Expression of sarcoplasmic reticulum Ca2+ transport proteins in cold-acclimating ducklings

The relationship between the cold-induced increase in sarcoplasmic reticulum (SR) Ca2+ transport proteins and the development of muscular nonshivering theermogenesis (NST) was investigated by determining the time course of expression of the sarcoplasmic or endoplasmic reticulum Ca(2+)-ATPase (SERCA), Ca2+ release channel, and calsequestrin in control and cold-acclimating ducklings. 45Ca2+ uptake and [3H]ryanodine binding measurements with skeletal muscle homogenates showed that a cold acclimation period of approximately 4 wk was required to observe a substantial increase in Ca(2+)-ATPase activity and Ca2+ release channel content, which correlates well with NST development Immunoblot analysis of muscle homogenates showed no differences in calsequestrin levels but revealed that the decrease in SERCA2a content was delayed in cold-acclimating birds and that the SERCA1 level was increased after 4 wk of cold acclimation. The persistence of SERCA2a may be related to shivering thermogenesis occurring preferentially in slow-twitch fibers. SERCA1 may account for most of the cold-induced increase in 45Ca2+ uptake activity, suggesting the preferential occurrence of a Ca(2+)-dependent NST in fast-twitch fibers.

Ca2+ release channels of pigs heterozygous for malignant hyperthermia

Porcine malignant hyperthermia (MH) is an autosomal recessive disorder resulting from a mutation in the skeletal muscle sarcoplasmic reticulum (SR) Ca2+ release channel. The Ca2+ release properties of SR vesicles isolated from pigs heterozygous for the MH gene have been demonstrated previously to be intermediate to those of vesicles isolated from MH-susceptible (MHS) and normal pigs. The Ca2+ release channel is tetrameric, so the intermediate Ca2+ release properties of heterozygous pig SR preparations could result either from populations of MHS and normal homotetramers, or populations of heterotetrameric Ca2+ release channels with properties unique from those of the two types of homozygous channels. To discriminate between these possibilities, the single channel percent open time (Po) and channel dwell time distributions of SR Ca2+ release channels were analyzed. These data suggest that the heterozygous porcine Ca2+ release channel population must contain heterotetramers with properties distinct from those of either MHS or normal channels. The data also imply that the Ca2+ release channel population in MHS humans who are heterozygous for a dominant mutation in this protein also contains heterotetrameric channels.

Effects of uridine triphosphate on skinned skeletal muscle fibers of the rat

Chemically skinned muscle fibers from rat extensor digitorum longus muscle were used to study the effects of uridine triphosphate (UTP) on Ca2+ uptake and release by the sarcoplasmic reticulum (SR) and on Ca2+-activated tensions. Total replacement (2.5 mM) of adenosine triphosphate (ATP) with UTP (i) increased submaximal Ca2+-induced tension (pCa 6.2-5.8) but diminished Po, the maximum tension elicited by pCa 4.2, by ca. 15%, (ii) markedly reduced Ca2+ uptake by the SR (evaluated by caffeine-elicited tension); and (iii) induced tension in Ca2+-loaded fibers. The UTP-induced tension averaged 55% of Po and its rates of development and decay were considerably slower than those of caffeine-evoked tension. The UTP-induced tension (i) depended on the Ca2+-loading conditions; (ii) was reversibly blocked by brief (15 s) exposures of Ca2+-loaded fibers to 5 mM EGTA or by pretreatment with caffeine; (iii) was abolished by functional disruption of the SR with the nonionic detergent Brij-58; and (iv) persisted after blockade of the SR Ca2+ release channels with ruthenium red. Exposure of Ca2+-loaded fibers to UTP depressed the tension elicited subsequently by caffeine, and enhanced the rate of depletion of caffeine-sensitive Ca2+ stores during soaking in relaxing solutions containing 5 mM EGTA. The UTP-induced tension is attributed to increased release of Ca2+ from the SR, via a ruthenium red insensitive pathway(s), combined with reduced Ca2+ uptake by the SR and increased Ca2+ affinity of the contractile proteins.

