Alterations in intracellular calcium handling associated with the inverse force-frequency relation in human dilated cardiomyopathy

BACKGROUND

The present study was performed to test the hypothesis that the altered force-frequency relation in human failing dilated cardiomyopathy may be attributed to alterations in intracellular calcium handling.

METHODS AND RESULTS

The force-frequency relation was investigated in isometrically contracting ventricular muscle strip preparations from 5 nonfailing human hearts and 7 hearts with end-stage failing dilated cardiomyopathy. Intracellular calcium cycling was measured simultaneously by use of the bioluminescent photoprotein aequorin. Stimulation frequency was increased stepwise from 15 to 180 beats per minute (37 degrees C). In nonfailing myocardium, twitch tension and aequorin light emission rose with increasing rates of stimulation. Maximum average twitch tension was reached at 150 min-1 and was increased to 212 +/- 34% (P < .05) of the value at 15 min-1. Aequorin light emission was lowest at 15 min-1 and was maximally increased at 180 min-1 to 218 +/- 39% (P < .01). In the failing myocardium, average isometric tension was maximum at 60 min-1 (106 +/- 7% of the basal value at 15 min-1, P = NS) and then decreased continuously to 62 +/- 9% of the basal value at 180 min-1 (P < .002). In the failing myocardium, aequorin light emission was highest at 15 min-1. At 180 min-1, it was decreased to 71 +/- 7% of the basal value (P < .01). Including both failing and nonfailing myocardium, there was a close correlation between the frequencies at which aequorin light emission and isometric tension were maximum (r = .92; n = 19; P < .001). Action potential duration decreased similarly with increasing stimulation frequencies in nonfailing and end-stage failing myocardium. Sarcoplasmic reticulum 45Ca2+ uptake, measured in homogenates from the same hearts, was significantly reduced in failing myocardium (3.60 +/- 0.51 versus 1.94 +/- 0.18 (nmol/L).min-1.mg protein-1, P < .005).

CONCLUSIONS

These data indicate that the altered force-frequency relation of the failing human myocardium results from disturbed excitation-contraction coupling with decreased calcium cycling at higher rates of stimulation.

Sarcoplasmic reticulum and myofilament function in chemically-treated ventricular trabeculae from patients with heart failure

OBJECTIVES

Assessment of sarcoplasmic reticulum calcium-loading ability, myofilament force production and myofilament calcium sensitivity in ventricular trabeculae from patients with heart failure.

METHODS

Right ventricular trabeculae (diameter 150-250 microns) were obtained from 18 patients undergoing elective cardiac transplantation. These were mounted for isometric tension measurement and treated with saponin which permeabilises the sarcolemma leaving the sarcoplasmic reticulum (SR) functionally intact. The trabecula was bathed in a mock intracellular solution containing ATP and weakly buffered [Ca2+] at various concentrations (150-400 nM). The amplitude of caffeine-induced contractures was used as a quantitative measure of the SR calcium content and was correlated with the clinical severity of heart failure. The same trabecula was then exposed to a solution containing Triton-X100 (1%) which destroys all cell membranes leaving only the myofilaments intact. The maximum calcium-activated force (Cmax) and myofilament responsiveness to calcium was assessed.

RESULTS

Patients with ischaemic heart disease (IHD) and severe heart failure (PCWP > 20 mm Hg, ejection fraction < 15%, n = 8) demonstrated low SR Ca(2+)-loading ability compared with patients with IHD and moderate heart failure (PCWP-20 mmHg, LV ejection fraction > 20%, n = 6). Patients with dilated cardiomyopathy (DCM) (n = 4) demonstrated SR Ca(2+)-loading ability which was lower than either of the two IHD groups. Myofilament force production (per unit cross-sectional area) was not significantly different between the three groups. Myofilament responsiveness to Ca2+ demonstrated no relationship with severity of heart failure.

CONCLUSIONS

In human heart failure, SR Ca(2+)-loading ability diminishes with increasing severity of heart failure. Myofilament force production and sensitivity to calcium are unaffected by severity of heart failure.

Conversion between permeability states of IP3 receptors in cultured smooth muscle cells

The kinetics of the effect of inositol 1,4,5-trisphosphate (IP3) on Ca2+ in the sarcoplasmic reticulum (SR) were studied in saponin-permeabilized A7r5 cells. At 0.1 microM, IP3 elicited slow monoexponential declines in SR free Ca2+ concentration ([Ca2+]SR). For IP3 concentration ([IP3]) = 0.2-100 microM, evoked declines in [Ca2+]SR were biphasic and best fit as the sum of two first-order processes with rate constants kfast and kslow. The kfast varied as a function of [IP3] over the range tested, whereas kslow was already maximal when [IP3] = 0.1 microM. To analyze SR Ca2+ release elicited by IP3, the rate constants for IP3-induced changes in the total SR Ca2+ content (kR) were calculated. kR was accurately described only when both [Ca2+]SR and [IP3] were considered together. kR was dependent on IP3 binding to receptors that existed in either of two states, a high-affinity low-conductance state (IP3RH) and a low-affinity high-conductance state (IP3RL). The permeability of IP3RL was 12.28 times larger than that of IP3RH, and the conversion between permeability states as well as changes in both the affinity and cooperativity with which IP3 was bound to IP3RL were mediated by SR Ca2+. This SR Ca(2+)-dependent modulation of the characteristics of IP3 receptors forms the basis for the biphasic time course characteristic of IP3-evoked SR Ca2+ release.

