Remote-refocusing light-sheet fluorescence microscopy enables 3D imaging of electromechanical coupling of hiPSC-derived and adult cardiomyocytes in co-culture

Improving cardiac function through stem-cell regenerative therapy requires functional and structural integration of the transplanted cells with the host tissue. Visualizing the electromechanical interaction between native and graft cells necessitates 3D imaging with high spatio-temporal resolution and low photo-toxicity. A custom light-sheet fluorescence microscope was used for volumetric imaging of calcium dynamics in co-cultures of adult rat left ventricle cardiomyocytes and human induced pluripotent stem cell-derived cardiomyocytes. Aberration-free remote refocus of the detection plane synchronously to the scanning of the light sheet along the detection axis enabled fast dual-channel 3D imaging at subcellular resolution without mechanical sample disturbance at up to 8 Hz over a ∼300 µm × 40 µm × 50 µm volume. The two cell types were found to undergo electrically stimulated and spontaneous synchronized calcium transients and contraction. Electromechanical coupling improved with co-culture duration, with 50% of adult-CM coupled after 24 h of co-culture, compared to 19% after 4 h (p = 0.0305). Immobilization with para-nitroblebbistatin did not prevent calcium transient synchronization, with 35% and 36% adult-CM coupled in control and treated samples respectively (p = 0.91), indicating that electrical coupling can be maintained independently of mechanotransduction.

Activation of GPER limits adverse changes to Ca2+ signalling and arrhythmogenic activity in cardiomyocytes of ovariectomised guinea pigs

Background

Post-menopausal women have an enhanced risk of developing cardiovascular disease and disturbances of cardiac rhythm, generally attributed to declining oestrogen levels during menopause. In an animal model that mimics menopause, the long-term withdrawal of oestrogen dysregulated Ca2+ signalling and increased the formation of a pro-arrhythmic substrate. Selective targeting of the membrane bound G-protein coupled oestrogen receptor 1 (GPER) eliminated such arrhythmogenic activity associated with the loss of oestrogen.

Purpose

We aim to assess the “cardioprotective” role of GPER in response to oestrogen withdrawal.

Methods

Ovariectomy (OVx) or sham surgeries were conducted on female guinea pigs. Left ventricular cardiomyocytes were isolated 150-days post-operatively for experimental use. GPER expression was quantified by western blot. Myocytes were incubated in solutions containing GPER agonist G-1 for >2h before recording some electrophysiological and Ca2+ regulatory parameters.

Results

GPER expression was 32% higher in OVx. OVx cardiomyocytes had prolonged action potential duration (APD) compared with sham and in the presence of G-1, APD90 shortened by 12% and 22% in sham and OVx, respectively. G-1 reduced early after depolarisation (EAD) formation by >99% in OVx. OVx cells had larger sarcoplasmic reticulum (SR) Ca2+ content (by 13%), compared with sham. While G-1 had little effect on SR content, it reduced Ca2+ transient amplitude (by 40%), SR fractional release (by 11%) and sarcomere shortening (by 29%) in OVx cells. The frequency of occurrence of spontaneous Ca2+ waves evoked by periods of rapid stimulation reduced by 40% and wave-free survival time prolonged in OVx cells incubated with G-1.

Conclusions

In the hearts of an animal species whose electrophysiology and intracellular Ca2+ regulation is akin to humans, we show that following oestrogen deficiency, GPER expression increased and its activation induces negative inotropic responses in cardiomyocytes. It limits the adverse changes to Ca2+ signalling and induces anti-arrhythmogenic behaviours in OVx.

Funding Acknowledgement

Type of funding sources: Private grant(s) and/or Sponsorship. Main funding source(s): British Heart Foundation

Changes in cellular Ca2+ and Na+ regulation during the progression towards heart failure in the guinea pig

Key points

During compensated hypertrophy in vivo fractional shortening (FS) remains constant until heart failure (HF) develops, when FS decreases from 70% to 39%.

Compensated hypertrophy is accompanied by an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current. These changes persist as HF develops.

SR Ca²⁺ content increases during compensated hypertrophy then decreases in HF. In healthy cells, increases in SR Ca²⁺ content and Ca²⁺ transients can be achieved by the same amount of inhibition of the Na⁺,K⁺‐ATPase as measured in the diseased cells.

SERCA function remains constant during compensated hypertrophy then decreases in HF, when there is also an increase in spark frequency and spark‐mediated Ca²⁺ leak.

We suggest an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current and function alters the balance of Ca²⁺ flux mediated by the Na⁺/Ca²⁺ exchange that limits early contractile impairment.

Abstract

We followed changes in cardiac myocyte Ca²⁺ and Na⁺ regulation from the formation of compensated hypertrophy (CH) until signs of heart failure (HF) are apparent using a trans‐aortic pressure overload (TAC) model. In this model, in vivo fractional shortening (FS) remained constant despite HW:BW ratio increasing by 39% (CH) until HF developed 150 days post‐TAC when FS decreased from 70% to 39%. Using live and fixed fluorescence imaging and electrophysiological techniques, we found an increase in I_Na,late from –0.34 to –0.59 A F⁻¹ and a decrease in Na⁺,K⁺‐ATPase current from 1.09 A F⁻¹ to 0.54 A F⁻¹ during CH. These changes persisted as HF developed (I_Na,late increased to –0.82 A F⁻¹ and Na⁺,K⁺‐ATPase current decreased to 0.51 A F⁻¹). Sarcoplasmic reticulum (SR) Ca²⁺ content increased during CH then decreased in HF (from 32 to 15 μm l⁻¹) potentially supporting the maintenance of FS in the whole heart and Ca²⁺ transients in single myocytes during the former stage. We showed using glycoside blockade in healthy myocytes that increases in SR Ca²⁺ content and Ca²⁺ transients can be driven by the same amount of inhibition of the Na⁺,K⁺‐ATPase as measured in the diseased cells. SERCA function remains constant in CH but decreases (τ for SERCA‐mediated Ca²⁺ removal changed from 6.3 to 3.0 s⁻¹) in HF. In HF there was an increase in spark frequency and spark‐mediated Ca²⁺ leak. We suggest an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current and function alters the balance of Ca²⁺ flux mediated by the Na⁺/Ca²⁺ exchange that limits early contractile impairment.

During compensated hypertrophy in vivo fractional shortening (FS) remains constant until heart failure (HF) develops, when FS decreases from 70% to 39%.

Compensated hypertrophy is accompanied by an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current. These changes persist as HF develops.

SR Ca²⁺ content increases during compensated hypertrophy then decreases in HF. In healthy cells, increases in SR Ca²⁺ content and Ca²⁺ transients can be achieved by the same amount of inhibition of the Na⁺,K⁺‐ATPase as measured in the diseased cells.

SERCA function remains constant during compensated hypertrophy then decreases in HF, when there is also an increase in spark frequency and spark‐mediated Ca²⁺ leak.

We suggest an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current and function alters the balance of Ca²⁺ flux mediated by the Na⁺/Ca²⁺ exchange that limits early contractile impairment.

Diastolic dysfunction and abnormality of the Na+/Ca2+ exchanger in single uremic cardiac myocytes

Cardiovascular disease is the most common cause of death in patients with end-stage renal disease, possibly due to a specific "uremic cardiomyopathy". This study investigated the function of the Na(+)/Ca(2+) exchanger in single cardiac myocytes from a model of early renal impairment. Mild uremia was induced by partial (5/6) nephrectomy in male Wistar rats. After 4 weeks, ventricular myocytes were isolated, loaded with the fluorescent Ca(2+) indicator indo-1, and contractile function and calcium transients recorded following electrical pacing at 0.2 Hz. Relaxation from rapid cooling contractures (RCCs) was also studied. Cells from uremic animals (U) were hypertrophied compared with controls (C), with a significant increase in width (14%; P<0.02) and cross-sectional area (13%; P<0.03). There was a significant increase in diastolic intracellular Ca(2+) ratio in the uremic cells (C, 0.33+/-0.00 vs U, 0.37+/-0.02; P<0.02), although the amount of calcium released per twitch was similar. Uremic cells were slower to relax following RCCs, however when Na(+)/Ca(2+) exchange was inhibited using a Na(+)-free/Ca(2+)-free solution, this difference was abolished. Under these conditions, there was little difference in the relaxation rate of control cells, indicating that the Na(+)/Ca(2+) exchanger plays only a minor role in relaxation in normal rat myocytes. However in uremia, the data indicate that the Na(+)/Ca(2+) exchanger actively interfered with relaxation, possibly by working in reverse rather than forward mode. These results indicate that myocyte relaxation and Ca(2+) handling are abnormal in early uremia and may provide further evidence for the existence of a specific "uremic cardiomyopathy".

