ entry on the L-type Ca2+ current. From the measurement of Ca2+ fluxes, it was calculated that, during acidosis, the cell gains 121.6+/-16.2 micromol l(-1) of Ca2+. This is equal to the measured increase of sarcoplasmic reticulum (SR) calcium content obtained by applying caffeine (20 mM) and integrating the resulting Na+-Ca2+ exchange current. We conclude that the recovery of the amplitude of the systolic Ca2+ transient is due to decreased SR calcium release, resulting in reduced Ca2+ efflux from the cell leading to increased SR calcium content.

Effects of the Ca2+ sensitizer EMD 57033 on intracellular Ca2+ in rat ventricular myocytes: relevance to arrhythmogenesis during positive inotropy

We have investigated the effects of the calcium-sensitizing inotropic agent EMD 57033 on Ca(2+) handling in intact and skinned rat ventricular myocytes. Intracellular Ca(2+) was monitored using fura 2. Myocytes were saponin-skinned, allowing study of sarcoplasmic reticulum (SR) function. In intact myocytes EMD 57033 (1-10 micromol/l) produced a concentration-dependent decrease in the amplitude of the Ca(2+) transient and prolonged its declining phase, but had no effect on the rise time. In skinned myocytes, the amplitude of spontaneous Ca(2+) release from the SR was decreased by EMD 57033 (5 and 10 micromol/l), although this agent had no significant effect on the frequency of spontaneous Ca(2+) release. In the presence of the cross-bridge inhibitor 2,3-butanedione monoxime (5 mmol/l), or in a low bathing Ca(2+) concentration (1 mmol/l), EMD 57033 (10 micromol/l) had smaller effects on both the amplitude and time course of the Ca(2+) transient in intact cells than in the absence of 2,3-butanedione monoxime or in the presence of 2 and 5 mmol/l Ca(2+) respectively. These data suggest that the effects of EMD 57033 on Ca(2+) are due to changes in Ca(2+) binding to troponin C, secondary to cross-bridge formation. Thus, during positive inotropy, EMD 57033 is unlikely to provoke arrhythmias due to effects on SR Ca(2+) handling. In intact cells, its effects on Ca(2+) handling would be expected to protect against arrhythmias.

Inhibition of the K+ channel kv1.4 by acidosis: protonation of an extracellular histidine slows the recovery from N-type inactivation

1. Acidosis alters the transient outward current, ito, in the heart. We have studied the mechanism underlying the effect of acidosis on one of the K+ channels, Kv1.4 (heterologously expressed in Xenopus laevis oocytes), known to underlie ito. 2. At pH 6.5, wild-type Kv1.4 current was inhibited during repetitive pulsing, in part as a result of a slowing of recovery from N-type inactivation. 3. Acidosis still caused slowing of recovery after deletion of just one (either the first or second) of the N-terminal inactivation ball domains. However, deletion of both the N-terminal inactivation ball domains greatly reduced the inhibition. 4. As well as the N-terminus, other parts of the channel are also required for the effect of acidosis, because, whereas the transfer of the N-terminus of Kv1.4 to Kv1.2 conferred N-type inactivation, it did not confer acidosis sensitivity. 5. Replacement of an extracellular histidine with a glutamine residue (H508Q) abolished the slowing of recovery by acidosis. Reduction of C-type inactivation by raising the bathing K+ concentration or by the mutation K532Y also abolished the slowing. 6. It is concluded that binding of protons to H508 enhances C-type inactivation and this causes a slowing of recovery from N-type inactivation and, thus, an inhibition of current during repetitive pulsing.

Effect of acidosis on transient outward potassium current in isolated rat ventricular myocytes

The effect of acidosis on the transient outward K(+) current (I(to)) of rat ventricular myocytes has been investigated using the perforated patch-clamp technique. When the holding potential was -80 mV, depolarizing pulses to potentials positive to -20 mV activated I(to) in subepicardial cells but activated little I(to) in subendocardial cells. Exposure to an acid solution (pH 6.5) had no significant effect on I(to) activated from this holding potential in either subepicardial or subendocardial cells. When the holding potential was -40 mV, acidosis significantly increased I(to) at potentials positive to -20 mV in subepicardial cells but had little effect on I(to) in subendocardial cells. The increase in I(to) in subepicardial cells was inhibited by 10 mM 4-aminopyridine. In subepicardial cells, acidosis caused a +8.57-mV shift in the steady-state inactivation curve. It is concluded that in subepicardial rat ventricular myocytes acidosis increases the amplitude of I(to) as a consequence of a depolarizing shift in the voltage dependence of inactivation.

Excitation-contraction coupling in rat ventricular myocytes after formamide-induced detubulation

Formamide-induced osmotic shock has been used to detubulate isolated adult rat ventricular myocytes (i.e., disrupt the surface membrane-T tubule junction). Cell volume, calculated from cell length and width, rapidly decreased and increased upon application and removal of formamide, respectively. After treatment with formamide, membrane capacitance decreased by 26.4% (from 199.4 +/- 18.7 pF in control cells to 146.7 +/- 6.4 pF in formamide-treated cells; n = 13, P < 0.05). However, the amplitude of the L-type Ca(2+) current (I(Ca)) decreased by a greater extent (from 0.75 +/- 0.14 to 0.18 +/- 0.03 nA; n = 5, P < 0.05) so that the density of I(Ca) decreased by 74.5%. Simultaneous measurements of I(Ca) and Ca(2+) transients (monitored using fura 2) showed that both decreased rapidly upon removal of formamide. However, the Ca(2+) content of the sarcoplasmic reticulum showed little change. Cross-striations, visualized with the fluorescent dye di-8-aminonaphthylethenylpyridinium, were sparse or absent in cells that had been treated with formamide, suggesting that formamide can successfully detubulate cardiac cells and that I(Ca) is concentrated in the T tubules, which therefore play an important role in excitation-contraction coupling.