Effects of peroxide and superoxide on coronary artery: ANG II response and sarcoplasmic reticulum Ca2+ pump

The sarcoplasmic reticulum (SR) Ca2+ pump in membranes isolated from arterial smooth muscle is damaged by reactive oxygen species (ROS). Because angiotensin II (ANG II) contracts arterial smooth muscle by mobilizing intracellular Ca2+ concentrations ([Ca2+])i, we determined the effects of ROS pretreatment on ANG II-induced contractions in coronary artery rings and [Ca2+]i transients in smooth muscle cells (SMC) cultured from them. This experimental design eliminates direct ROS interference in assay solutions, thus monitoring only the tissue damage. Pretreating the arteries with peroxide inhibited the ANG II contractions with the concentration for half-maximal activation (K0.5) = 74 +/- 5 microM. Peroxide (250 microM) inhibited the contractions to ANG II and cyclopiazonic acid (CPA, SR Ca(2+)-pump inhibitor) by 78.3 +/- 5.1 and 67.4 +/- 6.3%, respectively, but did not significantly affect the contractions by 60 mM KCl. Pretreating SMC with peroxide inhibited the ANG II-induced increase in [Ca2+]i with K0.5 = 24 +/- 3 microM for peroxide. Peroxide (100 microM) inhibited the increase in [Ca2+]i in response to ANG II and CPA by 78.9 +/- 5.1 and 38.3 +/- 4.9%, respectively. The SR Ca(2+)-pump activity was also measured as the Ca(2+)-dependent formation of 115-kDa acylphosphate. Pretreating SMC with 100 microM peroxide inhibited the acylphosphate levels by 36.3 +/- 3.2%. Peroxide (100 microM) pretreatment of SMC did not significantly affect their ANG II binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of ischemia and ischemia--reperfusion on ryanodine binding and Ca2+ uptake of cardiac sarcoplasmic reticulum

The effect of 15 min of global, normothermic ischemia on 3H-ryanodine binding and the oxalate-supported Ca2+ uptake of cardiac sarcoplasmic reticulum (SR) was investigated in parallel using ventricular homogenates of isolated perfused rat hearts. Ischemia increased the Ca2+ efflux under the uptake assay conditions, as demonstrated by the greater stimulation of Ca2+ uptake by high concentrations of ryanodine (+RY) to close the SR Ca2+ channel. This effect was partially reversed by reperfusion. Ischemia depressed Ca2+ uptake rate -RY at free [Ca2+] of 0.4 microM and above, while the depression + RY was significant only above 10 microM Ca2+. We tested the hypothesis that inhibition of the Ca-ATPase alone, by adding thapsigargin or cyclopiazonic acid, could reproduce the effects of ischemia on the homogenate Ca2+ uptake rate. Thapsigargin or cyclopiazonic acid proportionally depressed Ca2+ uptake rate +RY and -RY and produced distinctly different effects of ischemia. Ischemia did not change the Bmax or Kd for equilibrium 3H-ryanodine binding, or the Hill coefficient or KCa for the [Ca2+]-dependence of equilibrium 3H-ryanodine binding. The rate of ryanodine binding, measured under the uptake conditions, was increased by ischemia and further increased by reperfusion. The effect of ischemia on the rate and extent of equilibrium binding to the high-affinity ryanodine binding site were unrelated to the highly reproducible effects on SR Ca2+ uptake rates measured in the homogenate.

beta-Adrenergic stimulation induces acetylcholine to activate ATP-sensitive K+ current in cat atrial myocytes