Tissue-specific alternative promoters regulate the expression of the two sarco/endoplasmic reticulum Ca-ATPase isoforms from Artemia franciscana

The sarco/endoplasmic reticulum Ca-ATPase gene from Artemia franciscana is transcribed into two mRNAs of 4.5 and 5.2 kb that code for protein isoforms differing at their carboxyl terminus. Northern blot assays and anchored polymerase chain reaction (PCR) experiments have shown that these two mRNAs also differ at the initial part of their 5' untranslated region. The 5.2-kb mRNA-specific 5' untranslated region is present as an independent exon whose transcription is regulated by a promoter different from the one previously described that regulates the expression of the 4.5-kb mRNA. The nucleotide sequence of the 5.2-kb mRNA promoter and the transcription initiation site have been determined. These results suggest that the expression of the two protein isoforms is regulated in A. franciscana at the transcription initiation step, in contrast with the vertebrates sarco/endoplasmic reticulum Ca-ATPase genes 1 and 2 which have unique promoters for transcription of the two isoforms encoded by each gene.

Decreased inotropic response to beta-adrenergic stimulation and normal sarcoplasmic reticulum calcium stores in the spontaneously hypertensive rat heart

Cardiac hypertrophy in the spontaneously hypertensive rat has been shown to be accompanied by a diminished inotropic response to beta-adrenergic stimulation. This diminished response has been attributed to abnormalities in various components of the beta-adrenergic signaling system. There is also evidence that regulation of intracellular Ca2+ cycling may be altered in the hypertrophied heart of the spontaneously hypertensive rat. We proposed that the diminished response to beta-adrenergic stimulation may reflect abnormalities in Ca2+ cycling, specifically alterations in the ability of the sarcoplasmic reticulum to effectively release and resequester Ca2+. We have used the unique combination of functional measurements on isolated, isometrically contracting papillary muscles from hearts of 26-week-old spontaneously hypertensive rats and their Wistar-Kyoto controls, together with electron probe microanalysis measurements of sarcoplasmic reticulum Ca2+ content in the same muscles after rapid freezing, to determine the availability of Ca2+ for activation of contraction, following beta-adrenergic stimulation. We observed a significant decrease in the inotropic response to beta-adrenergic stimulation in papillary muscles from the spontaneously hypertensive rats, as compared with Wistar-Kyoto controls, however in these same muscles, frozen during relaxation, there was no evidence of an accompanying decrease in the size of the sarcoplasmic reticulum Ca2+ store. In an additional group of muscles which were frozen during contraction, the amount of Ca2+ remaining in the sarcoplasmic reticulum after stimulated release was also not different in the two strains. These results indicate that the decreased inotropic response to beta-adrenergic stimulation in hypertrophied hearts of the spontaneously hypertensive rat is unlikely to be due to decreased availability of Ca2+ for activation of contraction. Additionally, to determine whether there is intracellular Ca2+ overload in the cardiac muscle cells of hearts of spontaneously hypertensive rats, we measured the amount of Ca2+ in mitochondrial and at the level of the myofilaments by electron probe microanalysis. These results indicate that intracellular Ca2+ overload does not accompany cardiac hypertrophy in the spontaneously hypertensive rat. This study therefore shows no correlation between altered intracellular Ca2+ cycling and the decreased inotropic response to isoproterenol in the spontaneously hypertensive rat at 26 weeks of age.