Effects of the protein kinase A inhibitor H-89 on Ca2+ regulation in isolated ferret ventricular myocytes

We investigated the effects of a protein kinase A (PKA) inhibitor, H-89 {N-[2-(p-bromocinnamylamino)ethyl]-5-iso-quinolinesulphonamide}, on Ca2+ regulation in Fura-2-loaded ferret myocytes. H-89 (10 micromol/l) decreased the amplitude of the Fura-2 transient to 28. 2+/-4.3% (P<0.001) of control and prolonged its duration, characterized by a decrease in the rate of decline of Ca2+ to diastolic levels: t1/2 increased from 311+/-35 ms to 547+/-43 ms (P<0.001, n=7). Reduced Ca2+ uptake by the sarcoplasmic reticulum (SR) in the presence of H-89 was also indicated by a decrease in the SR Ca2+ content, as assessed with caffeine. The apparent slowing of the SR Ca2+-ATPase was not caused by changes in phosphorylation of phospholamban (PLB). However, Ca2+ uptake in microsomal vesicles prepared from canine hearts and fast-twitch rat skeletal muscle (which lacks PLB) was decreased by 34.1 and 46.8% (n=3), respectively, suggesting that H-89 has a direct inhibitory effect on the SR Ca2+-ATPase. In electrophysiological experiments, 5.0 micromol/l H-89 decreased the L-type Ca2+ current (ICa) by 39.5% (n=6) and slowed the upstroke of the action potential and, in some cases, caused loss of excitability without changes in the resting membrane potential. In summary, data show that [Ca2+ ]i regulation, and hence contraction, is sustained by PKA-mediated phosphorylation, even in the absence of beta-agonists. However, the use of H-89 as a tool to study the role of this signalling pathway is limited by the non-specific effects of H-89 on the SR Ca2+-ATPase.

Effect of acidosis on Ca2+ uptake and release by sarcoplasmic reticulum of intact rat ventricular myocytes

The effect of acidosis on Ca2+ uptake and release by the sarcoplasmic reticulum (SR) of rat ventricular myocytes has been investigated. Intracellular Ca2+ concentration ([Ca2+]i) was monitored using fura 2; the L-type Ca2+ current (ICa) was monitored using the perforated patch-clamp technique. Acidosis was produced either by superfusing the cells with an acid solution (intracellular and extracellular acidosis) or by NH4Cl withdrawal (intracellular acidosis). Both types of acidosis increased the amplitude, and slowed the declining phase, of the Ca2+ transient. Application of caffeine produced a rise of [Ca2+]i, which declined in the continued presence of caffeine; the declining phase was slowed by the acid solution but was unaffected by NH4Cl withdrawal. Acidosis decreased the fraction of the caffeine-induced release that was released by electrical stimulation but had no effect on ICa. It is concluded that acidosis inhibits SR Ca2+ uptake and Ca2+-induced Ca2+ release in intact myocytes but that these effects are compensated by an increase in SR Ca2+ content secondary to a rise in cytoplasmic [Ca2+].

Cs+ inhibits spontaneous Ca2+ release from sarcoplasmic reticulum of skinned cardiac myocytes

The effect of Cs+ on the function of the cardiac sarcoplasmic reticulum (SR) has been investigated in skinned cardiac myocytes. Isolated rat ventricular myocytes were permeabilized using saponin and then perfused with a solution containing 150 nmol/l Ca2+ and 10 micromol/l fura 2. Fura 2 fluorescence from the skinned cell was monitored to assess SR Ca2+ release. The frequency of spontaneous Ca2+ release from the SR decreased when K+ in the bathing solution was completely replaced with Cs+. Cs+ had little effect on the amplitude of spontaneous release but prolonged both the rise time and decay time. The SR Ca2+ content, assessed by application of caffeine, was reduced in the Cs+ solution. Cyclopiazonic acid produced effects similar to those of Cs+. Extracellular Cs+ (20 mmol/l) increased the amplitude of the Ca2+ transient and the SR Ca2+ content in intact field-stimulated cells but had little effect on the Ca2+ transient when the amplitude and duration of depolarization were kept constant using voltage clamp. These data suggest that Cs+ slows Ca2+ movement across the SR membrane, possibly by blocking the SR K+ channel, but has additional effects in intact cells that overcome its inhibitory effects on the SR.

A method for reversible permeabilization of isolated rat ventricular myocytes

A method is described that enables the cell membrane of isolated rat ventricular myocytes to be permeabilized and resealed while maintaining cell viability. Streptolysin O, a cholesterol-binding cytolysin, was used to form pores in the surface membrane; subsequent incubation with 5% fetal bovine serum was used to reverse this permeabilization. The efficacy of membrane permeabilization and resealing was ascertained using a simultaneous double-staining technique using propidium iodide, a marker for cells with permeabilized membranes, and fluorescein diacetate, a marker for viable cells. This procedure allowed a distinction to be made between dead cells, unpermeabilized cells and viable cells that had been successfully permeabilized and resealed. The accessibility of the cell interior during permeabilization was investigated by including fluorescein isothiocyanate (FITC)-labelled dextrans (11, 38 and 148 kDa) and bovine serum albumin (67 kDa) in the permeabilization buffer, and localizing the FITC label using confocal microscopy following resealing. The confocal images showed that these molecules entered the cells and were retained after resealing. Following the permeabilization-resealing protocol, cells appeared to have both normal morphology and response to electrical stimulation. Thus this appears to be a cheap, simple and effective method to introduce relatively large molecules into cardiac myocytes.