The voltage-sensitive release mechanism of excitation contraction coupling in rabbit cardiac muscle is explained by calcium-induced calcium release

The putative voltage-sensitive release mechanism (VSRM) was investigated in rabbit cardiac myocytes at 37 degrees C with high resistance microelectrodes to minimize intracellular dialysis. When the holding potential was adjusted from -40 to -60 mV, the putative VSRM was expected to operate alongside CICR. Under these conditions however, we did not observe a plateau at positive potentials of the cell shortening versus voltage relationship. The threshold for cell shortening changed by -10 mV, but this resulted from a similar change of the threshold for activation of inward current. Cell shortening under conditions where the putative VSRM was expected to operate was blocked in a dose dependent way by nifedipine and CdCl2 and blocked completely by NiCl2. "Tail contractions" persisted in the presence of nifedipine and CdCl2 but were blocked completely by NiCl2. Block of early outward current by 4-aminopyridine and 4-acetoamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid (SITS) demonstrated persisting inward current during test depolarizations despite the presence of nifedipine and CdCl2. Inward current did not persist in the presence of NiCl2. A tonic component of cell shortening that was prominent during depolarizations to positive potentials under conditions selective for the putative VSRM was sensitive to rapidly applied changes in superfusate [Na+] and to the outward Na+/Ca2+ exchange current blocking drug KB-R7943. This component of cell shortening was thought to be the result of Na+/Ca2+ exchange-mediated excitation contraction coupling. Cell shortening recorded under conditions selective for the putative VSRM was increased by the enhanced state of phosphorylation induced by isoprenaline (1 microM) and by enhancing sarcoplasmic reticulum Ca2+ content by manipulation of the conditioning steps. Under these conditions, cell shortening at positive test depolarizations was converted from tonic to phasic. We conclude that the putative VSRM is explained by CICR with the Ca2+ "trigger" supplied by unblocked L-type Ca2+ channels and Na+/Ca2+ exchange.

SERCA2A overexpression decreases the incidence of aftercontractions in adult rabbit ventricular myocytes

K. Davia, E. Bernobich, H. K. Ranu, F. del Monte, C. M. N. Terracciano, K. T. MacLeod, D. L. Adamson, B. Chaudhri, R. J. Hajjar and S. E. Harding. SERCA2a Overexpression Decreases the Incidence of Aftercontractions in Adult Rabbit Ventricular Myocytes. Journal of Molecular and Cellular Cardiology (2001) 33, 1005-1015. Slow relaxation and poor contractile response to increasing stimulation frequency in failing human heart have been strongly linked to a decrease in the activity of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA2a). Restoration of SERCA2a levels using gene transfer has beneficial effects on contractile function but, like beta -adrenoceptor stimulation, could potentially produce excess SR Ca(2+), arrhythmias and cell death. We have examined the effects of SERCA2a overexpression in adult rabbit cardiac myocytes, and compared changes in relaxation with those following beta -adrenoceptor stimulation. Myocytes were infected with an adenovirus carrying both SERCA2a and green fluorescent protein (GFP) for positive identification of infected cells. Myocyte survival was significantly enhanced in the infected cultures. There was a reduction in both time-to-peak contraction and time-to-50% relaxation (R50) 48 h after infection. Time-to-90% relaxation (R90) was particularly improved (non-infected 516+/-41 ms, AD.SERCA2a-GFP 230+/-23 ms, n=7 preparations, P<0.001). There was also a decreased incidence of aftercontractions in Ad.SERCA2a-GFP infected myocytes (21+/-5%v 41+/-4% in controls, P<0.01). This contrasts with beta -adrenoceptor stimulation, which reduced R50 but prolonged R90 by 158+/-76 ms (P<0.02, n=16). At higher stimulation frequencies (2-3 Hz) contraction amplitude and SR calcium content were increased and diastolic contracture was reduced following SERCA2a overexpression. Overall, increasing levels of SERCA2a resulted in an improvement in systolic and diastolic function and a reduction in cell death and arrhythmic aftercontractions. SERCA2a overexpression therefore lacks the detrimental effects associated with some other inotropic interventions.

Reduced ryanodine receptor to dihydropyridine receptor ratio may underlie slowed contraction in a rabbit model of left ventricular cardiac hypertrophy

Cardiac hypertrophy is associated with contractile dysfunction, a feature of which is a slowing of the time to reach peak contraction. We have examined the main mechanisms involved in the initiation of contraction and investigated if their functions are changed during cardiac hypertrophy. Cardiac hypertrophy was induced by constriction of the ascending aorta in the rabbit. After 6 weeks left ventricular myocytes were isolated or left ventricular and septal mixed membrane preparations were produced for electrophysiological and radioligand binding studies, respectively. Aortic constriction resulted in a 24% and 23% increase in heart weight to body weight ratio and cell capacitance, respectively. Action potential duration and time-to-reach 50% and 90% peak contraction (TTP(50)and TTP(90), respectively) were significantly prolonged in myocytes from hypertrophied hearts. The prolongation of TTP(50)and TTP(90)could not be explained by altered peak calcium current density or SR calcium content which were unchanged in hypertrophy. Radioligand binding studies performed on tissue preparations from the same hearts, revealed a 34% reduction in ryanodine receptor (RYR) density with no change in dihydropyridine receptor (DHPR) density. This resulted in a reduction in the ratio of RYR to DHPR from 4.4:1 to 3.3:1 in hypertrophy. Ryanodine receptor Ca(2+)-sensitivity was unchanged between sham operated and hypertrophied groups. A reduction in the ratio of RYRs to DHPRs may result in a degree of "functional uncoupling" causing defective release of Ca(2+)from the SR. These findings may underlie the slowed TTP of myocyte contraction in hypertrophy.

Low-dose ramipril treatment improves relaxation and calcium cycling after established cardiac hypertrophy

Rapid cooling contractures were used in this study to test whether low-dose ramipril improves sarcoplasmic reticulum (SR) Ca(2+) uptake and Na(+)/Ca(2+) exchanger function in isolated hypertrophied rat myocytes. Compensated cardiac hypertrophy was induced by abdominal aortic constriction for 5 wk followed by administration of ramipril (50 microg x kg(-1) x day(-1)) or vehicle for 4 wk. Myocyte cell length and cell width were significantly (P < 0.05) increased in both hypertrophied groups (+/-ramipril). Myocytes were loaded with indo 1, and relaxation was investigated after rapid cooling. Hypertrophied myocyte relaxation in Na(+)-free/Ca(2+)-free solution was 63% slower (P < 0.01) and the fall in intracellular Ca(2+) was 60% slower (P < 0.05) than the relaxation of control cells. After ramipril treatment both relaxation and the decline in intracellular Ca(2+) returned to control rates through improved SR Ca(2+)-ATPase function. Relaxation in caffeine showed no change after hypertrophy; however, after ramipril treatment the time to 50% relaxation in caffeine decreased by 30% (P < 0.05). The improvement in Ca(2+) extrusion across the sarcolemmal membrane occurred independently of changes in Na(+)/Ca(2+) exchanger mRNA and protein abundance. These data demonstrate that ramipril improves both SR-dependent and non-SR-dependent calcium cycling after established cardiac hypertrophy. However, the improvements in function are independent of transcriptional activation and likely to involve altered intracellular ion concentrations.

Evidence for the action potential mediating the changes to contraction observed in cardiac hypertrophy in the rabbit

BACKGROUND

We investigated the effects of cardiac hypertrophy on intracellular calcium (Ca(2+)) homeostasis, the amounts of proteins involved in calcium regulation and the influence of the action potential on such changes.

METHODS

Cardiac hypertrophy was induced in rabbits by constriction of the ascending aorta. They were kept for 6 weeks then the heart was removed and left ventricular myocytes isolated. A portion of these myocytes was immediately frozen and stored for subsequent protein analyses using Western blotting.