Our previous work on atrial myocytes suggested that the effect of acetylcholine (ACh) to increase K+ conductance can be potentiated by prior loading of the sarcoplasmic reticulum (SR) with Ca2+. The present study, therefore, sought to determine whether prior exposure to isoproterenol (ISO) potentiates ACh-induced increases in K+ conductance and the underlying mechanisms. A nystatin-perforated patch whole-cell configuration was used to record from cat atrial myocytes. Voltage-clamp ramps (40 mV/s) were used to assess total membrane conductance. The experimental protocol consisted of two consecutive 30-second ACh exposures (ACh1 and ACh2) separated by a 6-minute recovery period in ACh-free solution. In general, experimental interventions, such as exposure to ISO, were imposed during the period between ACh1 and ACh2 to determine their effects on the response to ACh2 in relation to ACh1. Under control conditions, K+ conductances induced by ACh1 and ACh2 were not different from one another with or without activation of L-type Ca2+ current (ICa,L) during the recovery period. When 1 mumol/L ISO plus ICa,L activation was imposed during the recovery period, ACh2 induced a significantly larger increase in K+ conductance than ACh1. The ACh2-induced K+ current potentiated by ISO was time independent and selectively blocked by 10 mumol/L glibenclamide and therefore identified as ATP-sensitive K+ current (IK,ATP). The effect of ISO to induce ACh2-activated IK,ATP was mimicked by 1 mumol/L forskolin or 200 mumol/L 8-(4-chlorophenylthio)-cAMP, but not by 0.5 mumol/L BAY K 8644, and was selectively abolished by (1) 5 mumol/L thapsigargin or 1 mumol/L ryanodine, agents that prevent accumulation of SR Ca2+, (2) inhibition of L-type Ca2+ current (ICa,L) by 1 mumol/L nisoldipine or zero external Ca2+, (3) 50 mumol/L Rp-cAMPs, an inhibitor of cAMP-dependent protein kinase A, or (4) 2 mumol/L propranolol. Atropine (1 mumol/L) abolished all ACh-induced currents. Moreover, ACh2-activated IK,ATP was selectively blocked by 0.2 mumol/L pirenzepine, an M1 muscarinic receptor antagonist, or 0.1 mumol/L calphostin C, a selective inhibitor of protein kinase C. AFDX116 (100 mumol/L), an M2 muscarinic receptor antagonist, blocked the conventional ACh-activated K+ current and revealed ACh2-activated IK,ATP. These results indicate that prior exposure to ISO potentiates ACh-induced increases in K+ current via ACh-activated IK,ATP.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of caffeine on intracellular calcium, force and the rate of relaxation of mouse skeletal muscle

1. Intracellular calcium concentration ([Ca2+]i) and force were measured from isolated single fibres of mouse skeletal muscle. The effects of 5 mM caffeine on muscle fibres at rest and during short tetani were examined. 2. Caffeine increased tetanic tension and slowed the rate of relaxation. [Ca2+]i was increased in the presence of caffeine both in the resting muscle and during tetani. The time course of decline of [Ca2+]i after a tetanus is complex with a large, early, rapid phase followed by a smaller and slower phase. Caffeine accelerated the early phase but slowed the later phase. 3. The sensitivity of the myofibrillar proteins to Ca2+ measured in the intact fibre was increased in the presence of caffeine, confirming earlier findings on skinned muscle fibres. 4. Analysis of the late phase of the decline of [Ca2+]i after a tetanus provides information about the properties of the sarcoplasmic reticulum (SR) Ca2+ pump. Caffeine slowed the pump to 60-70% of the control value at a given [Ca2+]i but had no effect on the Ca2+ leak from the SR. 5. Analysis of relaxation made use of the Ca(2+)-derived force in which the [Ca2+]i during relaxation was converted to the Ca(2+)-derived force by means of the steady-state relation between [Ca2+]i and force. The Ca(2+)-derived force fell more slowly in the presence of caffeine but the lag between Ca(2+)-derived force and measured force was unaffected. Thus, the slowed relaxation was caused by changes in Ca2+ handling and not by slowed cross-bridge kinetics. 6. A model of the Ca2+ movements and force production of muscle was used to examine independently the effects of increased Ca2+ sensitivity, slowing of the SR Ca2+ pump and increased SR Ca2+ permeability. The effects of caffeine on [Ca2+]i, tetanic force and relaxation could be explained by a combination of these three effects.

The force-interval relationship in human myocardium

The force-interval relationship is an important modulator of contractility in mammalian myocardium. In a number of mammalian species, increasing the frequency of stimulation results in an increase in force of contraction. Over the last 10 years, the effects of atrial pacing have been closely examined in normal human subjects and in patients with dilated cardiomyopathy, and the effects of the stimulation frequency have been investigated in isolated preparations from nonfailing and failing human hearts. An abnormal force-interval relationship in vivo and in vitro has been a consistent finding in patients with dilated cardiomyopathy, whereby an increase in stimulation frequency fails to increase the contractile response. The force-interval relationship of cardiac muscle has been shown to reflect intracellular calcium cycling and sarcoplasmic reticulum function. Therefore, agents that affect excitation-contraction coupling, in particular intracellular calcium mobilization and sarcoplasmic reticulum function, modulate the response of contraction force to stimulation frequency.