Effects of cytosolic Ca2+ on the Ca2+ content of the sarcoplasmic reticulum in saponin-permeabilized rat ventricular trabeculae

Rat ventricular trabeculae were mounted for isometric tension recording, and then permeabilized with saponin. The Ca2+ concentration ([Ca2+]) within the permeabilized preparation (cytosolic [Ca2+]) was monitored continuously using Indo-1 and the integrals of Ca2+ transients resulting from brief caffeine application used as an index of the sarcoplasmic reticulum (SR) Ca2+ content. The relationship between SR Ca2+ content and cytosolic [Ca2+] was studied within the reported physiological range (i.e. 50-250 nmol . l-1 Ca2+). Increasing cytosolic [Ca2+] from 50 nmol . l-1 to 250 nmol . l-1 increased the steady-state SR Ca2+ content about threefold. However, increasing [Ca2+] above 250 nmol . l-1 typically resulted in spontaneous SR Ca2+ release, with no further increase in SR Ca2+ content. The SR Ca2+ content increased only slowly when cytosolic [Ca2+] was increased; it was unchanged 20 s after a rapid increase in cytosolic [Ca2+], but increased progressively to a new steady-state level during the following 1-2 min. In a parallel series of experiments using intact papillary muscles, increasing extracellular [Ca2+] (from 0.5 to 5 mmol . l-1) significantly increased twitch tension within 20 s of the solution change. These results support previous suggestions that the SR Ca2+ content may increase when diastolic cytosolic [Ca2+] rises during inotropic interventions such as increased stimulus rate or extracellular [Ca2+]. However, the rate at which SR Ca2+ responds to changes in cytoplasmic [Ca2+] within the diastolic range does not appear rapid enough to explain the early potentiation of twitch tension in intact preparations after an increase in extracellular [Ca2+].

The effect of the venom of a Chilean tarantula, Phrixotrichus spatulatus, on isolated guinea pig ventricular myocytes

We have investigated the effect of the venom of a Chilean tarantula, Phrixotrichus spatulatus, on cell contraction, intracellular [Ca2+] ([Ca2+]i), and the L-type Ca2+ current (ICa,L) in cells isolated from the ventricles of guinea pig hearts. Whole-cell voltage clamp techniques were used to monitor ICa,L. The action potential was recorded using whole cell current clamp. [Ca2+]i was monitored using the fluorescent indicator indo-1. The venom of P. spatulatus decreased ICa,L in a dilution-dependent manner, with half-maximal inhibition at a dilution of 1.1/10(4) (v/v). At a dilution of 1/10(4), this inhibition occurred at all potentials so that the current-voltage relationship of ICa,L was depressed. However, inhibition of ICa,L by the venom was relieved by positive potentials, the venom being less effective following pulses to positive potentials. The venom reduced the duration of the action potential (at 50% repolarization) by between 26 and 43%. The venom also decreased the amplitude of the [Ca2+]i transient and cell contraction. It is concluded that the venom of P. spatulatus is a potent, voltage-dependent inhibitor of ICa,L; this inhibition of ICa,L may account for the effects of the venom on action potential duration, [Ca2+]i, and contraction.

Sarcoplasmic reticulum Ca2+ content, L-type Ca2+ current and the Ca2+ transient in rat myocytes during beta-adrenergic stimulation

1. The effect of beta-adrenergic stimulation on the relationship between the intracellular Ca2+ transient and the amplitude of the L-type Ca2+ current (ICa) has been investigated in ventricular myocytes isolated from rat hearts. Intracellular [Ca2+] was monitored using fura-2 during field stimulation and while membrane potential was controlled using voltage clamp techniques. 2. The increase in the amplitude, and the rate of decline, of the Ca2+ transient produced by isoprenaline (1.0 mumol l-1) was not significantly different in myocytes generating action potentials and in those voltage clamped with pulses of constant duration and amplitude. 3. Under control conditions, the current-voltage (I-V) relationship for ICa was bell shaped. The amplitude of the Ca2+ transient also showed a bell-shaped voltage dependence. In the presence of isoprenaline, the amplitude of both ICa and the Ca2+ transient was greater at all test potentials and the I-V relationship maintained its bell-shaped voltage dependence. However, the size of the Ca2+ transient was no longer graded with changes in the amplitude of ICa: a small ICa could now elicit a maximal Ca2+ transient. 4. Rapid application of caffeine (10 mmol l-1) was used to elicit Ca2+ release from the sarcoplasmic reticulum (SR). Isoprenaline increased the integral of the subsequent rise in cytoplasmic [Ca2+] to 175 +/- 13% of control. 5. Abbreviation of conditioning pulse duration in the presence of isoprenaline was used to reduce the amplitude of the Ca2+ transient to control levels. Under these conditions, the amplitude of the Ca2+ transient was again graded with the amplitude of ICa in the same way as under control conditions. 6. Nifedipine (2 mumol l-1) was also used to decrease Ca2+ transient amplitude in the presence of isoprenaline. In the presence of isoprenaline and nifedipine, the amplitude of the Ca2+ transient again showed a bell-shaped voltage dependence. 7. The SR Ca(2+)-ATPase inhibitor thapsigargin (2.5 mumol l-1) reduced the effect of isoprenaline on the amplitude of the Ca2+ transient. In the presence of thapsigargin, the size of the Ca2+ transient increased as ICa increased in response to isoprenaline. 8. These data suggest that the increase in the amplitude of the Ca2+ transient produced by beta-adrenergic stimulation in cardiac muscle is due to an increase in the gain of the SR Ca2+ release process, due principally to an increase in the Ca2+ content of the SR.

Rate-dependent abbreviation of Ca2+ transient in rat heart is independent of phospholamban phosphorylation

The mechanisms underlying the accelerated decline of the intracellular Ca2+ transient that occurs in cardiac muscle when stimulation rate is increased have been investigated in ventricular myocytes from rat hearts. Increasing stimulation rate from 0.1 to 0.5 and 1 Hz decreased the time taken for the Ca2+ transient to decline from its peak to 50% of its peak value in cells generating action potentials, when the duration of depolarization was held constant by voltage clamp, and when Na/Ca exchange was inhibited. The sarcoplasmic reticulum Ca2+ adenosinetriphosphatase inhibitor thapsigargin inhibited rate-dependent abbreviation of the Ca2+ transient. However, neither a chemical inhibitor of Ca(2+)-calmodulin-dependent protein kinase II (KN62) nor a peptide inhibitor of this enzyme (calmodulin-binding domain peptide) had a significant effect on rate-dependent abbreviation of the Ca2+ transient. Analysis of the phosphorylation of the regulatory sites Ser16 and Thr17 of phospholamban showed no significant change in phosphorylation with changes of stimulation rate. These data suggest that rate-dependent shortening of the Ca2+ transient is due predominantly to enhanced Ca2+ uptake by the sarcoplasmic reticulum without changes in phospholamban phosphorylation.