RESULTS

After aortic banding, cardiac myocyte two-dimensional area and membrane capacitance were increased by 53% and 23% respectively. Hypertrophy prolonged cell contraction and relaxation and the corresponding Indo-1 Ca(2+) transients. Hypertrophied cells displayed longer action potentials but Ca(2+) current densities were unchanged compared with myocytes from sham hearts. If Ca(2+) was released from the sarcoplasmic reticulum using rapid cooling, so bypassing the normal mechanisms involved in excitation-contraction coupling, then no functional differences between hypertrophied and control cells could be observed. Western blot analysis showed that the amounts of sarcoplasmic reticulum Ca(2+) ATPase, its regulatory protein phospholamban and the sodium/calcium exchanger were unchanged whereas the amount of calsequestrin was increased by 65% and the alpha(1) subunit of the sodium/potassium ATPase was reduced by 72%. These changes do not appear to evoke functional consequences under these conditions.

CONCLUSION

In this model of cardiac hypertrophy, the increase in action potential duration is responsible for changes in contraction and relaxation.

Overexpression of the Na(+)/Ca(2+) exchanger and inhibition of the sarcoplasmic reticulum Ca(2+)-ATPase in ventricular myocytes from transgenic mice

BACKGROUND

Myocytes from failing hearts produce slower and smaller Ca(2+) transients associated with reduction in expression of sarcoplasmic reticulum (SR) Ca(2+) ATPase and an overexpression of Na(+)/Ca(2+) exchanger. Since the physiological role of both these proteins is competing for, and removing, Ca(2+) from the cytoplasm, overexpression of the exchanger may compensate for less effective SR Ca(2+) uptake. This study demonstrates this compensatory effect and provides a quantitative description of the results.

METHODS

Ventricular myocytes from transgenic mice overexpressing the Na(+)/Ca(2+) exchanger (TR) and nontransgenic littermates (NON) were used. Cell shortening, cytoplasmic [Ca] (using indo-1 AM) and electrophysiological parameters were monitored.

RESULTS

TR myocytes displayed faster Ca(2+) transients and twitches compared with NON myocytes. Superfusion with thapsigargin prolonged the time-course of Ca(2+) transients of TR myocytes until these were equal to the ones measured in NON myocytes. The amount of SR Ca(2+)-ATPase (SERCA) inhibition needed to obtain such transients was calculated as a function of V(max) for the Ca(2+) flux via SERCA and found to be 28%. In TR myocytes V(max) for the Ca(2+) flux via Na(+)/Ca(2+)exchange was 240% of NON myocytes. When Ca(2+) transients in TR myocytes were slowed by thapsigargin to similar values to the ones recorded in NON myocytes, SR Ca(2+) content was also correspondingly reduced.

CONCLUSIONS

The results suggest that in pathophysiological conditions where there is a reduction in SERCA function, overexpression of Na(+)/Ca(2+) exchanger can compensate and allow normal Ca(2+) homeostasis to be maintained. In mouse ventricular myocytes a 2.4-fold increase in Na(+)/Ca(2+) exchange activity compensates for a reduction in SERCA function by 28% so maintaining the duration of the Ca(2+) transient.

The role of L-type calcium current in the generation of repolarization-induced contraction in cardiac myocytes

OBJECTIVE

Early experiments into the arrhythmogenic transient inward current frequently showed apparent coupling of this current to repolarization from a depolarizing voltage clamp step. Calcium transients have subsequently been shown to couple to such repolarization and are the result of calcium release from the sarcoplasmic reticulum. We have investigated whether this phenomenon is due to calcium entry via non-inactivated calcium channels or to voltage-activated SR release.

METHODS

Voltage clamp steps were imposed on isolated guinea pig and rabbit cardiac myocytes. Calcium release was monitored by tracking cell contraction. L-type calcium current at the moment of repolarization was manipulated by the rapid application of 2 mM cadmium or 10 mM calcium.

RESULTS

Repolarization-induced contraction was abolished by the rapid application of 2 mM cadmium immediately prior to repolarization, and was augmented by the rapid change of extracellular calcium concentration from 2 mM to 10 mM immediately prior to repolarization. There is no evidence of coupling of drive train-induced aftercontractions to repolarization from the final action potential of the drive train and 2 mM cadmium does not alter the appearance or timing of these aftercontractions. Simulation of phase 1 repolarization in the mammalian cardiac action potential decreases rather than increases twitch amplitude.

CONCLUSION

Repolarization-induced contraction results from calcium entry through non-inactivated calcium channels, not from voltage-activated release. It plays no physiological role in contributing to the stimulated twitch and no pathological role in generating drive train-induced aftercontractions.

Sarcoplasmic reticulum Ca content, sarcolemmal Ca influx and the genesis of arrhythmias in isolated guinea-pig cardiomyocytes

Early afterdepolarizations are seen during the repolarization phases of the action potential and delayed afterdepolarizations appear later, usually following complete repolarization of the cell membrane potential. Both forms of afterdepolarization are linked to the occurrence of aftercontractions, seem to play a role in the generation of ventricular arrhythmias and are believed to be the result of abnormalities of intracellular Ca handling. Suggestions for the mechanisms responsible vary from both types of afterpotential being mediated by Ca release from the sarcoplasmic reticulum, to early afterdepolarization formation being due to reactivation of the L-type sarcolemmal Ca channels during the action potential. We tried to assess the functional importance of the sarcoplasmic reticulum or Ca influx in the development of afterpotentials and abnormal contractile activity in guinea-pig cardiac myocytes. Ca influx was increased using isoproterenol, Bay K8644 or increasing extracellular [Ca]. Sarcoplasmic reticulum Ca content was measured using rapid cooling contractures or caffeine-induced Na/Ca exchange current and the sarcoplasmic reticulum was inhibited using caffeine or thapsigargin. Aftercontractions associated with either early or delayed afterdepolarizations could be induced by increasing Ca influx. The increased Ca influx produced increases in sarcoplasmic reticulum Ca content and aftercontractions were associated with a larger SR Ca content. However, the sarcoplasmic reticulum was no more loaded with Ca when aftercontractions occurred than when aftercontractions did not occur. Preventing Ca sequestration by the sarcoplasmic reticulum inhibited the formation of aftercontractions. The results suggest that alterations to both Ca influx and sarcoplasmic reticulum Ca content are required to produce aftercontractions.

Calcium extrusion during aftercontractions in cardiac myocytes: the role of the sodium-calcium exchanger in the generation of the transient inward current

Spontaneous release of calcium from the sarcoplasmic reticulum leads to delayed afterdepolarizations which may represent an arrhythmogenic mechanism in the intact heart. The current underlying delayed afterdepolarizations is the transient inward current, but how this is triggered by a spontaneous rise in cytoplasmic calcium concentration is a matter of debate. We have investigated this by rapid application of caffeine to isolated guinea-pig cardiac myocytes, before and after drive train-induced aftercontractions. Mean (+/- s.e.m.) sarcoplasmic reticulum content reduced from 85 +/- 11 micromol/l accessible cell volume to 53 +/- 9 micromol/l accessible cell volume (n=11) during the course of the aftercontraction. The charge movement expected to result from extrusion of this calcium via the sodium-calcium exchanger was 70.1 +/- 5.4 pC, compared with charge measured during the transient inward current of 70.1 +/- 10.8 pC in the same cells (P=0.9969). Rapid inhibition of the sodium-calcium exchanger, by replacement of the superfusate with a sodium and calcium free solution between the end of the drive train and the aftercontraction, completely abolished the transient inward current (from 90.4 +/- 10.2 pA inward current to 23.8 +/- 14.9 pA outward current, P<0.001). We conclude that the transient inward current in this species is explained entirely by sodium-calcium exchange current without the need to invoke other calcium-activated conductances.

