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W H Dillmann

Publications and source records attributed to W H Dillmann.

At least 19 recordsLinked to original sources
PubMed2000

Leukemia Inhibitory Factor and Interleukin-6 downregulate sarcoplasmic reticulum Ca2+ ATPase (SERCA2) in cardiac myocytes.

Alterations in gene expression are a hallmark of cardiac hypertrophy and heart failure. Among these, the decreased expression of the sarcoplasmic reticulum calcium ATPase (SERCA2) has been described. Elevated levels of cytokines in particular, Leukemia Inhibitory Factor (LIF) and Interleukin-6 (IL-6) have been shown to have the capacity to elicit hypertrophic responses in cultured cardiac myocytes. In this study, we investigated the effects of these cytokines (LIF & IL-6) on the regulation of SERCA2 levels in cardiac myocytes. Cultured neonatal rat ventricular myocytes were transfected with a 3.2 kb promoter plasmid construct containing the SERCA2 promoter linked to a chloramphenicol acetyltransferase (CAT) reporter gene, and subsequently treated with 10 ng/ml LIF or 10 ng/ml IL-6. LIF and IL-6 independently caused a significant (p < or = 0.05) 23-36% inhibition in SERCA2 promoter activity. LIF and IL-6 induced inhibition was also evident in SERCA2 mRNA levels as assessed by Northern analysis. Time course of inhibition of SERCA2 mRNA levels showed the most prominent decrease occurring after 48 hours of treatment, with both cytokines having a dose dependent effect on the inhibitory response. Western analysis using a polyclonal antibody to SERCA2 protein indicate a significant, 60% decrease in the amount of total SERCA2 protein in cultured myocytes treated with 10 ng/ml LIF or IL-6. In conclusion, the cytokines LIF and IL-6 downregulate SERCA2 gene expression and protein levels. The molecular mechanism responsible for cytokine induced downregulation of SERCA2 is at least partly transcriptional.

Animals↗
PubMed2000

Thyroid hormone-induced stimulation of the sarcoplasmic reticulum Ca(2+) ATPase gene is inhibited by LIF and IL-6.

We investigated the effects of the leukemia inhibitory factor (LIF) and interleukin-6 (IL-6) on 3,3', 5-triiodo-L-thyronine, or thyroid hormone (T(3))-stimulated sarcoplasmic reticulum Ca(2+) ATPase (SERCA2) gene expression on cultured neonatal rat cardiac myocytes. A reduction of T(3) induced increases in SERCA2 mRNA levels after co-treatment with LIF or IL-6. To investigate for the molecular mechanism(s) responsible for the blunted gene expression, a 3.2-kb SERCA2 promoter construct containing a reporter gene was transfected into cardiac myocytes. T(3) treatment stimulated transcriptional activity twofold, whereas co-treatment with T(3) and either of the cytokines caused an inhibition of T(3)-induced SERCA2 transcriptional activity. A T(3)-responsive 0.6-kb SERCA2 construct also showed a similar inhibition by cytokines. Cytokine inhibition of SERCA2 transcriptional activity was also evident when a 0.6-kb SERCA2 mutant, T(3)-unresponsive promoter construct was used. Treatment with T(3) and cytokines showed a significant decrease in transcription when a reporter construct was used that was comprised of direct repeats of SERCA2 thyroid response element I. These data provide evidence for cytokine-mediated inhibitory effects on the SERCA2 promoter that may be mediated by interfering with T(3) action.

Animals↗
PubMed2000

Phospholamban: a major determinant of the cardiac force-frequency relationship.

The cardiac force-frequency relationship has been known for over a century, yet its mechanisms have eluded thorough understanding. We investigated the hypothesis that phospholamban, a potent regulator of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), determines the cardiac force-frequency relationship. Isolated left ventricular papillary muscles from wild-type (WT) and phospholamban knockout (KO) mice were stimulated at 2 to 6 Hz. The force-frequency relationship was positive in WT but negative in KO muscles, i.e., it was inverted by ablation of phospholamban (P < 0.01, n = 6 mice). From 2 to 6 Hz, relaxation accelerated considerably (by 10 ms) in WT muscles but only minimally (by 2 ms) in KO muscles (WT vs. KO: P < 0. 0001, n = 6). To show that the lack of frequency potentiation in KO muscles was not explained by the almost maximal basal contractility, twitch duration was prolonged in six KO muscles with the SERCA inhibitor cyclopiazonic acid to WT values. Relaxation still failed to accelerate with increased frequency. In conclusion, our results clearly identify phospholamban as a major determinant of the cardiac force-frequency relationship.

Animals↗
PubMed1999

A role for HSP27 in sensory neuron survival.