Acidosis alters the phosphorylation of Ser16 and Thr17 of phospholamban in rat cardiac muscle

The effect of acidosis on the phosphorylation of Ser16 and Thr17 of phospholamban in rat cardiac muscle has been investigated using phosphorylation-site-specific antibodies to this protein. Ventricular myocytes were stimulated at 0.5 Hz for 5 min, in either control (pH 7.4) or acid (pH 6.5) physiological salt solution, in the absence or presence of isoprenaline. Site-specific phosphorylation of phospholamban was determined by Western blotting. Acidosis reduced phosphorylation of Ser16 in the absence of isoprenaline, but did not alter the isoprenaline-induced phosphorylation of Ser16. In contrast, acidosis increased Thr17 phosphorylation in the absence and presence of isoprenaline. Buffering intracellular Ca2+ ([Ca2+]i) with BAPTA inhibited the increase in Thr17 phosphorylation during acidosis but had no effect on Ser16 phosphorylation. We conclude that acidosis can alter the phosphorylation of Ser16 and Thr17 by inhibition of protein kinase A, and by an acidosis-induced increase in [Ca2+]i and the subsequent activation of a Ca2+/calmodulin-dependent protein kinase, respectively. The possible effect of these changes in phosphorylation on the activity of the Ca2+-ATPase of the cardiac sarcoplasmic reticulum during acidosis is discussed.

Changes in [Ca2+]i, [Na+]i and Ca2+ current in isolated rat ventricular myocytes following an increase in cell length

Isolated rat ventricular myocytes were stretched using carbon fibres to investigate the mechanisms underlying the increase in contraction following stretch. 2. [Ca2+]i and [Na+]i were monitored using the fluorescent indicators fura-2 and sodium-binding benzofuran isophthalate, respectively. The L-type Ca2+ current was recorded simultaneously with contraction using the perforated patch-clamp technique. 3. Mechanical stretch caused an immediate increase in contraction, followed by a slow increase. Contraction was prolonged immediately after the stretch, but did not change during the slow phase. 4. The Ca2+ transient did not change immediately after the stretch. The slow increase in contraction was accompanied by an increase in the amplitude of the Ca2+ transient. However, diastolic [Ca2+]i did not change significantly following stretch. 5. [Na+]i did not change significantly either immediately, or during the slow increase in contraction, after the stretch. 6. The L-type Ca2+ current was not significantly altered either by mechanical loading of the cell with carbon fibres or by stretching the cell. 7. These results suggest that: (1) the rapid increase in contraction following a stretch is due to an increase in myofilament Ca2+ sensitivity rather than to changes in the L-type Ca2+ current or [Na+]i; and (2) a slow increase in the Ca2+ transient underlies the slow increase in contraction in isolated myocytes, but is not caused by either an increase in diastolic [Ca2+]i or a change in [Na+]i (and hence Ca2+ influx via Na(+)-Ca2+ exchange) or a change in myofilament Ca2+ sensitivity.

Effects of acidosis on phosphorylation of phospholamban and troponin I in rat cardiac muscle

Acidosis inhibits Ca2+ transport by the sarcoplasmic reticulum of cardiac muscle and decreases the Ca2+ sensitivity of the contractile proteins, although the mechanisms underlying these changes are unclear. We have investigated the hypothesis that changes in the phosphorylation of the regulatory proteins phospholamban and troponin I might play a role in the acidosis-induced changes in the function of the sarcoplasmic reticulum and the myofilaments, respectively. Langendorff-perfused rat hearts were labeled with 32P and then perfused with either control (pH 7.4) or acid (pH 6.8) physiological salt solution, in both the absence and presence of isoproterenol. The incorporation of 32P into phospholamban and troponin I was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of sarcoplasmic reticulum and myofibrillar proteins, followed by autoradiography and liquid scintillation counting. The data show that acidosis has no effect on the phosphorylation of phospholamban in the absence of isoproterenol but that, in the presence of isoproterenol, acidosis increased the phosphorylation of phospholamban. However, acidosis increased the phosphorylation of troponin I, in both the absence and the presence of isoproterenol. Acidosis did not alter the adenosine 3',5'-cyclic monophosphate content of the hearts but did inhibit type 1 phosphatase. These data show that acidosis can alter the phosphorylation of these two proteins and suggest that these changes underlie, in part the changes observed in cardiac muscle during acidosis.

Effect of stretch on contraction and the Ca2+ transient in ferret ventricular muscles during hypoxia and acidosis

The effect of stretch on cardiac muscle contraction and the Ca2+ transient was studied during hypoxia and acidosis in isolated ferret ventricular muscles. In control conditions, a maintained stretch produced an immediate increase in tension followed by a slow increase in tension and the Ca2+ transient. A stretch between contractions (diastolic stretch) caused only a slow increase in tension and the Ca2+ transient, whereas a stretch during the period of contraction (systolic stretch) produced an immediate increase in tension followed by a small slow increase in tension and the Ca2+ transient. In hypoxia, the immediate percent increase in tension was the same as in control. However, the slow increase was smaller during all three types of stretch. In acidosis, the immediate percent increase in tension was larger than in control. The slow change was the same during maintained stretch. However, the slow increase in tension was smaller during diastolic stretch and larger during systolic stretch. Thus the stretch-dependent increase in contraction is inhibited during hypoxia and modulated by acidosis.