Na+-Ca2+ exchange and sarcoplasmic reticular Ca2+ regulation in ventricular myocytes from transgenic mice overexpressing the Na+-Ca2+ exchanger

1. The contribution of the sarcoplasmic reticulum (SR) and Na+-Ca2+ exchanger to intracellular Ca2+ regulation in mouse cardiac myocytes was investigated by measuring contraction after variable rest intervals, rapid cooling contractures (RCCs) and fast application of caffeine. The results obtained showed differences from other species in the roles played by the SR and the Na+-Ca2+ exchanger. They suggest that in mouse ventricular myocytes there is significant Ca2+ entry via the exchanger during rest and during the latter part of the Ca2+ transient. 2. In cardiac myocytes isolated from transgenic mice overexpressing the cardiac Na+-Ca2+ exchanger the time to peak and relaxation of twitches and RCCs were faster than in control littermates. The decline of Ca2+, assessed by indo-1 fluorescence, was faster in transgenic myocytes even in the absence of Na+ and Ca2+ in the superfusing solution. This suggests that SR Ca2+ uptake is faster in these myocytes. However, no difference in the expression of SERCA2a, phospholamban or calsequestrin measured with Western blotting could be found in the two groups. 3. We measured SR Ca2+ content by integrating the caffeine-induced transient inward current. The amount of Ca2+ stored in the SR of transgenic mouse myocytes was 69 % greater than in non-transgenic littermates. The increased SR Ca2+ content may be responsible for the faster rate of SR Ca2+ release and uptake in cells from transgenic mice. 4. We performed experiments to assess whether the reversal potential of the Na+-Ca2+ exchanger (ENa-Ca) was different in transgenic cardiac cells. We measured a Ni2+-sensitive current elicited by voltage ramps in non-dialysed myocytes. The current-voltage relationship showed no difference in the reversal potential of the Na+-Ca2+ exchanger in transgenic and control myocytes. This suggests that the effects on the SR Ca2+ content in transgenic cardiac myocytes can be ascribed to the overexpression of the exchanger and are not secondary to changes in intracellular diastolic Ca2+ and Na+.

Effects of hypoxia and metabolic inhibition on increases in intracellular Ca2+ concentration induced by Na+/Ca2+ exchange in isolated guinea-pig cardiac myocytes

We have assessed if the sarcolemmal Na+/Ca2+ exchange of isolated cardiac myocytes becomes inhibited when the cells are subjected to hypoxia and metabolic inhibition. Function of the exchange was assessed by exposing single cardiac myocytes to Na+-free solutions and measuring the increases in [free Ca2+]i using the fluorescent indicator Fura-2. Removal of extracellular Na+ reverses the Na+ gradient and causes Ca2+ influx via Na+/Ca2+ exchange. This influx may trigger further Ca2+ release from the sarcoplasmic reticulum. Hypoxia and metabolic inhibition produced either by (1) vigorously bubbling glucose-free Tyrode with N2, or (2) bubbling with N2 and adding 10 mm 2-deoxy-D-glucose to the Tyrode or (3) using 0.1-1.0 mm sodium dithionite (sodium hyposulphite, Na2S2O4) in glucose-free Tyrode caused a significant decline in the rates of increase in [free Ca2+]i during Na2+-free superfusion. The decreases in [free Ca2+]i on re-application of Na+ were also significantly slowed. This suggests that reverse-mode Na+/Ca2+ exchange and the processes leading to release of Ca2+ from the sarcoplasmic reticulum are inhibited in hypoxia and metabolic inhibition.

Voltage-dependent binding and calcium channel current inhibition by an anti-alpha 1D subunit antibody in rat dorsal root ganglion neurones and guinea-pig myocytes

1. The presence of calcium channel alpha 1D subunit mRNA in cultured rat dorsal root ganglion (DRG) neurones and guinea-pig cardiac myocytes was demonstrated using the reverse transcriptase-polymerase chain reaction. 2. An antipeptide antibody targeted at a region of the voltage-dependent calcium channel alpha 1D subunit C-terminal to the pore-forming SS1-SS2 loop in domain IV (amino acids 1417-1434) only bound to this exofacial epitope if the DRG neurones and cardiac myocytes were depolarized with 30 mM K+. 3. Incubation of cells under depolarizing conditions for 2-4 h with the antibody resulted in a maximal inhibition of inward current density of 49% (P < 0.005) for DRGs and 30% (P < 0.05) for cardiac myocytes when compared with controls. 4. S-(-)-Bay K 8644 (1 microM) enhanced calcium channel currents in DRGs by 75 +/- 19% (n = 5) in neurones incubated under depolarizing conditions with antibody that had been preabsorbed with its immunizing peptide (100 micrograms ml-1). This was significantly (P < 0.05) larger than the enhancement by S-(-)-Bay K 8644 that was seen with cells incubated under identical conditions but with antibody alone, which was 15 +/- 4% (n = 5). 5. These results demonstrate the presence of calcium channel alpha 1D subunits in rat DRG neurones and guinea-pig cardiac myocytes. They also show that amino acids 1417-1434 of the alpha 1D subunit are only exposed to the extracellular face of the membrane following depolarization and that the binding of an antibody to these amino acids attenuates calcium channel current and reduces the ability of S-(-)-Bay K 8644 to enhance this current, indicating that it is an L-type current that is attenuated.

Effects of lactate on the relative contribution of Ca2+ extrusion mechanisms to relaxation in guinea-pig ventricular myocytes

1. The aim of this study was to investigate the effects of 20 mM extracellular lactate on Ca2+ regulation mechanisms in enzymatically isolated single guinea-pig cardiac myocytes. 2. The activities of the Ca2+ regulation mechanisms during application of lactate were studied using rapid cooling contractures (RCCs) and fast application of caffeine. Cytoplasmic Ca2+ was monitored using the fluorescent indicator indo-1. 3. After application of 20 mM lactate for 5 min, the diastolic level of Ca2+ was increased. The change in cytoplasmic Ca2+ elicited by stimulation (Ca2+ transient) was also changed. With lactate, the amplitude of the Ca2+ transient was smaller, and its time course was slower compared with control. 4. The recovery of cytoplasmic Ca2+ during rewarming after rapid cooling in lactate was slower than under control conditions. When the rewarming was performed either in Na(+)- and Ca(2+)-free solution or in the presence of 10 mM caffeine, the rate of recovery of cytoplasmic Ca2+ in lactate was slower than under control conditions, suggesting that the activity of both SR Ca2+ uptake and Na(+)-Ca2+ exchange is affected by lactate. 5. Cytoplasmic Ca2+ recovery during application of 10 mM caffeine in lactate was slower than in the control. The rate of recovery of the caffeine-induced transient inward current was also slower supporting the hypothesis of a slower Ca2+ extrusion brought about by Na(+)-Ca2+ exchange. 6. The relative contribution of the Ca2+ extrusion mechanisms in the presence of lactate was investigated using paired RCCs. In lactate, a second RCC (RCC2) induced immediately after recovery from the first (RCC1) was greatly reduced compared with the control. RCC2/RCC1 x 100 in lactate was 39% and RCC2/RCC1 x 100 in control conditions was 60%, suggesting that the net sarcoplasmic reticulum Ca2+ uptake is smaller in the presence of lactate. 7. When Na(+)-free Ca2+ solution was used during the paired RCCs and rewarming, RCC2/RCC1 x 100 was increased to 96 and 95% in lactate and control conditions, respectively, implying that Ca2+ efflux from the cell can be maintained by the Na(+)-Ca2+ exchanger and that other Ca2+ removal mechanisms (mitochondria and sarcolemmal Ca(2+)-ATPase) remain largely unchanged in the presence of lactate.

The effect of alterations to action potential duration on beta-adrenoceptor-mediated aftercontractions in human and guinea-pig ventricular myocytes

Aftercontractions induced by beta-adrenoceptor stimulation in human and guinea-pig cardiomyocytes may be related to changes in action potential duration (APD). We investigated the effects of altering APD during the occurrence of isoproterenol-induced aftercontractions, using the KATP channel openers cromakalim and lemakalim or the action potential voltage clamp technique, in guinea-pig and human ventricular cardiomyocytes. Contractile responses were measured at 32 degrees C using a video-based edge-detection system. In guinea-pig myocytes, action potentials, Indo-1 fluorescence and contraction were measured at 22 degrees C. Isoproterenol (< or = 12 nM) had variable effects on APD but induced aftercontractions, the majority (14/19 cells) of which occurred during the action potential. Short action potentials were produced using K+ channel openers. These compounds reduced or completely abolished the isoproterenol-induced aftercontractions. Increasing isoproterenol in the presence of K+ channel opener restored the main contraction to a level similar to or above those with isoproterenol alone, but without the reappearance of aftercontractions. When cells were stimulated to contract under action potential voltage clamp, isoproterenol-induced aftercontractions were abolished by voltage clamping with action potentials of short duration. It was possible to induce aftercontractions in some cells without application of isoproterenol if voltage clamp-imposed action potentials of very long duration were used. These aftercontractions were also abolished by shortening action potential duration. We conclude that K+ channel openers or the imposition of action potentials of short duration can dissociate positively inotropic beta-adrenoceptor stimulation from aftercontraction formation and that action potentials of long duration can be pro-arrhythmic.