Cytoplasmic Ca2+ inhibits the ryanodine receptor from cardiac muscle

Ca(2+)-dependent inhibition of native and isolated ryanodine receptor (RyR) calcium release channels from sheep heart and rabbit skeletal muscle was investigated using the lipid bilayer technique. We found that cytoplasmic Ca2+ inhibited cardiac RyRs with an average Km = 15 mM, skeletal RyRs with Km = 0.7 mM and with Hill coefficients of 2 in both isoforms. This is consistent with measurements of Ca2+ release from the sarcoplasmic reticulum (SR) in skinned fibers and with [3H]-ryanodine binding to SR vesicles, but is contrary to previous bilayer studies which were unable to demonstrate Ca(2+)-inhibition in cardiac RyRs (Chu, Fill, Stefani & Entman (1993) J. Membrane Biol. 135, 49-59). Ryanodine prevented Ca2+ from inhibiting either cardiac or skeletal RyRs. Ca(2+)-inhibition in cardiac RyRs appeared to be the most fragile characteristic of channel function, being irreversibly disrupted by 500 mM Cs+, but not by 500 mM K+, in the cis bath or by solublization with the detergent CHAPS. These treatments had no effect on channel regulation by AMP-PNP, caffeine, ryanodine, ruthenium red, or Ca(2+)-activation. Ca(2+)-inhibition in skeletal RyRs was retained in the presence of 500 mM Cs+. Our results provide an explanation for previous findings in which cardiac RyRs in bilayers with 250 mM Cs+ in the solutions fail to demonstrate Ca(2+)-inhibition, while Ca(2+)-inhibition of Ca2+ release is observed in vesicle studies where K+ is the major cation. A comparison of open and closed probability distributions from individual RyRs suggested that the same gating mechanism mediates Ca(2+)-inhibition in skeletal RyRs and cardiac RyRs, with different Ca2+ affinities for inhibition. We conclude that differences in the Ca(2+)-inhibition in cardiac and skeletal channels depends on their Ca2+ binding properties.

Role of calcium feedback in excitation-contraction coupling in isolated triads

There is a considerable controversy in the literature concerning the effects of higher concentrations of calcium chelators (e.g. BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) or fura-2) on the intracellular Ca2+ transients in muscle. We induced calcium release from sarcoplasmic reticulum (SR) in the triad preparation by chemical depolarization of the T-tubule in the presence of various concentrations of BAPTA-calcium buffer ([Ca2+] = 0.1 microM) and investigated the effects of the BAPTA concentration on the time courses of conformational changes in the junctional foot protein (JFP) and calcium release from SR. Upon stimulation, the JFP underwent biphasic conformational changes, as determined by stopped-flow fluorometry of the JFP-bound conformational probe. The first phase of protein conformational change, which preceded calcium release from SR, was virtually unaffected by the BAPTA concentration. However, the magnitude of the second phase increased in an inversely proportional fashion to the BAPTA concentration. An abrupt increase in [Ca2+] from 0.1 microM up to 1.0 microM (delta Ca2+), concurrently with T-tubule depolarization, produced biphasic protein conformational changes: a delta Ca(2+)-independent first phase and a delta Ca(2+)-dependent second phase. Similar Ca2+ jump experiments under non-depolarizing conditions produced a slow monophasic conformational change equivalent to the second phase described above. These results suggest that the first phase of protein conformational change represents the activation of JFP by T-tubule depolarization to induce calcium release, and the second phase the secondary activation by the released Ca2+. Activation of the JFP by the released Ca2+ resulted in an acceleration of both (i) the rate of initial calcium release, and (ii) the subsequent attenuation of calcium release. The acceleration of both was suppressed by higher concentrations of BAPTA. These results provide a reasonable explanation for both of the apparently contradictory views in the literature; high concentrations of calcium buffer (a) suppress the initial activation and (b) prevent the subsequent attenuation of calcium release.

Alterations of sarcoplasmic reticulum proteins in failing human dilated cardiomyopathy

BACKGROUND

Previous studies provide considerable evidence that excitation-contraction coupling may be disturbed at the level of the sarcoplasmic reticulum (SR) in the failing human heart. Disturbed SR function may result from altered expression of calcium-handling proteins.

METHODS AND RESULTS

Levels of SR proteins involved in calcium release (ryanodine receptor), calcium binding (calsequestrin, calreticulin), and calcium uptake (calcium ATPase, phospholamban) were measured by Western blot analysis in nonfailing human myocardium (n = 7) and in end-stage failing myocardium due to dilated cardiomyopathy (n = 14). The levels of the ryanodine receptor, calsequestrin, and calreticulin were not significantly different in nonfailing and failing human myocardium. Phospholamban protein levels (pentameric form) normalized per total protein were decreased by 18% in the failing myocardium (P < .05). However, phospholamban protein levels were not significantly different in failing and nonfailing myocardium when normalization was performed per calsequestrin. Protein levels of SR calcium ATPase, normalized per total protein or per calsequestrin, were decreased by 41% (P < .001) or 33% (P < .05), respectively, in the failing myocardium. Furthermore, SR calcium ATPase was decreased relative to ryanodine receptor by 37% (P < .05) and relative to phospholamban by 28% (P < .05).