The effect of mechanical loading on the response of rat ventricular myocytes to acidosis

The effect of mechanical loading on the negative inotropic effect of acidosis in isolated rat ventricular myocytes was investigated. The mechanical loading of the myocytes was changed by attaching carbon fibres to the ends of the cell. To monitor intracellular Ca2+ and Na+ concentrations, cells were loaded with fluorescent dyes (fura-2 for Ca2+ and SBFI for Na+) using the acetoxymethyl (AM) esters. Mechanical loading reduced cell shortening by 73.0 +/- 3.5% (mean +/- S.E.M., n = 16) and abbreviated the time course of contraction. CO2-induced acidosis caused a rapid decrease in contraction followed by a slow partial recovery. The percentage changes in contraction were not significantly different in mechanically loaded and unloaded conditions. Mechanical loading had little effect on the time course of contraction during acidosis. Changes in intracellular Ca2+ and Na+ concentrations during acidosis were unaffected by mechanical loading. The mechanical loading conditions of a region of the heart can be modified by ischaemia and the subsequent acidosis. Our results suggest that the response of cardiac muscle to acidosis is not markedly modified by such changes in mechanical loading.

The mechanism underlying the cardiotoxic effect of the toxin from the jellyfish Chironex fleckeri

We have investigated the mechanisms underlying the cardiac effects of the toxin from the box jellyfish Chironex fleckeri. Papillary muscles isolated from the hearts of ferrets and ventricular myocytes isolated from the hearts of ferrets and rats were used. Force, intracellular [Ca2+], and membrane potential were monitored in the papillary muscles; contraction, intracellular [Ca2+], intracellular [Na+], and membrane currents were monitored in the isolated myocytes. Application of the toxin to these preparations resulted in a large increase in intracellular [Ca2+] and the adverse symptoms of Ca2+ overload (aftercontractions, spontaneous contractions, a decrease in developed force, and an increase in resting force). The response of papillary muscles to the toxin was not inhibited by blockers of Ca2+ or Na+ channels or by inhibitors of the sarcoplasmic reticulum, Na+/K+ ATPase, or Na+/H+ exchange. The response to the toxin was, however, blocked by prior exposure to a solution which contained no Na+ and by Ni2+. In the isolated myocytes, as well as an increase in intracellular [Ca2+], the toxin also caused an increase in intracellular [Na+] and the appearance of a current which was inward at negative potentials and reversed at about -10 mV. These data can be explained by the toxin increasing Na+ influx into the cell. The increase in intracellular [Na+] will then increase intracellular [Ca2+] via the Na+/Ca2+ exchange mechanism, thus producing the observed Ca2+ overload.

The effects of mechanical loading and changes of length on single guinea-pig ventricular myocytes

1. The effects of mechanical loading and changes of length on the contraction of single guinea-pig ventricular myocytes has been investigated. 2. Cell shortening was monitored during isotonic contractions (in which the cell shortened freely) and after attaching carbon fibres of known compliance to the ends of the cell, so that the cell contracted auxotonically (the cell both shortened and developed force). 3. Mechanically loading the cells decreased the amount of shortening during a contraction and abbreviated the contraction. There were, however, no consistent changes in the action potential or the [Ca2+]i transient (measured with the fluorescent dye fura-2). 4. Increasing stimulation rate increased the size of the contraction and the [Ca2+]i transient in both isotonic and auxotonic conditions. The increase in the size of the contraction induced by an increase in stimulation rate was greater in auxotonic conditions but the increase in the size of the [Ca2+]i transient was not. 5. When cells were stretched, there was a step increase in the size of the contraction and a prolongation of its time course. However, neither the size nor the time course of the accompanying [Ca2+]i transient was significantly altered by this intervention. 6. When a stretch was maintained, a further, slow increase in the size of the contraction occurred during the following 3-11 min, in about half the cells studied. The probability of this slow response occurring was increased if the initial degree of activation of the cell was decreased. 7. These data suggest that the mechanisms underlying the responses to mechanical loading and changes of length are the same in both multicellular and single cell preparations of cardiac muscle.

The role of the sarcoplasmic reticulum in the response of isolated ferret cardiac muscle to beta-adrenergic stimulation

beta-Adrenergic stimulation of cardiac muscle leads to an increase in the strength of contraction and an abbreviation of its time course. We have investigated the role of the sarcoplasmic reticulum in these changes by monitoring force and cytoplasmic [Ca2+] in ferret papillary muscles, and the Ca2+ current in isolated ferret myocytes, during the application of isoprenaline in the absence and presence of the sarcoplasmic reticulum inhibitor ryanodine (10(-6) mol/l). Isoprenaline (10(-6) mol/l) led to a marked increase in the size of both the twitch and Ca2+ transient, and a decrease in their duration. In the presence of ryanodine, application of isoprenaline had no significant effect on either the size or the time course of the twitch. However, the increase in the Ca2+ current in response to isoprenaline was the same in the absence and presence of ryanodine. Increasing bathing [Ca2+] led to a prolongation of both the twitch and the Ca2+ transient. In the presence of ryanodine, increasing bathing [Ca2+] still increased the size, but decreased the duration, of the twitch. These data provide direct evidence that both the inotropic and lusitropic effects of isoprenaline are mediated via the sarcoplasmic reticulum.

Acidosis and arrhythmias in cardiac muscle

Acidosis is a well recognised consequence of myocardial ischaemia. In this brief article we have reviewed the consequences of acidosis that might be arrhythmogenic. These include early afterdepolarisations and triggered activity, delayed afterdepolarisations, pulsus alternans, and reentry. In each case we have described the evidence that acidosis can provoke such behaviour and then discussed the possible mechanisms and consequences of each behaviour. It seems likely that changes of pH may contribute to arrhythmogenesis during ischaemia by these mechanisms.

Effects of acidosis and hypoxia on the response of isolated ferret cardiac muscle to inotropic agents

OBJECTIVE

The aim was to study the effects of acidosis and hypoxia on the response of cardiac muscle to inotropic agents which (a) act predominantly by increasing intracellular [Ca2+] (raising extracellular [Ca2+], noradrenaline, isoprenaline) and (b) act partly (phenylephrine) or predominantly (EMD 57033) by increasing myofilament calcium sensitivity.