Measurements of Ca2+ entry and sarcoplasmic reticulum Ca2+ content during the cardiac cycle in guinea pig and rat ventricular myocytes

This study investigates the contribution of Ca2+ entry via sarcolemmal (SL) Ca2+ channels to the Ca2+ transient and its relationship with sarcoplasmic reticulum (SR) Ca2+ content during steady-state contraction in guinea pig and rat ventricular myocytes. The action potential clamp technique was used to obtain physiologically relevant changes in membrane potential. A method is shown that allows calculation of Ca2+ entry through the SL Ca2+ channels by measuring Cd(2+)-sensitive current during the whole cardiac cycle. SR Ca2+ content was calculated from caffeine-induced transient inward current. In guinea pig cardiac myocytes stimulated at 0.5 Hz and 0.2 Hz, Ca2+ entry through SL Ca2+ channels during a cardiac cycle was approximately 30% and approximately 50%, respectively, of the SR Ca2+ content. In rat myocytes Ca2+ entry via SL Ca2+ channels at 0.5 Hz was approximately 3.5% of the SR Ca2+ content. In the presence of 500 nM thapsigargin Ca2+ entry via SL Ca2+ channels in guinea pig cardiac cells was 39% greater than in controls, suggesting a larger contribution of this mechanism to the Ca2+ transient when the SR is depleted of Ca2+. These results provide quantitative support to the understanding of the relationship between Ca2+ entry and the SR Ca2+ content and may help to explain differences in the Ca2+ handling observed in different species.

The effects of changes to action potential duration on the calcium content of the sarcoplasmic reticulum in isolated guinea-pig ventricular myocytes

We have estimated sarcoplasmic reticulum calcium content using rapid application of caffeine on voltage clamped, isolated guinea-pig ventricular myocytes. Caffeine induces the release of calcium from the sarcoplasmic reticulum and this calcium is extruded from the cells by the sarcolemmal Na/Ca exchange. Integrating the inward Na/Ca exchange current thus allows estimations of sarcoplasmic reticulum calcium content. Ventricular myocytes were stimulated to reach new steady-states by action potential voltage clamps of varying duration. Once contractile steady-state had been reached caffeine was rapidly applied in place of the next action potential and sarcoplasmic reticulum calcium content measured. Prolonging the action potential duration increased sarcoplasmic reticulum calcium content and vice-versa. This calcium loading may underlie the positive inotropic effect of increased action potential duration.

Reloading of Ca(2+)-depleted sarcoplasmic reticulum during rest in guinea pig ventricular myocytes

The effects of rest on a Ca(2+)-depleted sarcoplasmic reticulum (SR) in guinea pig ventricular myocytes were investigated. Cell shortening was measured using a video edge-detection system, and cytoplasmic Ca2+ was monitored using the fluorescent indicator indo-1. Rapid cooling and rewarming in the presence of 10 mM caffeine were used to deplete the SR of Ca2+. The resting cell was then superfused for variable time intervals with a normal Tyrode solution containing 2 mM Ca2+. Another rapid cooling in caffeine was performed to assess the SR Ca2+ load at the end of rest. Rapid cooling after 1- and 2-min rest elicited an increase of indo-1 fluorescence of 51.9 +/- 7.7 (n = 17) and 72.7 +/- 6.7% of control (n = 9), respectively. This increase was not detectable when Ca2+ was absent from the superfusing solution. In contrast, the increase was larger when external Ca2+ was elevated to 4 mM. Nickel (5 mM) and nifedipine (20 microM) added to the superfusing solution during the rest interval did not alter the increase in indo-1 fluorescence. We conclude that Ca2+ is reaccumulated by a depleted SR during rest. Although this Ca2+ seems to originate from the extracellular space, its route from there to the SR is unclear.

Effects of rest interval on the release of calcium from the sarcoplasmic reticulum in isolated guinea pig ventricular myocytes

Guinea pig cardiac myocytes were loaded with the fluorescent dye indo 1, and cell contraction was measured by a video edge-detection system. Ca2+ was released from the sarcoplasmic reticulum (SR) by rapidly cooling the myocytes or by rapid application of 10 mmol/L caffeine. Estimates of the amount of Ca2+ released from the SR after different rest intervals (ie, under different loading conditions) were obtained by measuring the current evoked by rapid application of 10 mmol/L caffeine, which we call Na+/Ca2+ exchange current. This current is completely inhibited by removal of extracellular Na+ and Ca2+ or by application of 5 mmol/L Ni2+. SR Ca2+ release after rest intervals of 5 to 120 seconds (assuming cell volume to be 30 x 10(-12) L) was estimated to be 57.8 +/- 5.7 to 25.7 +/- 4.5 mumol/L accessible cell volume, respectively, equivalent to 23 to 10 mumol/kg wet wt, respectively. There was an exponential decline in Ca2+ release from the SR after rest intervals of 2 to 120 seconds (rate constant, 0.029 s-1; t1/2, 24 seconds); thereafter, there remained a portion (56%) of Ca2+ releasable to caffeine application. We found a similar exponential decay (rate constant, 0.020 s-1; t1/2, 35 seconds) of the size of rapid cooling contractures with increasing rest intervals. The time to peak of the Na+/Ca2+ exchange current in the presence of caffeine slowed at long rest intervals, ie, at smaller SR loads. A decrease in SR load of 50% increased the time to peak of the exchange current by 213 +/- 37% (n = 6).(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormalities of the myocytes in ischaemic cardiomyopathy

Acute myocardial ischemia and subsequent reperfusion result in biochemical and ionic changes in cardiac myocytes which cause contracture of the muscle and a reduced contractile force. Whether changes observed in single myocytes isolated from ischaemic ventricles are a direct consequence of the acute insult, or develop more slowly due to subsequent alterations in load and neurohumoural environment, is controversial. Myocytes from ischemic hearts have a similar contraction amplitude to those from non-failing hearts at physiological or maximally activating levels of ca2+. This could be partly due to the method of cell selection, or could represent the detection of a population of myocytes that have recovered from the original insult. However, there are significant decreases in the velocities of contraction and, particularly, relaxation in myocytes from the ischaemic heart. These resemble alterations caused by anoxia/reperfusion, but similar changes have also been observed in non-ischaemic causes of heart failure. Responses of beta-adrenoceptor stimulation are reduced in single cells from the failing heart, and a post-receptor defect has also been detected. Treatment with pertussis toxin, which reduces the activity of the inhibitory guanine-nucleotide binding protein (Gi) was able to restore beta-adrenoceptor responses to normal. The hypothesis that alterations in the beta-adrenoceptor/Gi/cAMP pathway represent the response of the myocyte to continued exposure to noradrenaline, because of the high sympathetic drive in these patients, is supported by the strong parallels observed with catecholamine-treated animals, and by the fact that non-ischemic aetiologies exhibit similar desensitization. It is concluded that the surviving myocytes in an ischaemic heart are damaged by the neurohumoral alterations that represent the body's attempt to restore cardiac output.

Effects of anoxia on intracellular Ca2+ and contraction in isolated guinea pig cardiac myocytes

Single, enzymatically isolated guinea pig ventricular myocytes were exposed to 3-min periods of anoxia with glucose-free Tyrode solution containing 1 mM sodium dithionite (Na2S2O4) and were then reoxygenated for 10 min. The myocytes were exposed to rapid applications of 10 mM caffeine during the control, anoxic, and reoxygenation periods. Intracellular Ca2+ concentration ([Ca2+]i) was measured ratiometrically using indo 1 with simultaneous measurements of cell length. The effects of anoxia on Ca2+ were compared with those of hypoxia and metabolic inhibition. The amplitude of the electrically stimulated (Ca transient) and caffeine-evoked Ca2+ (Caff-Ca) transients decreased during anoxia and recovered after reoxygenation. Diastolic [Ca2+]i did not change during 3 min of anoxia but rose progressively after prolonged anoxia and remained at this higher level on reoxygenation. During metabolic inhibition the Ca transients decreased, while the Caff-Ca transients showed no change in amplitude. During hypoxia the Ca transients decreased. Anoxia slowed the time to peak of the Ca transient, the time to 50% relaxation, and the time to 90% relaxation. The decline of indo 1 fluorescence on rapid caffeine application was slowed during anoxia, metabolic inhibition, and hypoxia and partially recovered after reoxygenation.