CONCLUSIONS

Levels of SR proteins involved in calcium binding and release are unchanged in failing dilated cardiomyopathy. In contrast, protein levels of calcium ATPase involved in SR calcium uptake are reduced in the failing myocardium. Moreover, SR calcium ATPase is decreased relative to its inhibitory protein, phospholamban.(ABSTRACT TRUNCATED AT 250 WORDS)

Lumenal Ca2+ dissociation from the phosphorylated Ca(2+)-ATPase of the sarcoplasmic reticulum is sequential

Once two radioactive Ca2+ coming from the cytoplasm are bound to the transport sites of the nonphosphorylated ATPase, excess EGTA induces rapid dissociation of both ions, whereas excess nonradioactive Ca2+ only reaches one of the two bound Ca2+. This difference has been explained assuming that the two Ca2+ sites are in a single file channel in which the superficial Ca2+ is freely exchangeable from the cytoplasm, whereas the deeper Ca2+ is exchangeable only when the superficial site is vacant. The same experiment was done using phosphorylated ATPase to determine whether Ca2+ dissociation toward the lumen is sequential as well. Under conditions that allow ADP-sensitive phosphoenzyme to accumulate (leaky vesicles, 5 degrees C, pH 8, 300 mM KC1), we found the same two pools of Ca2+. Excess EGTA induced dissociation of both ions together with dephosphorylation. Excess nonradioactive Ca2+ induced the exchange of half the radioactive Ca2+ without any effect on the phosphoenzyme level. Our results show a close similarity between the transport sites of the nonphosphorylated and the phosphorylated enzymes, although the orientation, affinities, and dissociation rate constants are different.

Identification of dantrolene binding sites in porcine skeletal muscle sarcoplasmic reticulum

Dantrolene, an intracellularly acting skeletal muscle relaxant, inhibits Ca2+ release from the sarcoplasmic reticulum during excitation-contraction coupling by an unknown mechanism. The drug is used to treat malignant hyperthermia, a genetic sensitivity to volatile anesthetics which results in the massive release of intracellular Ca2+ from affected skeletal muscle. We hypothesize that determination of the site of action of dantrolene will lead to further understanding of the regulation of sarcoplasmic reticulum calcium release. We report the identification of specific dantrolene binding sites in porcine skeletal muscle sarcoplasmic reticulum using a rapid filtration binding assay for [3H]dantrolene. The binding isotherm in the heavy sarcoplasmic reticulum fraction indicates a single binding site with a Kd of 277 +/- 25 nM and a Bmax of 13.1 +/- 1.5 pmol/mg of protein. Pharmacological specificity is characterized by inhibition of [3H]dantrolene binding with unlabeled dantrolene, or azumolene, a physiologically active congener, but not with aminodantrolene, which is physiologically inactive. Drug binding is maximal at pH 6.5-7.5, requires no Ca2+ or Mg2+, and is inhibited by salt concentrations above 100 mM. [3H]Dantrolene binding is greatest in the sarcoplasmic reticulum, which contains the ryanodine receptor, the primary calcium release channel. No binding is detected in the fractions enriched for sarcolemma or transverse tubules. We suggest that dantrolene inhibits calcium release from the sarcoplasmic reticulum by either direct or indirect interaction with the ryanodine receptor.

Phosphorylation of dihydropyridine receptor II-III loop peptide regulates skeletal muscle calcium release channel function. Evidence for an essential role of the beta-OH group of Ser687

In vertebrate skeletal muscle, excitation-contraction coupling may occur by a mechanical coupling mechanism involving protein-protein interactions between the dihydropyridine receptor (DHPR) of the transverse tubule membrane and the ryanodine receptor (RYR)/Ca2+ release channel of the sarcoplasmic reticulum membrane. We have previously shown that the cytoplasmic II-III loop peptides of the skeletal and cardiac muscle DHPR alpha 1 subunits (SDCL and CDCL, respectively) activate the skeletal muscle RYR. We now report that cyclic AMP-dependent protein kinase-mediated phosphorylation of Ser687 of SDCL yields a peptide that fails to activate the RYR, as determined in [3H]ryanodine binding and single channel measurements. The phosphorylated SDCL bound to the skeletal muscle but not cardiac muscle RYR, and the binding could be displaced by the unphosphorylated SDCL. A mutant SDCL with a Ser687-->Ala substitution failed to activate the RYR, but was still able to bind. Similarly, a Ser813-->Ala substitution in CDCL yielded a peptide that failed to activate the skeletal RYR. Use of three smaller overlapping peptides within the SDCL region identified an amino acid region from 666 to 726 including Ser687, which bound to and activated the skeletal muscle RYR. These results suggest that cyclic AMP-dependent protein kinase-mediated phosphorylation of the DHPR alpha 1 subunit may play a role in the functional interaction of the DHPR and RYR in skeletal muscle.