Ketamine, at clinical concentrations, does not alter the function of cardiac sarcoplasmic reticulum calcium release channels

In the absence of sympathetically mediated stimulation, ketamine depresses myocardial contractility. This results from a decrease in the availability of intracellular Ca2+ for excitation-contraction coupling. Although sites of action other than the Ca2+ release channel of sarcoplasmic reticulum have been implicated, ketamine-induced alterations in Ca2+ efflux from the sarcoplasmic reticulum remain contentious. The purpose of the present study was to identify interactions of ketamine with the calcium release channel using sarcoplasmic reticulum enriched vesicles from porcine left ventricle. Ketamine did not alter [3H]ryanodine binding at concentrations of 1 mM or less, while binding was almost completely inhibited at 10 mM. Gating and conductance of SR Ca2+ channels studied in planar bilayers was not altered by clinical concentrations of ketamine over the range of physiologic cytoplasmic free Ca2+ concentrations. Channel inactivation was observed at 10 mM ketamine, well in excess of clinical concentrations. These findings indicate that clinical concentrations of ketamine do not alter the function of the Ca2+ release channel. Alterations in intracellular Ca2+ homeostasis that result in depression of myocardial contractility must therefore result from effects at other sites along the excitation-contraction coupling pathway.

Altered [Ca2+]i mobilization in diabetic cardiomyocytes: responses to caffeine, KCl, ouabain, and ATP

To study the mechanisms mediating intracellular calcium transients involved in diabetic cardiac dysfunction, changes in intracellular calcium concentration ([Ca2+]i) in response to stimulation by caffeine, ouabain, KCl and ATP were studied in single cardiomyocytes (quiescent or electrically-stimulated) isolated from streptozotocin (STZ) diabetic rats. [Ca2+]i was measured by fluorescence microscopy using fura-2. Peak [Ca2+]i response to caffeine (20 mM) and decline of [Ca2+]i (-peak d[Ca2+]i/dt) were decreased in diabetic myocytes. Insulin treatment corrected these depressed [Ca2+]i responses. The data suggest a reduced sarcoplasmic reticulum (SR) calcium content and a depressed Na-Ca exchange activity in diabetic myocytes. Insulin deficiency may play a causal role in these changes. The maximum [Ca2+]i increase in response to ouabain was reduced in diabetic cells while the sensitivity of diabetic myocytes to ouabain was increased. This may be a result of depressed Na-K ATPase and elevated [Na+]i as previously reported. The KCl (12.5-50 mM)-induced [Ca2+]i increase was enhanced in diabetic cells. Caffeine (20 mM) and dichlorobenzamil (DCB, 10 microM) blocked this [Ca2+]i transient to a smaller degree in diabetic cells, but nitrendipine effects were similar in diabetic and control cells. These effects may be due to the increased L-channel activity and altered features, such as different responses to Ca-channel blockers, in diabetes which has previously been reported. The maximum response of [Ca2+]i to exogenous ATP was increased in diabetic cells while the sensitivity remained unchanged. The mechanisms underlying this enhanced response may be similar to the KCl-induced [Ca2+]i changes in diabetes.

[Relative changes between calcium content and number of atrial specific granules induced by water and salt loading]

Rats prepared by either deprivation of water intake for 5 d or drinking 2% NaCl solution for 4 d were sacrificed for dissection of auricle, and making ultrathin sections. The numerical density (Nv) of atrial specific granules (ASG) were counted by using electron microscopic morphometry, and Ca concentration in ASG and in terminal cisterna of sarcoplasmic reticulum (TSR) was determined by using electron microscopic X-ray microanalysis. In the drinking 2% NaCl solution group, the Nv of ASG decreased from 6.02 +/- 2.30 to 2.96 +/- 1.62/microns 3 (P < 0.01) and all companied by a decrease of Ca concentration from 64 +/- 16 to 38 +/- 21 mmol/kg (P < 0.01), while the secretion of atrial natriuretic peptide (ANP) showed some increase. Water deprivation induced all the contrary results. However, the Ca concentration in TSR was not affected by water-salt loading. It is suggested that when the atrial cardiocytes were stimulated by salt loading, Ca in ASG was released into cytosol and thus promoted fusion of ASG membrane with sarcolemma which resulted in release of ANP. Therefore, it is considered that ASG as an intracellular Ca store participates in the stimulus-secretion coupling of ANP through the release of Ca ions.