METHODS

The experiments were performed on isometrically contracting, isolated ferret papillary muscles (n = 45). For each intervention dose-response curves were performed in control solution (pH 7.35), in hypercapnic acidosis (pH 6.85), and in hypoxia (produced by replacing O2 with N2 in the superfusing solution). In some experiments, the photoprotein aequorin was microinjected into superficial cells of the preparation in order to measure intracellular [Ca2+] as well as force.

RESULTS

The results were broadly similar for both classes of inotropic agent. Acidosis caused a shift of the pCa-tension curve to the right (desensitisation of the myofilaments to calcium), but had no significant effect on maximum force. A sufficient inotropic stimulus supplied by either class of inotropic agent could completely reverse the negative inotropic effects of acidosis. The main difference between the two inotropic mechanisms was that the enhanced force produced by calcium sensitisers was associated with a reduction in calcium transient amplitude, while the other inotropes increased the amplitude. The main effect of hypoxia was to decrease maximum force. All the inotropes tested were relatively ineffective in reversing the force depression due to hypoxia.

CONCLUSIONS

The negative inotropic effects of acidosis can be reversed by a sufficiently large inotropic stimulus. Since calcium transient amplitude is already increased in acidosis, the results suggest that calcium sensitisers are likely to be less arrhythmogenic in this situation. The relative ineffectiveness of the inotropes in hypoxia indicates that the main mechanisms causing reduced force in this situation lie downstream of the mechanisms of action of the inotropic agents tested.

The role of Na(+)-Ca2+ exchange in paired pulse potentiation of ferret ventricular muscle

1. Stimulation of cardiac muscle with pairs of stimuli ('paired pulse stimulation') results in a large inotropic effect and experiments have been carried out on ferret ventricular muscle to investigate the underlying mechanism. 2. Aequorin was used to measure sarcoplasmic Ca2+ in papillary muscles. During paired pulse stimulation the first aequorin light transient (i.e. Ca2+ transient) and contraction of the pair increased in amplitude, whereas the second aequorin light transient and contraction were small. When the interval between the pair was decreased, the second aequorin light transient and contraction of the pair were smaller, but the increase in the first aequorin light transient and contraction was greater. 3. The relationship between contraction and the aequorin light transient was the same during paired pulse stimulation and on raising the bathing Ca2+ concentration. It is concluded that there was no change in the myofilament sensitivity to Ca2+ during paired pulse stimulation. 4. The increase in the aequorin light transient and contraction during paired pulse stimulation was prevented by ryanodine, an inhibitor of the sarcoplasmic reticulum (SR). 5. During paired pulse stimulation of ventricular myocytes there was little change in the first action potential of the pair, but the second action potential was shorter than control when the interval between the pair was short. During paired pulse stimulation of ventricular myocytes under voltage clamp control there was little change in the first Ca2+ current (iCa) of the pair, but the second iCa was smaller than control when the interval between the pair was short. Because paired pulse potentiation was greatest when the interval between the pair was short, it is concluded that paired pulse potentiation was not the result of a prolongation of the action potential or increase in iCa. 6. During paired pulse stimulation of ventricular myocytes under voltage clamp control the increase in contraction was greater, the more positive the membrane potential during the second pulse of the pair. This voltage dependence is consistent with a role for the Na(+)-Ca2+ exchanger in paired pulse potentiation. 7. During paired pulse stimulation of ventricular myocytes under voltage clamp control, changes in putative Na(+)-Ca2+ exchange current were observed consistent with a decrease of Ca2+ efflux (or increase of Ca2+ influx) via the exchanger during the second pulse of the pair. 8. A computer model of excitation-contraction coupling (Harrison, McCall & Boyett, 1992) has been used to simulate paired pulse stimulation and the results described above.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential effects of the optical isomers of EMD 53998 on contraction and cytoplasmic Ca2+ in isolated ferret cardiac muscle

EMD 53998 (a thiadiazinone) is a novel inotropic substance that increases the Ca2+ sensitivity of the myofilaments in skinned cardiac fibers and has been found to have similar effects in intact cardiac muscle. However, the compound also possesses the ability to inhibit phosphodiesterase III, indicating that its actions in intact cardiac muscle are likely to be complex. The present study was carried out to investigate the possibility that the optical isomers of EMD 53998--(+)EMD 57033 and (-)EMD 57439--which have recently been shown to possess a separation of sensitization and phosphodiesterase inhibition in subcellular preparations, might also demonstrate this separation of activities in intact cardiac muscle. The experiments were performed on isolated ferret papillary muscles, which were contracting isometrically. In some preparations, the photoprotein aequorin was injected into superficial cells to measure intracellular Ca2+ as well as force. (+)EMD 57033 caused a substantial positive inotropic effect that was associated with prolongation of the twitch, reduction in the amplitude of the Ca2+ transient, and abbreviation of the Ca2+ transient. This is the profile expected of a Ca(2+)-sensitizing compound. Conversely, (-)EMD 57439 caused a less marked positive inotropic effect that was associated with an abbreviation of the twitch, an increase in the amplitude of the Ca2+ transient, and an abbreviation of the Ca2+ transient. This is the profile expected of an agent producing its inotropic effect by increasing cAMP (e.g., phosphodiesterase inhibition). The results indicate that the optical isomers of EMD 53998 possess a remarkable separation of Ca(2+)-sensitizing and phosphodiesterase-inhibiting activities in intact cardiac muscle. These actions were additive and could account for the effects observed with EMD 53998. (+)EMD 57033 appears to be the first inotropic agent that acts predominantly by increasing myofilament calcium sensitivity.

A novel thiadiazinone derivative fully reverses acidosis-induced depression of force in cardiac muscle by a calcium-sensitizing effect

1. We examined the effects of the novel thiadiazinone derivative EMD 57033 on developed force and intracellular [Ca2+] in cardiac muscle during control conditions (pH 7.35) and in acidosis (pH 6.8). 2. In the control solution, application of EMD 57033 fully activated the muscle. Acidosis reduced developed force to 18% of maximum, but application of EMD 57033 in acid solution was able to fully reverse this effect and restore force to its previous maximum. 3. During the positive inotropic effect in acidosis, the Ca2+ transients declined to 62% of their initial amplitude, whereas force increased to 557%. 4. These observations suggest that EMD 57033 increases force by a Ca(2+)-sensitizing action in intact cardiac muscle. Since EMD 57033 is able to fully reverse the effects of acidosis on force without increasing the amplitude of the Ca2+ transients, this compound and others with similar mechanisms of action appear to hold particular promise for heart failure therapy.