[Impaired calcium uptake by cardiac sarcoplasmic reticulum and its underlying mechanism during rat septic shock]

The underlying mechanism of Ca2+ uptake function of cardiac sarcoplasmic reticulum (SR) was investigated in the rat septic shock model produced by cecal ligation and puncture (CLP). The results are as follows. During the early phase of sepsis, the initial rate of ATP-dependent Ca2+ uptake by SR was decreased, while both the capacity of Ca2+ uptake and the activity of Ca(2+)-ATPase were unaffected. In the late sepsis, the impairment in SR function was even greater as the initial rate and the capacity of Ca2+ uptake by SR were significantly decreased, and this was paralleled by a reduction in Ca(2+)-ATPase activity. Although Ca2+ affinity (Km value) to calcium pump and the A0.5 values for Mg2+ and ATP activation on the Ca2+ uptake rate were unchanged, during sepsis the phosphorylation of SR vesicles by adding of catalytic subunit of the cAMP-dependent protein kinase (PKA), calmodulin, or the fragment of PKC into Ca2+ uptake buffer, failed to stimulate Ca2+ uptake activities of SR isolated from early or late septic rats. These data suggest that depression of cardiac SR function is aggravated as sepsis develops, the impairment of SR Ca2+ uptake is possibly based on a mechanism of defective phosphorylation of SR rather than the ionic and energic regulatory actions of Ca2+, Mg2+, ATP on cardiac SR.

Calcium inactivation of calcium release in frog cut muscle fibers that contain millimolar EGTA or Fura-2

Cut muscle fibers from Rana temporaria (sarcomere length, 3.4-4.2 microns) were mounted in a double Vaseline-gap chamber (14-15 degrees C) and equilibrated with end-pool solutions that contained 20 mM EGTA and 1.76 mM Ca. Sarcoplasmic reticulum (SR) Ca release was estimated from changes in pH (Pape, P. C., D.-S. Jong, and W.K. Chandler. 1995. Journal of General Physiology. 106:000-000). Although the amplitude and duration of the [Ca] transient, as well as its spatial spread from the release sites, are reduced by EGTA, SR Ca release elicited by either depolarizing voltage-clamp pulses or action potentials behaved in a manner consistent with Ca inactivation of Ca release. After a step depolarization to -20 or 10 mV, the rate of SR Ca release, corrected for SR Ca depletion, reached a peak value within 5-15 ms and then rapidly decreased to a quasi-steady level that was about half the peak value; the time constant of the last half of the decrease was usually 2-4 ms. Immediately after an action potential or a 10-15 ms prepulse to -20 mV, the peak rate of SR Ca release elicited by a second stimulation, as well as the fractional amount of release, were substantially decreased. The rising phase of the rate of release was also reduced, suggesting that at least 0.9 of the ability of the SR to release Ca had been inactivated by the first stimulation. There was little change in intramembranous charge movement, suggesting that the changes in SR Ca release were not caused by changes in its voltage activation. These effects of a first stimulation on the rate of SR Ca release elicited by a second stimulation recovered during repolarization to -90 mV; the time constant of recovery was approximately 25 ms in the action-potential experiments and approximately 50 ms in the voltage-clamp experiments. Fura-2, which is able to bind Ca more rapidly than EGTA and hence reduce the amplitude of the [Ca] transient and its spatial spread from release sites by a greater amount, did not prevent Ca inactivation of Ca release, even at concentrations as large as 6-8 mM. These effects of Ca inactivation of Ca release can be simulated by the three-state, two-step model proposed by Schneider, M. F., and B. J. Simon (1988, Journal of Physiology. 405:727-745), in which SR Ca channels function as a single uniform population of channels. (ABSTRACT TRUNCATED AT 400 WORDS)

Reciprocal changes in the postnatal expression of the sarcolemmal Na+-Ca(2+)-exchanger and SERCA2 in rat heart

The aim of this study was to examine the relationship between sarcolemmal Na(+)-Ca2+ exchangers and sarcoplasmic reticulum (SR) Ca(2+) -ATPase (SERCA2) expression and the developmental differences in cardiac Ca2+ handling. Postnatal steady-state mRNA and protein levels were analysed in rat ventricular myocardium by Northern and immunoblot analysis, respectively. This was compared to Na+ gradient-induced and SR oxalate-supported Ca2 transport in isolated membranes. Na(+)-Ca2+ exchanger mRNA declined by 75% between day 1 and 30, whereas SR Ca2+ ATPase mRNA levels increased by 97% during this period. The Na(+)-Ca2+ exchanger mRNA/Ca(2+)-ATPase mRNA ratio was found to be inversely related to post-natal age. The changes in mRNA levels were associated with corresponding developmental differences in the Ca2+ transport activities of the respective membrane proteins. In crude membranes, the Na(+)-dependent Ca2+ transport activity (at 75 microM Ca2+) declined gradually (P < 0.01; mean +/- S.E.) from 17.7 +/- 2.4 nmoles Ca2+/g wet tissue/2s at day 1-3 (n = 5) to a value of 4.2 +/- 1.1 at day 40 (n =4). Conversely, SR Ca2+ uptake increased (P < 0.01) 2.6-fold during this period. The inversely related changes in the post-natal expression and function of the Na(+)-Ca2+ exchanger and SR Ca(2+)-ATPase suggest a coordinated control at the pretranslational level of the cellular Ca2+ transport processes mediated by the two membrane proteins.