[Expression of the heat shock protein 70 in inflammatory myopathies]

Heat shock protein 70 (HSP 70) expression was immunohistochemically observed in diseased muscle fibers of 35 patients with dermatomyositis (DM) and 7 with polymyositis (PM). In DM, HSP 70 was localized in the sarcoplasm of type 1 fibers adjacent to the small vessels showing deposits of complement components in 13 patients and in the atrophic fibers at perifascicular regions in 7. HSP 70 was also expressed more preferentially in the small vessels rather than in the sarcoplasm in 13 DM patients. In PM, the expression of HSP 70 was blurred in all fibers including non-necrotic fibers invaded by T cells. In conclusion, HSP 70 is likely more frequently to be expressed in the sarcoplasm of DM than PM due to probable ischemic insults.

Differential changes in cardiac phospholamban and sarcoplasmic reticular Ca(2+)-ATPase protein levels. Effects on Ca2+ transport and mechanics in compensated pressure-overload hypertrophy and congestive heart failure

The objective of this study was to elucidate the role of the sarcoplasmic reticulum (SR) in the transition from compensated pressure-overload hypertrophy (increased left ventricular [LV] mass, normal LV function, and no pulmonary congestion) to congestive heart failure (increased LV mass, depressed LV function, and pulmonary congestion). To address this issue, the descending thoracic aorta was banded for 4 and 8 weeks in adult guinea pigs, and the changes in isovolumic LV mechanics, SR Ca2+ transport, and SR protein levels were determined and compared with age-matched sham-operated control animals. A subgroup of the 8-week banded animals manifested the congestive heart failure phenotype with diminished developed LV pressure normalized by LV mass, reduced rates of LV pressure development and relaxation, and markedly increased lung weight-to-body weight ratios. The cardiac mechanical and morphometric changes were associated with depressed protein levels of the SR Ca(2+)-ATPase (85% of the control) and phospholamban (65% of the control) assessed by quantitative immunoblotting. Resultant rates of SR Ca2+ uptake (Vmax) and the affinity of SR Ca(2+)-ATPase for Ca2+ (EC50) were significantly depressed [32 +/- 6 nmol Ca2+.min-1.mg-1 and 0.59 +/- 0.12 (mumol/L)/L, respectively] compared with the 8-week sham-operated control animals [40 +/- 1 nmol Ca2+.min-1.mg-1 and 0.40 +/- 0.05 (mumol/L)/L, respectively]. We conclude that this model of pressure overload-induced cardiac failure is associated with (1) diminished LV force development, rates of pressure development, and decay; (2) depressed protein expression of the Ca(2+)-cycling proteins SR Ca(2+)-ATPase and phospholamban; and (3) decreased Vmax and affinity of the SR Ca(2+)-ATPase for Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

The mechanism of coupling chemical and physical reactions by the calcium ATPase of sarcoplasmic reticulum and other coupled vectorial systems

The coupling of the chemical reaction of ATP hydrolysis to the transport of calcium from the cytoplasm into the lumen of sarcoplasmic reticulum vesicles can be defined by a set of rules that define alternating changes in the specificities of the enzyme for catalysis of chemical and physical reactions.

Ca2+ binding and translocation by the sarcoplasmic reticulum ATPase: functional and structural considerations

Three experimental systems are described including sarcoplasmic reticulum (SR) vesicles, reconstituted proteoliposomes, and recombinant protein obtained by gene transfer and expression in foreign cells. It is shown that the Ca(2+) ATPase of sarcoplasmic reticulum (SR) includes an extramembranous globular head which is connected through a stalk to a membrane bound region. Cooperative binding of two calcium ions occurs sequentially, within a channel formed by four clustered helices within the membrane bound region. Destabilization of the helical cluster is produced following enzyme phosphorylation by ATP at the catalytic site in the extramembranous region. The affinity and orientation of the Ca2+ binding site are thereby changed, permitting vectorial dissociation of bound Ca2+ against a concentration gradient. A long range linkage between phosphorylation and Ca2+ binding sites is provided by an intervening peptide segment that retains high homology in cation transport ATPases, and whose function is highly sensitive to mutational perturbations.