Effects of acidosis on Na+/Ca2+ exchange and consequences for relaxation in guinea pig cardiac myocytes

We investigated the effect of intracellular acidosis (imposed by NH4Cl prepulses) on the relaxation and decline in intracellular Ca2+ (using indo 1 fluorescence) of isolated cardiac myocytes from the guinea pig. Acidosis produced a decrease in contraction and a prolongation of the fluorescence transient. The rate of decline in fluorescence after a rapid-cooling contracture was slower in acidosis compared with control. The decline in fluorescence after a rapid-cooling contracture in the presence of 10 mM caffeine was greatly slowed during acidosis, suggesting that Na+/Ca2+ exchange is affected. We recorded indo 1 fluorescence and the transient inward current in voltage-clamped cells on rapid application of 10 mM caffeine under control conditions and in acidosis. The amplitude of the transient increase in fluorescence was reduced in acidosis and the decline in fluorescence slowed. The current showed no difference in amplitude in acidosis, but the time to 50% recovery was increased by 57%. When amiloride or ethylisopropylamiloride was present, no differences in the current were found between control and acidosis, and the times to 50% recovery were similar. We conclude that intracellular acidosis slows Ca2+ efflux via Na+/Ca2+ exchange because of an increase in intracellular Na+ due to enhanced Na+/H+ exchange activity.

Effect of lemakalim on action potentials, intracellular calcium, and contraction in guinea pig and human cardiac myocytes

OBJECTIVE

The aim was to investigate the effects of lemakalim on action potential duration, intracellular free calcium ([free Ca2+]i), and cell contraction in human and guinea pig cardiac myocytes. In addition, the possible modulation by pH of lemakalim induced activation of ATP sensitive potassium (KATP) channels was assessed.

METHODS

Single ventricular myocytes were enzymatically dissociated from adult male guinea pigs (300-600 g). Single myocytes were isolated from human ventricular tissues. Cells were loaded with the acetoxymethyl ester form of fura-2 to monitor changes in [free Ca2+]i and subjected to conventional electrophysiological techniques.

RESULTS

In guinea pig cells, lemakalim (3, 10, 30 microM) reduced action potential duration in a concentration dependent manner. This decrease was accompanied by hyperpolarisation of the resting membrane potential. Lemakalim (3, 10, 30 microM) reduced the systolic fura-2 fluorescence ratio without having a significant effect on diastolic fluorescence and also reduced the cell contraction in concentration dependent manner. Glibenclamide (1 microM), a specific inhibitor of KATP channels, did not affect action potential duration, fura-2 fluorescence ratio, or cell contraction in the absence of lemakalim. However, the same dose of glibenclamide markedly inhibited the lemakalim induced decrease in action potential duration, fura-2 fluorescence ratio, and cell contraction. Reducing extracellular pH enhanced the decrease in action potential duration induced by lemakalim. In human ventricular myocytes, lemakalim (3, 10 and 30 microM) caused a decrease in action potential duration and systolic fura-2 fluorescence ratio. The reduction in action potential duration and fura-2 fluorescence ratio was also reversed by glibenclamide (1 microM).

CONCLUSIONS

These results suggest that lemakalim reduces systolic [free Ca2+]i by activating ATP sensitive potassium channels which results in a decrease of action potential duration in guinea pig and human ventricular myocytes. The reduction in [free Ca2+]i mediates the negative inotropic effect induced by lemakalim. In addition, pH may modulate the KATP channel activation by the channel opener.

Characteristics of myocytes isolated from hearts of renovascular hypertensive guinea pigs

A model of renovascular hypertension has been developed in the guinea pig using the Goldblatt (2-kidney, 1-clip) operation. Systolic and diastolic blood pressures were significantly increased 3 and 7 wk after the operation, but levels fell to control values at 11 wk. The two-dimensional areas of myocytes isolated from the hearts of Goldblatt-operated (GB) animals were larger than those in control cells at 3 wk (cf. 3,397 +/- 87 and 2,208 +/- 125 microns 2, P < 0.01), and the difference was maintained at 7 and 11 wk. No change in cell contraction or relaxation characteristics were seen at either 3 or 7 wk after clipping. Myocytes from the 11-wk GB group showed a significantly reduced contraction amplitude and velocity at 32 degrees C in maximally activating Ca2+ or isoproterenol concentrations (%cell shortening in Ca2+, cf. 6.8 +/- 0.4 and 10.0 +/- 0.9, P < 0.01). Concentrations eliciting 50% of maximal response for Ca2+ or isoproterenol were unchanged, as was the ratio of isoproterenol to Ca2+ effect in the same cell. Increases in time to peak contraction (TTP) and time to 50% relaxation (R50) were observed in 11-wk GB myocytes, but only at room temperature. There was no lengthening of TTP or R50 of the Ca2+ transient, nor was there any change in Ca2+ current density or inactivation kinetics in these myocytes.

Effect of 17 beta-oestradiol on contraction, Ca2+ current and intracellular free Ca2+ in guinea-pig isolated cardiac myocytes

1. The effect of 17 beta-oestradiol on cardiac cell contraction, inward Ca2+ current and intracellular free Ca2+ ([free Ca2+]i) was investigated in guinea-pig single, isolated ventricular myocytes. The changes of cell length were measured by use of a photodiode array, the voltage-clamp experiments were performed with a switch clamp system and [free Ca2+]i was measured with the Ca2+ indicator, Fura-2. 2. 17 beta-Oestradiol (10, 30 microM) caused a decrease in cell shortening at both 22 and 35 degrees C. This negative inotropic effect was accompanied by a decrease in action potential duration mainly brought about by a shortening of the plateau region of the action potential. 17 beta-Oestradiol (10, 30 microM) induced a similar decrease in cell shortening in voltage-clamped and current-clamped cells. 3. In Fura-2 loaded cells, 17 beta-oestradiol (10 and 30 microM) decreased systolic Fura-2 fluorescence to 72 +/- 7% and 47 +/- 4% (n = 6, P less than 0.001) of control respectively. 17 beta-Oestradiol (10 microM) had no significant effect on diastolic Fura-2 fluorescence, but at higher concentration (30 microM) induced a slight decrease in resting Fura-2 fluorescence. The effect of 17 beta-oestradiol was reversible after 1-2 min of washout of the steroid. 4. 17 beta-Oestradiol (10 and 30 microM) decreased the peak inward Ca2+ current (ICa), which was sensitive to [Ca2+]o, dihydropyridines and isoprenaline, to 59 +/- 3% and 39 +/- 5% (n = 7 approximately 9, P less than 0.01) respectively, without producing any significant change in the shape of the current-voltage relationship.5. The recovery time of ICa from inactivation was delayed by 17beta-oestradiol (10microM). The inhibitory effect of 17beta-oestradiol on ICa was less at a holding potential of -80 mV than at -40 mV.6. We conclude that 17beta-oestradiol has a negative inotropic effect on guinea-pig single ventricular myocytes by inhibiting ICa and so reducing systolic [Ca2+]i. 17beta-Oestradiol may therefore have a Ca2+ channel blocking property in guinea-pig isolated ventricular myocytes.