The effect of acidosis on beta-adrenergic receptors in ferret cardiac muscle

Acidosis decreases the force of contraction of cardiac muscle in response to noradrenaline. The role of beta-adrenergic receptors in this response to acidosis was investigated. Radioligand techniques were used to determine beta-adrenergic receptor number and the degree of G-protein coupling, and to see whether these were altered in tissues subject to acidosis. The effect of pH on agonist and antagonist binding to these receptors was also investigated. Tissue pre-exposure to acidic conditions had no effect on numbers of beta-adrenergic receptors and no effect on the affinity of [125I]iodocyanopindolol (ICYP) for the receptors. Agonist competition experiments indicated that there was no change in the affinity of isoprenaline for these receptors, and there was no change in the relative proportions of high and low affinity binding sites. When radioligand experiments were performed under acidic conditions, however, the total number of beta-adrenergic receptors increased, and the affinity of these receptors for isoprenaline increased. This increase in agonist affinity might, therefore, minimize the shift to the right seen in the dose-response curve to noradrenaline during acidosis.

Contraction and intracellular Ca2+, Na+, and H+ during acidosis in rat ventricular myocytes

We have investigated the effect of a CO2-induced (respiratory) acidosis on contraction and on intracellular Ca2+, Na+, and pH (measured using the fluorescent dyes fura-2, sodium-binding benzofuran isophthalate, and 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein, respectively) in ventricular myocytes isolated from rat hearts. Initial exposure to acidosis led to a rapid decrease in intracellular pH that was accompanied by an abrupt decline in contractility. There were no consistent changes of intracellular Na+ or Ca2+ during this period. The rapid decline of contractility was followed by a slower partial recovery, which was accompanied by increases in intracellular Na+, systolic and diastolic Ca2+, and an increase in the Ca2+ content of the sarcoplasmic reticulum (estimated using caffeine). Intracellular pH did not change during this slow recovery. The slow rise of intracellular Na+ and the recovery of the twitch were blocked by the Na(+)-H+ exchange inhibitor amiloride. The sarcoplasmic reticulum inhibitor ryanodine blocked the recovery of the twitch but had no effect on the rise of intracellular Na+ induced during acidosis. It is concluded that a major cause of the initial decline of the twitch during acidosis is a decrease in the response of the contractile proteins to Ca2+ due to the decrease of intracellular pH. The subsequent slow recovery of the twitch is due to the decrease of intracellular pH activating the Na(+)-H+ exchange mechanism. This elevates intracellular Na+ and presumably, via the Na(+)-Ca2+ exchange mechanism, intracellular Ca2+. This in turn may lead to increased Ca2+ loading of, and hence release from, the sarcoplasmic reticulum, and it is this that underlies the partial recovery of contraction during acidosis in this preparation.

[Ca2+] and [Na+] in rat ventricular myocytes showing negative and positive force frequency relationships

Ventricular cells that show a positive force-frequency relationship show graded changes of intracellular [Ca2+] (Cai), [Na+] (Nai) and sarcoplasmic reticulum Ca2+ content with changes of stimulation frequency. Cells that show a negative force-frequency relationship show smaller changes of Nai and no change in the Ca2+ load of the sarcoplasmic reticulum as stimulation frequency is changed.

The effect of a calmodulin inhibitor on intracellular [Ca2+] and contraction in isolated rat ventricular myocytes

1. The effect of the calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide (W7; 10 microM) on intracellular [Ca2+] ([Ca2+]i) and [H+], and on contraction, has been studied in myocytes isolated from the ventricles of rat hearts. [Ca2+]i and [H+] were monitored using the fluorescent dyes Fura-2 and 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) respectively. 2. W7 decreased the size of both the Fura-2 fluorescence (a function of [Ca2+]i) transient and twitch, but had no effect on their time course. 3. The decrease in the size of the Fura-2 fluorescence transient in the presence of W7 was accompanied by a decrease in the increase of Fura-2 fluorescence that could be elicited by releasing Ca2+ from the sarcoplasmic reticulum using 10 mM-caffeine. 4. There was a decrease in the apparent sensitivity of the contractile proteins to Ca2+ in the presence of W7 which may account, in part, for the decrease in the twitch observed in the presence of W7. 5. Test beats were interpolated at different test intervals after a train of steady-state contractions. Mechanical restitution curves were constructed by plotting the size of the test beat against the test interval. Both the size and the duration of the twitch increased as the test interval was prolonged. W7 slowed this mechanical restitution but had no effect on the changes in the duration of the twitch. 6. Intracellular pH was not altered by W7. 7. These results are discussed in terms of the known actions of calmodulin and W7.

Ca2+ and Na+ in rat myocytes showing different force-frequency relationships

Intracellular [Ca2+] ([Ca2+]i), intracellular Na+ activity (aiNa), and contraction have been monitored in single myocytes isolated from the ventricles of rat hearts. Some of these cells showed an increase in the size of the twitch as stimulation frequency was increased (positive force-frequency relationship), while others showed a decrease in the strength of contraction as the frequency of stimulation was increased (negative force-frequency relationship). In cells that showed a positive force-frequency relationship, increasing stimulation frequency resulted in increases in aiNa, diastolic [Ca2+]i, systolic [Ca2+]i, and the amount of Ca2+ that could be released from the sarcoplasmic reticulum by caffeine. The rate of decline of the [Ca2+]i transient and the twitch also increased as stimulation frequency was increased. In cells that showed a negative force-frequency relationship, increasing stimulation frequency had less effect on aiNa and had either no effect or decreased systolic [Ca2+]i with no change in the amount of Ca2+ that could be released from the sarcoplasmic reticulum using caffeine. The rate of relaxation of the [Ca2+]i transient and the twitch again increased as stimulation frequency increased. The pattern and time course of mechanical restitution was the same in both cell types. Although these data are essentially descriptive, it is consistent with the hypothesis that the final contractile response observed during changes of stimulation frequency may be dependent on how the Ca2+ loading of the preparation varies with stimulation frequency.