Transmembrane Ca2+ gradient and function of membrane proteins

This review will focus on the recent advance in the study of effect of transmembrane Ca2+ gradient on the function of membrane proteins. It consits of two parts: 1. Transmembrane Ca2+ gradient and sarcoplasmic reticulum Ca(2+)-ATPase; 2. Effect of transmembrane Ca2+ gradient on the components and coupling of cAMP signal transduction pathway. The results obtained indicate that a proper transmembrane Ca2+ gradient may play an important role in modulating the conformation and activity of SR Ca(2+)-ATPase and the function of membrane proteins involved in the cAMP signal transduction by mediating the physical state change of the membrane phospholipids.

Ligand-gated channel of the sarcoplasmic reticulum Ca2+ transport ATPase

In resting muscle, cytoplasmic Ca2+ concentration is maintained at a low level by active Ca2+ transport mediated by the Ca2+ ATPase from sarcoplasmic reticulum. The region of the protein that contains the catalytic site faces the cytoplasmic side of the membrane, while the transmembrane helices form a channel-like structure that allows Ca2+ translocation across the membrane. When the coupling between the catalytic and transport domains is lost, the ATPase mediates Ca2+ efflux as a Ca2+ channel. The Ca2+ efflux through the ATPase channel is activated by different hydrophobic drugs and is arrested by ligands and substrates of the ATPase at physiological pH. At acid pH, the inhibitory effect of cations is no longer observed. It is concluded that the Ca2+ efflux through the ATPase may be sufficiently fast to support physiological Ca2+ oscillations in skeletal muscle, that occur mainly in conditions of intracellular acidosis.

Luminal calcium regulation of calcium release from sarcoplasmic reticulum

This article discusses how changes in luminal calcium concentration affect calcium release rates from triad-enriched sarcoplasmic reticulum vesicles, as well as single channel opening probability of the ryanodine receptor/calcium release channels incorporated in bilayers. The possible participation of calsequestrin, or of other luminal proteins of sarcoplasmic reticulum in this regulation is addressed. A comparison with the regulation by luminal calcium of calcium release mediated by the inositol 1,4,5-trisphosphate receptor/calcium channel is presented as well.

Imaging elementary events of calcium release in skeletal muscle cells

In skeletal muscle cells, calcium release to trigger contraction occurs at triads, specialized junctions where sarcoplasmic reticulum channels are opened by voltage sensors in the transverse tubule. Scanning confocal microscopy was used in cells under voltage clamp to measure the concentration of intracellular calcium, [Ca2+]i, at individual triads and [Ca2+]i gradients that were proportional to calcium release. In cells stimulated with small depolarizations, the [Ca2+]i gradients broke down into elementary events, corresponding to single-channel currents of about 0.1 picoampere. Because these events were one-tenth to one-fifth the size of calcium sparks (elementary release events of cardiac muscle), skeletal muscle control mechanisms appear to be fundamentally different.

Isolation and partial cloning of ryanodine-sensitive Ca2+ release channel protein isoforms from human myometrial smooth muscle

Partial cDNAs of the ryanodine receptor were cloned using PCR analysis from reverse transcribed total and mRNA, extracted from freshly isolated pregnant, non-pregnant, and cultured human myometrial smooth muscle. The identity of these clones was confirmed by nucleotide sequencing of the fragments and indicate the expression of both the skeletal and brain ryanodine receptor isoforms in these preparations. In freshly isolated non-pregnant myometrial tissue, membrane fractions displaying specific [3H]ryanodine binding activities were isolated using density gradient centrifugation. SDS-PAGE of the sucrose gradient fractions indicated the specific comigration of a polypeptide with a molecular mass of approximately 544 kDa with the ryanodine binding activity.