Calcium release and its voltage dependence in frog cut muscle fibers equilibrated with 20 mM EGTA

Sarcoplasmic reticulum (SR) Ca release was studied at 13-16 degrees C in cut fibers (sarcomere length, 3.4-3.9 microns) mounted in a double Vaseline-gap chamber. The amplitude and duration of the action-potential stimulated free [Ca] transient were reduced by equilibration with end-pool solutions that contained 20 mM EGTA with 1.76 mM Ca and 0.63 mM phenol red, a maneuver that appeared to markedly reduce the amount of Ca complexed by troponin. A theoretical analysis shows that, under these conditions, the increase in myoplasmic free [Ca] is expected to be restricted to within a few hundred nanometers of the SR Ca release sites and to have a time course that essentially matches that of release. Furthermore, almost all of the Ca that is released from the SR is expected to be rapidly bound by EGTA and exchanged for protons with a 1:2 stoichiometry. Consequently, the time course of SR Ca release can be estimated by scaling the delta pH signal measured with phenol red by -beta/2. The value of beta, the buffering power of myoplasm, was determined in fibers equilibrated with a combination of EGTA, phenol red, and fura-2; its mean value was 22 mM/pH unit. The Ca content of the SR (expressed as myoplasmic concentration) was estimated from the total amount of Ca released by either a train of action potentials or a depleting voltage step; its mean value was 2,685 microM in the action-potential experiments and 2,544 microM in the voltage-clamp experiments. An action potential released, on average, 0.14 of the SR Ca content with a peak rate of release of approximately 5%/ms. A second action potential, elicited 20 ms later, released only 0.6 times as much Ca (expressed as a fraction of the SR content), probably because Ca inactivation of Ca release was produced by the first action potential. During a depolarizing voltage step to 60 mV, the rate of Ca release rapidly increased to a peak value of approximately 3%/ms and then decreased to a quasi-steady level that was only 0.6 times as large; this decrease was also probably due to Ca inactivation of Ca release. SR Ca release was studied with small step depolarizations that open no more than one SR Ca channel in 7,000 and increase the value of spatially averaged myoplasmic free [Ca] by only 0.2 nM.

[Altered ryanodine receptor of rat cardiac sarcoplasmic reticulum and its underlying mechanism during septic shock]

The present study was undertaken to observe the changes of Ryanodine receptor of cardiac junctional sarcoplasmic reticulum (SR) in relation to membrane lipid microenvironment alteration during septic shock. The results showed that the Bmax for 3H-ryanodine binding to cardiac junctional SR was decreased by 41.3% (3.9 +/- 0.1 vs. sham 6.6 +/- 0.7 pmol/mg, P < 0.01) while the Kd value was unaffected during late septic shock (CLP 18 h). Ca2+ activated 3H-ryanodine binding significantly and reached a saturation value when Ca2+ concentration was 5 x 10(-5) mol/L, while the S0.5 and the Hill coefficient values remained unchanged during septic shock. Caffeine, ATP, and AMP-PCP activated while Mg2+, ruthenium red inhibited 3H-ryanodine binding in both groups but the A0.5 (concentration requires for half maximum activation) and the IC50 (concentration requires for half-maximum inhibition) for the above mentioned activators and inhibitors, were respectively unaffected during septic shock. Digestion of cardiac SR isolated from control rats with phospholipase A2 inhibited 3H-ryanodine binding, which could be dramatically recovered by the incorporation of phosphatidylcholine (PC), or phosphatidylserine (PS), or phosphatidylethanolamine (PE) into the isolated cardiac SR. Incorporation of above phospolipids into SR isolated from septic rats reversed shock-induced inhibition of 3H-ryanodine binding. It is concluded that the mechanism responsible for the inhibition of 3H-ryanodine binding of junctional SR during septic shock may be related to modification of membrane lipid microenvironment in response to PLA2 overactivation during septic shock.

Effect of the pyrethroid insecticide allethrin on membrane fluidity

Allethrin is a widely used pyrethroid insecticide with an alkenylmethylcyclopentenolone group in its structure. We have analyzed its interaction with model and native membranes using DPH and its polar derivative TMA-DPH fluorescence polarization. Allethrin modified the bilayer order in the temperature range of the phase transition when incorporated into liposomes made with dimyristoyl-(DMPC), dipalmitoyl-(DPPC) and distearoyl-(DSPC) phosphatidylcholine. In DMPC: allethrin mixtures the pyrethroid decreased the bilayer order in the gel phase, without altering the liquid-crystalline one. In native membranes, DPH and TMA-DPH fluorescence polarization remained unchanged after incubation with allethrin. The release of hemoglobin was notably facilitated by the incorporation of allethrin into human erythrocytes. The results are discussed in terms of a possible aggregation of the insecticide in the lipid bilayer to create special domains with a consequent increase in membrane instability.