Effects of phorbol ester on contraction, intracellular pH and intracellular Ca2+ in isolated mammalian ventricular myocytes

1. We have investigated the actions of certain phorbol esters on the intracellular pH, intracellular Ca2+ and contractility of isolated rat and guinea-pig cardiac myocytes. Intracellular pH was measured using 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) and intracellular Ca2+ was measured using Fura-2. 2. Application of the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (also called phorbol 12-myristate 13-acetate) (TPA) (which activates protein kinase C) to rat cardiac myocytes significantly increased cell shortening by 116 +/- 34% (n = 8) (p less than 0.02). The rate of change of cell length during contraction (i.e. +dL/dt) increased from 67.2 +/- 8.7 microns/s to 127.7 +/- 14.1 microns/s (n = 7). The rate of change of cell length during relaxation (-dL/dt) increased from 55.8 +/- 7.4 microns/s to 118.9 +/- 12.1 microns/s (n = 7). Time to peak shortening was unchanged. 3. Application of 4 alpha-phorbol 12,13-didecanoate, which does not activate protein kinase C, did not affect rat myocyte contractility. An insignificant decrease in contractility (by 7.5 +/- 7.5%) was observed (n = 5). The positive inotropic effect of TPA may therefore be evoked through an activation of protein kinase C. 4. In rat myocytes we have measured the changes of pHi and contractility (cell shortening) during an alkalosis and acidosis induced by exposure to and subsequent removal of NH4Cl both in the presence and absence of TPA. Recovery times from an acid load were significantly (p less than 0.05) enhanced by 15.1 +/- 6.9% (n = 13) in the presence of TPA. Recovery times of cell shortening were also more rapid (p less than 0.05) by an average of 59.1 +/- 10.6% (n = 5) in the presence of TPA. Recovery times were unchanged in the presence of 4-phorbol 12,13-didecanoate (which does not activate protein kinase C). 5. Since pHi recovery of an isolated myocyte from an acid load is partially inhibited by the presence of 1 mM-amiloride and inhibited by removing extracellular Na+ then it is suggested that, like pHi regulation in sheep heart Purkinje fibres, pHi recovery in rat cardiac ventricular myocytes is mainly through sarcolemmal Na(+)-H+ exchange. We suggest that in the presence of TPA the Na(+)-H+ exchange is stimulated. 6. The relationship between pHi and cell shortening is non-linear as has been observed by others in whole tissue preparations. The presence of TPA shifts the relationship upwards such that at any one pHi, cell shortening is greater.(ABSTRACT TRUNCATED AT 400 WORDS)

Contractile responses of myocytes isolated from patients with cardiomyopathy

Single cardiac myocytes isolated from failing and non-failing human ventricles were superfused at 32 degrees C and electrically stimulated at 0.2 Hz. Their contraction amplitude and velocities of contraction and relaxation were continuously during challenge with isoprenaline or high extracellular calcium. Action potentials were monitored with intracellular microelectrodes, and calcium transients followed using the fluorescent dye fura-2. Changes in contractility were correlated with severity of disease, as defined by New York Heart Association class, dose of diuretics, left ventricular ejection fraction and left ventricular end-diastolic pressure. Beta-adrenoceptor desensitization was detected in these cells as a decreased response to isoprenaline relative to that of calcium in the same cell. Significant correlations were obtained between reduction of beta-adrenoceptor sensitivity and all four indicators of disease severity. No correlation between the maximum contraction amplitude in high extracellular calcium and severity of disease was observed, the same was true for contraction and relaxation velocity in high calcium. Some significant decline in contractility with age of the patient was noted. Analysis with respect to aetiology of disease showed a subpopulation with dilated or hypertrophic cardiomyopathy where relaxation of the single cells was impaired. This was related to a prolonged calcium transient and action potential. Isoprenaline accentuated the lengthened second phase of relaxation, whereas high extracellular calcium reduced it. These interventions had similar effects on action potential duration. The actions of isoprenaline and calcium were similar on cells from failing and non-failing human hearts and on normal guinea-pig myocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation and interaction of intracellular calcium, sodium and hydrogen ions in cardiac muscle

The contractility of heart muscle is sensitive to the relative cytoplasmic concentrations of Na+, H+ and Ca++. The concentration of Na+ is mainly controlled by the Na+/K(+)-ATPase while the concentration of H+ is regulated by the Na+/H+ and Cl-/HCO3- exchanges. Cytoplasmic Ca++ concentration is mainly under the control of the sarcolemmal Na+/Ca++ exchange and Ca(++)-ATPase and sarcoplasmic reticular Ca(++)-ATPase. However, in heart muscle there is also a complex interaction between these ions such that altering the main regulation of one will affect the intracellular levels of the other two. Such interaction may thus enhance or attenuate the contractile response to the initial change. This review briefly describes the properties of the main regulatory mechanisms and focuses on their interactions and what consequences these have for contraction.

Sheep cardiac sarcoplasmic reticulum calcium-release channels: modification of conductance and gating by temperature

1. The gating and conduction properties of single calcium-release channels of sheep isolated cardiac junctional sarcoplasmic reticulum membranes incorporated into planar phospholipid bilayers were investigated under voltage clamp conditions at temperatures between 4 and 32 degrees C. 2. Single channel conductance was reduced linearly when temperature was decreased from 32 to 5 degrees C with a Q10 value of 1.5 between 10 and 20 degrees C. The apparent activation enthalpy for conductance between 32 and 5 degrees C was 6.16 +/- 1.2 kcal/mol. 3. Cooling the channel increased open probability (Po) when activating cytosolic calcium concentrations were varied within the range 0.1-100 microM. At an activating free calcium concentration of 10 microM, channel Po increased from 0.13 +/- 0.05 at 23 degrees C to 0.69 +/- 0.07 at 5-10 degrees C. 4. At sub-activating calcium concentrations (100 pM) or high concentrations of calcium (greater than or equal to 1000 microM), the calcium-release channel remained closed at 23 degrees C. Cooling the channel under these conditions did not increase Po. 5. Lifetime analysis indicates that with calcium as the sole activating ligand, the cooling-induced increase in Po results from an increase in channel open lifetimes with no significant alteration in the frequency of channel opening. At 23 degrees C, the open and closed lifetime distributions of the calcium-activated channel are best described by two and three exponentials respectively. At reduced temperatures (5-10 degrees C), both open and closed lifetime distributions were best described by three exponentials. 6. At sub-activating calcium concentrations, calcium-independent channel openings could be induced by sulmazole (AR-L 115 BS, 0.5-10 mM). At 23 degrees C, with sulmazole as the sole activating ligand, the best fits to both open and closed lifetime distributions were obtained with three exponentials. At reduced temperatures (5-10 degrees C), Po was increased. Gating was characterized by long open events, however the open and closed lifetime distributions were still best described by three exponentials. 7. The net effect of temperature reduction is an increase in calcium current through the channel. This finding is consistent with the suggestion that calcium release from the SR is a major factor in the initiation of rapid cooling contractures of mammalian cardiac muscle preparations.

Effects of hypoxia and metabolic inhibition on the intracellular sodium activity of mammalian ventricular muscle

1. Intracellular Na+ activity (aiNa) has been measured in Purkinje fibres from sheep heart and in ventricular muscle from rabbit heart during hypoxia and metabolic inhibition. The aiNa was measured using liquid sensor ion-sensitive microelectrodes. 2. Hypoxia, produced by replacement of O2 with N2 in the superfusate, produced an increase in aiNa. This increase was larger if sucrose replaced glucose in the superfusing Tyrode solution. The increase in aiNa was accompanied by a small depolarization. Upon reoxygenation aiNa decreased and cells rapidly repolarized. 3. When oxidative phosphorylation was inhibited by application of 2 mM-cyanide, aiNa increased. This increase was also accompanied by a small depolarization. Upon removal of cyanide, aiNa and membrane potential recovered to control levels. 4. After inhibiting glycolysis, by replacing glucose with 2-deoxy-D-glucose, inhibition of oxidative phosphorylation (by addition of cyanide or exposure to hypoxia) produced a much more rapid increase in aiNa and a large contracture. The rise in aiNa and the occurrence of a contracture could not be inhibited by application of amiloride (1 mM) or tetrodotoxin (1 microgram ml-1). Removal of cyanide or reoxygenation and replacement of glucose resulted in a rapid relaxation of the contracture and a slower decrease in aiNa. 5. The relative rates of increase in aiNa during metabolic inhibition were compared with the rate observed when Na+-K+-ATPase was inhibited by application of 10 mumols l-1 of the cardio-active steroid strophanthidin. The rate of increase of aiNa when both oxidative phosphorylation and glycolysis were inhibited was approximately twice that observed with only oxidative phosphorylation inhibited and approximately half that observed in the presence of 10 microM-strophanthidin. 6. Cyanide, applied when aiNa had been elevated (i.e. during exposure to 10 microM-strophanthidin to inhibit Na+-K+-ATPase), did not produce a contracture. The contracture observed in the presence of cyanide and 2-deoxy-D-glucose still occurred when Ca2+ was removed from the superfusate.