Modulation of cardiac ryanodine receptors of swine and rabbit by a phosphorylation-dephosphorylation mechanism

1. The regulation of the cardiac Ca2+ release channel-ryanodine receptor (RyR) by exogenous acid phosphatase (AcPh) and purified Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) was studied in swine and rabbit sarcoplasmic reticulum (SR) vesicles using [3H]ryanodine binding and planar bilayer reconstitution experiments. 2. Addition of AcPh (1-20 U ml-1) to a standard incubation medium increased [3H]ryanodine binding in a Ca(2+)-dependent manner. Stimulation was only readily apparent in media containing micromolar Ca2+ concentrations. 3. Scatchard analysis of [3H]ryanodine binding curves revealed that AcPh enhanced binding by increasing the affinity of the receptor for [3H]ryanodine without recruiting additional receptor sites (Kd, 9.8 +/- 0.85 and 3.9 +/- 0.65 nM; Bmax (the maximal receptor density), 1.45 +/- 0.14 and 1.47 +/- 0.12 pmol mg-1 for control and AcPh, respectively). The failure of AcPh to increase Bmax suggested that the number of receptors that were 'dormant' due to phosphorylation in the SR preparation was very small. 4. At the single channel level, AcPh increased the open probability (Po) of RyR channels by increasing the opening rate and inducing the appearance of a longer open state while having no effect on single channel conductance. Thus AcPh acted directly on RyR channels or a closely associated regulatory protein. 5. CaMKII decreased both [3H]ryanodine binding and Po of RyRs when added to medium supplemented with micromolar levels of Ca2+ and calmodulin (CaM). Addition of a synthetic peptide inhibitor of CaMKII, or replacement of ATP with the non-hydrolysable ATP analogue adenylyl[beta, gamma-methylene]-diphosphate (AMP-PCP), prevented CaMKII inhibition of RyRs, suggesting that CaMKII acted specifically through a phosphorylation mechanism. 6. The inhibition of RyR channel activity by CaMKII was reversed by the addition of AcPh. Thus we showed that an in vitro phosphorylation-dephosphorylation mechanism effectively regulates RyRs. 7. The results suggest that intracellular signalling pathways that lead to activation of CaMKII may reduce efflux of Ca2+ from the SR by inhibition of RyR channel activity. The Ca2+ dependence of CaMKII inhibition suggests that the role of the phosphorylation mechanism is to modulate the RyR response to Ca2+.

Enhancing effect of calmodulin on Ca(2+)-induced Ca2+ release in the sarcoplasmic reticulum of rabbit skeletal muscle fibres

1. We analysed the effect of calmodulin on Ca(2+)-induced Ca2+ release (CICR) in the sarcoplasmic reticulum (SR) using chemically skinned fibres of rabbit psoas muscle. Ca2+ release was measured using fura-2 microfluorometry. 2. In saponin-skinned fibres, calmodulin potentiated Ca2+ release at low Ca2+ concentrations (< 3 microM), while it showed an inhibitory effect at high Ca2+ concentrations (3-30 microM). 3. Co-application of ryanodine and calmodulin at 0.3 microM Ca2+, but not ryanodine alone, induced a decline in the Ca2+ uptake capacity of the SR, an effect expected from the open-lock of active CICR channels by ryanodine. Thus, potentiation of Ca2+ release by calmodulin at low Ca2+ concentrations can be regarded as a result of the activation of the ryanodine receptor. 4. Greater concentrations of calmodulin were required for potentiation of CICR at low Ca2+ concentrations (1 microM) than for inhibition at high Ca2+ concentrations (10 microM). 5. In beta-escin-permeabilized fibres in which intrinsic calmodulin was retained, the rates of CICR were similar to those measured in the presence of 1 microM calmodulin in saponin-permeabilized fibres. 6. These results suggest that calmodulin plays an important role in the regulation of CICR channels in intact skeletal muscle fibres.