Diastolic, systolic and sarcoplasmic reticulum [Ca2+] during inotropic interventions in isolated rat myocytes

1. The fluorescent indicator Fura-2 has been used to monitor intracellular [Ca2+] (Ca2+i) in myocytes isolated from the ventricles of rat hearts. 2. The relationships between diastolic Ca2+i, systolic Ca2+i and the Ca2+ content of the sarcoplasmic reticulum (SR; assayed using caffeine) have been studied during changes of stimulation rate and bathing [Ca2+] (Ca2+o). 3. When stimulation rate was increased, there were increases in diastolic Ca2+i, systolic Ca2+i and the Ca2+ content of the SR. 4. The SR inhibitor ryanodine (1 mumol l-1) decreased the size of the Ca2+i transient, and abolished the increase of Ca2+i produced by caffeine (10 mmol l-1). In the presence of ryanodine, increasing stimulation rate increased diastolic Ca2+i but not systolic Ca2+i. 5. Increasing Ca2+o led to increases of diastolic Ca2+i, systolic Ca2+i and SR Ca2+ content similar to those observed during changes in stimulation rate. 6. Ryanodine altered the relationship between systolic and diastolic Ca2+i during changes of Ca2+o. 7. These results are consistent with a change of diastolic Ca2+i leading to an increase in the Ca2+ content of the SR, and hence an increase in the size of the Ca2+i transient during changes in stimulation rate and Ca2+o.

The effect of acidosis on the relationship between Ca2+ and force in isolated ferret cardiac muscle

1. The relationship between force and intracellular [Ca2+] (monitored using the protein aequorin) has been investigated in papillary muscles isolated from ferret hearts, under control conditions (superfusate pH (pHo) 7.3) and during acidosis (pHo 6.8). 2. At pHo 7.3, increasing bathing [Ca2+] from 0.5 mmol l-1 to 8 mmol l-1 led to an increase in the size of the intracellular calcium transient. At the lower [Ca2+] this was accompanied by an increase in developed force; however, at the higher bathing [Ca2+] developed force reached a plateau. 3. Acidosis (produced by increasing the [CO2] of the gas with which the muscle superfusate was equilibrated) decreased maximum force and shifted the curve relating peak developed force to peak intracellular [Ca2+] to the right. 4. The mechanisms underlying the apparent decrease in the sensitivity of the contractile proteins to Ca2+ were investigated by applying rapid length changes to papillary muscles at control pHo, during acidosis, and after bathing [Ca2+] had been increased to match force during acidosis to that in control. 5. Acidosis decreased the change in force produced in response to a given length change (i.e. decreased muscle stiffness) but when bathing [Ca2+] was increased during acidosis, muscle stiffness returned to control. 6. Acidosis had no effect on muscle stiffness after the induction of rigor in the muscle (produced by metabolic inhibition). 7. It is suggested that in intact cardiac muscle the major effect of a mild acidosis is to decrease the sensitivity of the contractile proteins to Ca2+, hence decreasing the number of bound cross-bridges.

Mechanical alternans during acidosis in ferret heart muscle

Acidosis leads to mechanical alternans (i.e., alternation of large and small contractions) in ferret papillary muscles. This alternation in the size of the contraction is paralleled by alternation in the size of the intracellular Ca2+ transient (monitored using the photoprotein aequorin). In isolated myocytes, the large contraction is accompanied by a prolonged action potential. Mechanical alternans also can be induced by acidosis in isolated myocytes during a train of voltage-clamp pulses. Thus, it appears unlikely that the mechanical alternans is secondary to changes in action potential duration; it is more likely that the observed changes in action potential duration are secondary to changes in the size of the Ca2+ transient. The observation that a Ca2(+)-activated inward current also shows alternation during mechanical alternans provides a possible mechanism for the link between Ca2+ and action potential duration. The alternation in the size of the Ca2+ transient may be secondary to the slowed mechanical restitution observed in papillary muscles during acidosis. This also could explain the observation that decreasing stimulation rate can abolish the alternans.

The effect of acidosis on the interval-force relation and mechanical restitution in ferret papillary muscle

1. The effect of a respiratory acidosis on the interval-force relation and on mechanical restitution was investigated in ferret papillary muscles. 2. Acidosis (pH 6.85) decreased developed force over a range of stimulation frequencies (1.0.06 Hz); the percentage decrease was greatest at the lowest stimulation frequencies. Qualitatively similar effects of acidosis on developed force were observed in the presence of the sarcoplasmic reticulum (SR) inhibitor ryanodine. 3. Mechanical restitution curves were constructed by interpolating extra-systoles at different test intervals following a train of steady-state beats. Mechanical restitution in ferret papillary muscle was triphasic: an initial, rapid, exponential increase in force with test intervals to 2 s, a further increase with test intervals between 60 and 90 s and then a slow decline, with a plateau at about 30 min (0.33 Hz, 30 degrees C). 4. Acidosis slowed the initial phase of mechanical restitution. The degree of slowing depended on the steady-state stimulation frequency, being greatest at low frequencies. 5. Inhibition of the SR abolished the initial phase of mechanical restitution, suggesting that this phase depends on Ca2+ release from the SR. 6. The strength of the first contraction after the extra-systole varied inversely with the size of the extra-systole under all conditions studied. 7. It is concluded that acidosis may inhibit the SR by altering the time required for Ca2+ recycling between contractions. This effect may alter Ca2+ release from the SR during acidosis, and may underlie the mechanical alternans (the alternation of small and large contractions) that can occur during acidosis.