Effects of rest duration and ryanodine on changes of extracellular [Ca] in cardiac muscle from rabbits

Cumulative depletions of extracellular Ca were measured using double-barreled Ca-sensitive microelectrodes in the extracellular space of rabbit ventricular muscle. Depletions were produced by 1-Hz stimulation after rest intervals of 10 s to 10 min. With longer rest intervals, depletion size increased while the first postrest contraction decreased in a reciprocal manner. The depletions may represent refilling of sarcoplasmic reticulum (SR) Ca stores that have become partially depleted of Ca during the rest. Within this interpretive framework, the longer the rest interval the lower the SR Ca content, so the SR is then capable of taking up larger amounts of Ca. This may be related to the rest decay of tension of the first postrest beat, since this is thought to be SR dependent. Ryanodine (1 microM) increased the size of the depletions after short rest intervals (less than 2 min) but not after longer (greater than or equal to 2 min) intervals. Ryanodine also increased the rate of Ca loss from the cell on cessation of stimulation. This increased rate of Ca loss with ryanodine may deplete the SR of Ca such that more Ca can be taken up during subsequent stimulation than in untreated muscles. Thus cumulative depletions after short rest intervals are enhanced by ryanodine. When a Ca load was produced during 1) quiescence [by removal of extracellular Na (Nao)] or 2) continuous stimulation (in the presence of 3 microM acetylstrophanthidin), addition of ryanodine (5-10 microM) did not produce any apparent Ca loss. Caffeine (10 mM), added after ryanodine, induced contractures accompanied by Ca efflux, implying there was Ca in the SR after ryanodine exposure. The results of other investigators have suggested that ryanodine may inhibit cardiac SR Ca release. The present study suggests that ryanodine also enhances the loss of cellular (and probably SR) Ca on cessation of stimulation but not when applied during continuous stimulation or quiescence.

The mechanism of ryanodine action in rabbit ventricular muscle evaluated with Ca-selective microelectrodes and rapid cooling contractures

Cellular Ca uptake and efflux in rabbit ventricular muscle was measured using double-barreled Ca microelectrodes in the extracellular space. When repetitive stimulation was stopped there was a slow loss of cellular Ca. Upon resumption of stimulation Ca was taken up by the cells. These Ca movements are thought to represent the loss of Ca from the sarcoplasmic reticulum and the cell during rest and the refilling of the sarcoplasmic reticulum during stimulation. Ryanodine (100 nM) greatly enhanced both the efflux of Ca during rest and the uptake of Ca induced by stimulation. These results are consistent with the conclusions drawn below, but they are dependent upon the interpretation that these extracellular Ca depletions are indicative of sarcoplasmic reticulum Ca movements. To examine further this process, contractures induced by rapid cooling to 0 degrees C were used as an independent assay of sarcoplasmic reticulum Ca content. These rapid cooling contractures were smaller after longer rest intervals (declining with a half time of 1.5 min). In the presence of ryanodine, the rapid cooling contracture immediately after a contraction was greater than that seen under control conditions. However, in the presence of ryanodine these rapid cooling contractures decline as a function of rest duration with a half time of about 1 s. These results suggest that in the presence of ryanodine the sarcoplasmic reticulum can still take up Ca, but that it also loses this Ca very rapidly at the onset of rest.(ABSTRACT TRUNCATED AT 250 WORDS)

Cumulative depletions of extracellular calcium in rabbit ventricular muscle monitored with calcium-selective microelectrodes

Transient changes of extracellular free calcium in rabbit ventricular muscle under nonsteady state conditions were measured with double-barreled calcium microelectrodes. Resumption of stimulation after a rest interval produces a cumulative decrease of extracellular free calcium often by more than 10% (with bulk extracellular free calcium = 0.2 mM). The extracellular free calcium returns to the bulk value as a new steady state is achieved. The changes of extracellular free calcium recorded presumably represent net calcium uptake and loss by cardiac muscle cells. These cumulative extracellular free calcium depletions are blocked by 0.5 mM cobalt and 1 microM nifedipine and are increased to 167 +/- 11% of control by the calcium agonist Bay k 8644 (1 microM) and to 620 +/- 150% of control by increasing stimulus frequency from 0.2-2 Hz. Caffeine (10 mM) inhibits the cumulative extracellular free calcium depletions, probably by rendering the sarcoplasmic reticulum unable to accumulate calcium. It is proposed that the extracellular free calcium depletions recorded represent, in large part, calcium which has entered the cells and has been taken up by the sarcoplasmic reticulum (which had become depleted of calcium during the rest interval). Nifedipine and cobalt inhibit these cumulative depletions presumably by preventing the calcium entry which could subsequently be accumulated by the sarcoplasmic reticulum. The net cellular calcium uptake produced by such a post-rest stimulation protocol can also be inhibited by 1-3 microM acetylstrophanthidin and reduction of extracellular sodium to 70 mM. Acetylstrophanthidin and low extracellular sodium would be expected to shift the sodium-calcium exchange in favor of increased calcium uptake, which may, in turn, prevent the loss of sarcoplasmic reticulum calcium during the rest interval. This would limit the amount of calcium which the sarcoplasmic reticulum could take up with subsequent activation. In contrast to the results with caffeine, ryanodine (1 microM) increases the magnitude and rate of calcium uptake after a rest interval, indicative of a fundamental difference in the actions of caffeine and ryanodine. When stimulation is stopped in the presence of ryanodine, extracellular free calcium increases much faster than in control. This suggests that ryanodine may enhance calcium uptake by the sarcoplasmic reticulum during repetitive stimulation and may enhance calcium efflux from the sarcoplasmic reticulum during quiescence. These experiments provide insight into transsarcolemmal calcium movements and certain aspects of cellular calcium regulation.

Sodium-dependent control of intracellular pH in Purkinje fibres of sheep heart

Intracellular pH (pHi) of Purkinje fibres from sheep heart was recorded with pH-sensitive glass micro-electrodes. The cells were acidified by one of three methods: (1) exposure to and subsequent removal of NH4Cl, (2) exposure to solutions containing 5% CO2 or (3) exposure to an acidic Tyrode solution. The pHi recovery from these acidifications was studied. The time constant of recovery from an acidification induced by NH4Cl was almost twice as long as that from one induced by CO2 or acid extracellular pH. Following an acidification induced by exposure to CO2 the time constant of pHi recovery was not changed when the cell was depolarized to -40 mV (by replacement of some Na+ by K+). An intracellular acidification was produced when extracellular Na+ was removed and replaced by quaternary ammonium ions or K+. Such Na+-free solutions also inhibited pHi recovery from an acidification. A 50% inhibition of the rate of recovery was produced by lowering the [Na+]o to 8 mM. When used as a Na+ substitute, Li+ could permit recovery. Tris (22 mM) changed pHi in the alkaline direction. Amiloride (1 mM) or a decrease in temperature slowed the recovery from an acidification (Q10 = 2.65). There was no effect of SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid disodium salt; 100 microM) on the recovery. Na+-sensitive glass micro-electrodes were used to measure the intracellular Na+ activity when [Na+]o was lowered to levels used in our pHi recovery experiments. From these data we have calculated the apparent Na+ electrochemical gradient at different values of [Na+]o. If this gradient is responsible for H+ efflux from the cell then, by applying thermodynamic considerations, it can be shown that only low concentrations (1-2mM) of extracellular Na+ are required. Solutions containing a very low [Ca2+]o (less than 10(-8) M, buffered with EGTA) were used to prevent large rises of [Ca2+]i which may occur on removal of external Na+. Under these conditions pHi recovery is still dependent upon [Na+]o, and the apparent inhibition of pHi recovery by removal of Na+ is not simply due to rises in [Ca2+]i. The intracellular acidification which occurs on removal of Na+ does not occur when [Ca2+]o is very low (less than 10(-8) M).(ABSTRACT TRUNCATED AT 400 WORDS)