Measurement of SR free Ca2+ and Mg2+ in permeabilized smooth muscle cells with use of furaptra

The concentrations of intrasarcoplasmic reticulum (SR) free Ca2+ ([Ca2+]SR) and Mg2+ ([Mg2+]SR) were measured in furaptra-loaded saponin-permeabilized cultured aortic smooth muscle (A7r5) cells. Ca(2+)-independent fluorescence emitted by furaptra trapped within organelles, excited at 346 nm (isosbestic point), decreased with a half time of 30 min. All Ca2+ measurements appeared to be from SR, because the apparent Ca2+ distribution within permeabilized cells was uniform and therefore inconsistent with furaptra loading into mitochondria. Moreover, thapsigargin-induced SR Ca(2+)-adenosinetriphosphatase inhibition caused near-total depletion of Ca2+, and the metabolic poisons oligomycin and rotenone had no effect. Calibration curves relating 370 nm-to-346 nm ratios to [Ca2+] and to [Mg2+] were calculated in situ; dissociation constants for Ca2+ and Mg2+ binding were 49 microM and 6.8 mM, respectively. Resting [Ca2+]SR was 75-130 microM, with a mean of 97.2 +/- 2.2 microM (n = 376), whereas [Mg2+]SR, estimated in the absence of Ca2+, was 1.0 mM. Stimulation with inositol 1,4,5-trisphosphate resulted in time-dependent declines in [Ca2+]SR, and pretreatment with guanosine 5'-triphosphate caused a large increase in the rate of inositol 1,4,5-trisphosphate-evoked SR Ca2+ release, although guanosine 5'-triphosphate had no effect by itself. These observations indicate that furaptra will be a valuable tool with which to directly study [Ca2+]SR and SR function.

Effects of thapsigargin on aequorin-injected and skinned preparations of ferret ventricular muscles

OBJECTIVE

We investigated the effects of thapsigargin (TG) (0.1-1 microM) on the relation between intracellular Ca2+ concentration and tension in ferret papillary muscles using aequorin-injected and skinned preparations.

METHODS

Aequorin was injected into the superficial cells of ferret papillary muscles; the Ca2+ signals of aequorin and tension in twitch and those with the application of 15 mM caffeine were simultaneously measured. The alteration of Ca2+ sensitivity of the contractile elements was examined by measuring the pCa-tension relation in Triton-X-treated skinned preparations.

RESULTS

TG decreased the peak of the Ca2+ signal accompanied by a prolonged decay time. However, the tension was scarcely altered even at 1 microM TG. TG inhibited the caffeine-induced Ca2+ signal. Prolongation of decay of the Ca2+ signal by TG in twitch was further enhanced by isoprenaline (10 nM). The pCa-tension relation of the skinned preparation was slightly but significantly shifted to the right by TG.

CONCLUSIONS

The apparent dissociation of the effects of TG on the Ca2+ signal and tension in intact preparations is not a result of alteration of the Ca2+ sensitivity of the myofilaments. The effects of TG in multicellular preparations are probably limited to the outer layer of the preparation. The slower time course of the Ca2+ signal induced by TG is due to the inhibition of Ca2+ uptake by sarcoplasmic reticulum, which is more significantly observed when the intracellular Ca2+ transient is increased by isoprenaline.

Sarcoplasmic reticulum in cardiac length-dependent activation in rabbits

After a step increase in length of rabbit right ventricular papillary muscles, active stress increased immediately followed by a further slow increase in stress over 15-20 min. We studied the contribution of the sarcoplasmic reticulum (SR) to the slow change in stress (SCS) after changing muscle length from 85 to 95% of length at which active force development was maximal. SCS amounted to 32.5 +/- 12.2% mean +/- SD, n = 19) of the total increase in active stress. This was associated with a 13.2 +/- 8.7% increase in calcium content of the SR as estimated with rapid cooling contractures (P < 0.0001, n = 19). However, SCS was not dependent on SR calcium content. There was no significant attenuation in SCS after SR calcium depletion with ryanodine (n = 6), SR Ca(2+)-adenosinetriphosphatase inhibition with cyclopiazonic acid (n = 6), or combined treatment with ryanodine and cyclopiazonic acid (n = 3). We conclude that, in the rabbit, SR calcium content increases slowly after a step increase in cardiac muscle length but the slow changes in active stress are not dependent on the sarcoplasmic reticulum.