Intracellular calcium transients underlying interval-force relationship in whole rat hearts: effects of calcium antagonists

OBJECTIVES

Much of the understanding about the cardiac interval-force relationship of the whole heart, including mechanical restitution and postextrasystolic potentiation (PESP), has been inferred from isolated muscle studies. We tested whether results from isolated muscles about intracellular Ca2+([Ca2+]i) transients underlying the interval-force relationship can be substantiated in whole hearts. Additionally, we investigated whether Ca2+ antagonists could alter [Ca2+]i transients underlying mechanical restitution and postextrasystolic potentiation.

METHODS

[Ca2+]i transients were studied in isolated perfused rat hearts by surface fluorometry and Indo-1. Using computer-controlled pacing protocols, we performed restitution curves for left ventricular developed pressure and [Ca2+]i (developed pressure and [Ca2+]i plotted as a function of extrasystolic intervals). To quantify restitution curves, we fitted monoexponential functions to plots and analyzed their shift and slope. Then, we used Ca2+ antagonists, low extracellular Ca2+([Ca2+]o) and PESP to modify restitution curves. [Ca2+]i transients in isolated rat hearts were interpreted as Ca2+ released from the sarcoplasmic reticulum.

RESULTS

Interval-dependent changes in developed pressure were strongly correlated to interval-dependent changes in the amplitude of [Ca2+]i transients in isolated whole rat hearts. Additionally, nifedipine and low [Ca2+]o led to similar downward shifts but not to a changed slope of restitution curves for [Ca2+]i. On the other hand, PESP increased the slope of restitution curves for [Ca2+]i. Furthermore, the effect of PESP on developed pressure was blunted by high concentrations of Ca2+ antagonists.

CONCLUSIONS

The results from isolated muscles about [Ca2+]i transients underlying the interval-force relationship could be substantiated in whole hearts. Additionally, low [Ca2+]i (induced by nifedipine or low [Ca2+]o) decreased the maximal Ca2+ release of the sarcoplasmic reticulum but did not change the release kinetics. On the other hand, PESP presumably accelerated Ca2+ release kinetics of the sarcoplasmic reticulum.

A novel method for incorporation of ion channels into a planar phospholipid bilayer which allows solution changes on a millisecond timescale

We have developed a method of rapidly changing the solutions on one side of a planar phospholipid bilayer. Bilayers can be painted on glass pipettes of tip diameter > or = 50 microns. By modifying an established method for rapid exchange of solutions bathing excised membrane patches, solution changes can be made at the bilayer within 10 ms. After incorporation of channels into the bilayer, the bilayer is moved into one of two parallel streams of solution flowing from a length of double-barrelled glass theta tubing. Activation of a solenoid system rapidly moves the theta tubing so that the bilayer is in the flow of the adjacent solution. For various reasons, the single-channel gating mechanisms of many channels are studied in planar bilayer systems. The conventional bilayer technique only allows for steady-state single-channel gating to be monitored. This novel method now allows the effects of rapid changes in modulators of channels incorporated into planar phospholipid bilayers to be measured.

Examination of the role of phosphorylation and phospholamban in the regulation of the cardiac sarcoplasmic reticulum Cl- channel

Sarcoplasmic reticulum (SR) vesicles were prepared from either canine or sheep heart and fused into lipid bilayers to study their ionic channels. A 92 +/- 5 pS anion-selective channel was recorded in asymmetric 50 mM trans/250 mM cis CsCl buffer system. Reversal potentials and theoretical equilibrium potentials for Cl-ions obtained under various experimental conditions allowed us to confirm the Cl- selectivity of this SR channel. The majority (69%) of channel recordings (n = 45) displayed steady-state kinetics and a slight voltage dependency of the open probability. However, 31% of the channels inactivated after their incorporation. We now report that the channel might be reactivated by depolarizing voltage steps. Furthermore, the use of either PKA or PKG in association with adequate phosphorylating buffers lengthens the deactivation process at the end of the voltage pulses, but does not prevent the inactivation. It was assumed that the change in gating mode was due to a voltage-sensitive association/dissociation mechanism with a phosphorylated protein of the SR membrane such as phospholamban (PL). We demonstrated that a specific monoclonal antibody raised against canine PL inhibited the activity of the channel and prevented its reactivation by depolarizing steps. 400 to 800 ng/ml of Anti-PL Ab consistently and sequentially turned off the channel activities. In contrast, heat inactivated Anti-PL Ab had no effect. We propose that phospholamban may be a primer of the SR Cl- channel whereby Cl- anions would play the role of counter-charge carrier during rapid Ca2+ release and Ca2+ uptake by the SR.

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)