Skeletal muscle sarcoplasmic calcium regulation and sudden death syndrome in chickens

1. Sudden Death Syndrome (SDS) is a disease of well-developed, predominantly male broiler chickens where death appears to occur because of cardiovascular failure. The role of skeletal muscle sarcoplasmic calcium regulation as a potential cause of SDS has been investigated. 2. Calcium regulation of skeletal muscle sarcoplasmic reticulum was compared between broiler and Leghorn chickens. Calcium regulation matured from the 2nd to the 11th week and, at any age, broiler chickens showed significantly lower calcium transport rates and transport efficiencies. The mechanism of calcium transport in broiler chickens was more energy-consuming than that of the Leghorn chickens. 3. Sarcoplasmic calcium regulation is pivotal for muscle metabolism. As in porcine malignant hyperthermia, weaker calcium regulation might lead to hyperactivation of skeletal muscle, followed by elevated lactic acid concentrations and cardiovascular failure.

Characterization of a chloride channel reconstituted from cardiac sarcoplasmic reticulum

We have characterized a voltage-sensitive chloride channel from cardiac sarcoplasmic reticulum (SR) following reconstitution of porcine heart SR into planar lipid bilayers. In 250 mM KCl, the channel had a main conductance level of 130 pS and exhibited two substrates of 61 and 154 pS. The channel was very selective for Cl- over K+ or Na+ (PK+/PCl- = 0.012 and PNa+/PCl- approximately 0.040). It was permeable to several anions and displayed the following sequence of anion permeability: SCN- > I- > NO3- approximately Br- > Cl- > F- > HCOO-. Single-channel conductance saturated with increasing Cl- concentrations (Km = 900 mM and gamma max = 488 pS). Channel activity was voltage dependent, with an open probability ranging from approximately 1.0 around 0 mV to approximately 0.5 at +80 mV. From -20 to +80 mV, channel gating was time-independent. However, at voltages below -40 mV the channel entered a long-lasting closed state. Mean open times varied with voltage, from approximately 340 msec at -20 mV to approximately 6 msec at +80 mV, whereas closed times were unaffected. The channel was not Ca(2+)-dependent. Channel activity was blocked by disulfonic stilbenes, arylaminobenzoates, zinc, and cadmium. Single-channel conductance was sensitive to trans pH, ranging from approximately 190 pS at pH 5.5 to approximately 60 pS at pH 9.0. These characteristics are different from those previously described for Cl- channels from skeletal or cardiac muscle SR.

Dietary medium-chain triglycerides can prevent changes in myosin and SR due to CPT-1 inhibition by etomoxir

To define determinants of subcellular structures of heart, Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) were treated for 5 wk with 15 mg.kg-1.day-1 etomoxir [reduces mitochondrial carnitine palmitoyltransferase-1 (CPT-1) activity and fatty acid synthesis]. To bypass CPT-1 inhibition, etomoxir-treated rats were fed a medium-chain fatty acid (MCFA) diet. Etomoxir induced a proportionate growth of heart, which could partially (WKY, P < 0.05) or completely (SHR, P < 0.05) be prevented by the MCFA diet. Also the etomoxir-induced increase in myosin V1 was partially prevented (P < 0.05). Etomoxir increased (P < 0.05) rate of sarcoplasmic reticulum (SR) Ca2+ uptake of WKY and SHR ventricular homogenates in the presence or absence of the SR Ca2+ release inhibitor ruthenium red. The MCFA diet resulted in SR Ca2+ uptake rates that were in between those of etomoxir-treated and untreated rats. The in vitro 32P incorporation into phospholamban and troponin I did not differ significantly in WKY. Etomoxir induced, however, an increase (P < 0.05) in the phosphorylated intermediate of the Ca2+ adenosinetriphosphatase in WKY that was prevented by the MCFA diet. In SHR, etomoxir increased the in vitro phospholamban phosphorylation, which was reduced compared with WKY. The data show that myosin and SR are affected by a chronically altered substrate utilization of heart.