Peripheral nerve injury in neonatal rats results in the death of the majority of the axotomized sensory neurons by 7 d after injury. In adult animals, however, all sensory neurons survive for at least 4 months after axotomy. How sensory neurons acquire the capacity to survive axonal injury is not known. Here we describe how the expression of the small heat shock protein 27 (HSP27) is correlated with neuronal survival after axotomy in vivo and after NGF withdrawal in vitro. The number of HSP27-immunoreactive neurons in the L4 DRG is low at birth and does not change significantly for 21 d after postnatal day 0 (P0) sciatic nerve axotomy. In contrast, in the adult all axotomized neurons begin to express HSP27. One week after P0 sciatic nerve section the total number of neurons in the L4 DRG is dramatically reduced, but all surviving axotomized neurons, as identified by c-jun immunoreactivity, are immunoreactive for HSP27. In addition, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling reveals that very few HSP27-expressing neurons are dying 48 hr after neonatal axotomy. In vitro, a similar correlation exists between HSP27 expression and survival; in P0 DRG cultures, neurons that express HSP27 preferentially survive NGF withdrawal. Finally, overexpression of human HSP27 in neonatal rat sensory and sympathetic neurons significantly increases survival after NGF withdrawal, with nearly twice as many neurons surviving at 48 hr. Together these results suggest that HSP27 in sensory neurons plays a role in promoting survival after axotomy or neurotrophin withdrawal.

Aging↗
PubMed1999

Effects of mutant and antisense RNA of phospholamban on SR Ca(2+)-ATPase activity and cardiac myocyte contractility.

BACKGROUND: The delayed cardiac relaxation in failing hearts has been attributed to a reduced activity of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2). Phospholamban (PLB) inhibits SERCA2 activity and is therefore a potential target to improve the cardiac performance in heart failure. METHODS AND RESULTS: Mutants of PLB (Adv/mPLB) or antisense RNA of PLB (Adv/asPLB) was expressed in cardiac myocytes by recombinant adenovirus, and their effects on SERCA2 activity and myocyte contractility were studied. One mPLB, K3E/R14E, pentamerized with endogenous PLB in neonatal myocytes and resulted in a 45% increase in the affinity of SERCA2 for Ca(2+) and 27% faster diastolic Ca(2+) decline as determined by SR (45)Ca uptake assays and by indo 1-facilitated Ca(2+) transient measurement, respectively. Edge-detection analysis of adult myocyte contractility showed a 74% increase in fractional shortening, accompanied by 115% increase in velocity of relengthening and 25% decrease in time to half-maximal relengthening. In parallel, infection of neonatal cardiac myocytes by Adv/asPLB decreased the endogenous PLB level by 54%, which was associated with a 35% increase in Ca(2+) affinity of SERCA2 and 21% faster diastolic Ca(2+) decline. However, in adult cardiac myocytes, Adv/asPLB failed to significantly alter the endogenous PLB level, the SERCA2 activity, or most of the contractile parameters. CONCLUSIONS: K3E/R14E is a dominant negative mutant of PLB that disrupts the structural integrity and function of the endogenous PLB and consequently enhances SERCA2 activity and myocyte contractility. In neonatal myocytes, the decrease in steady-state abundance of PLB by asPLB also leads to increased SERCA2 activity.

Adenoviridae↗
PubMed1999

Overexpression of sarcoplasmic reticulum Ca(2+)-ATPase improves cardiac contractile function in hypothyroid mice.

OBJECTIVE: Prolonged cardiac contraction and relaxation in hypothyroidism are in part related to diminished expression of the gene coding for the calcium pump of the sarcoplasmic reticulum (SERCA2a). Therefore, we examined whether or not transgenic SERCA2a gene expression in mice may compensate for the cardiac effects of hypothyroidism. METHODS: SERCA2a mRNA and protein were analyzed from hearts of euthyroid and hypothyroid mice of wild-type or SERCA2a transgene status. Contractile function was studied in isolated left ventricular papillary muscles. RESULTS: We found significant decreases of SERCA2a mRNA and protein levels in hearts of hypothyroid wild-type mice in comparison with euthyroid wild-type mice (controls). Papillary muscles from hypothyroid wild-type mice showed significant increases in time to peak contraction and relaxation times compared with controls. In contrast, SERCA2a mRNA and protein levels were significantly higher in hypothyroid SERCA2a transgenic mice than in hypothyroid wild-type mice. The transgene led to a functional improvement by compensating for the prolonged contraction and relaxation of papillary muscles. CONCLUSIONS: Our murine model of hypothyroidism revealed decreases in SERCA2a gene expression accompanied by prolonged contraction and relaxation of papillary muscles, and an improvement of the contractile phenotype due to compensated SERCA2a gene expression in SERCA2a transgenic mice.

Animals↗
PubMed1999

Small heat shock proteins and protection against injury.

The small heat shock proteins alpha B crystallin and HSP27 exert a protective effect in response to simulated ischemia. A model is proposed whereby proteins not in their final folding state bind to the outside of the large oligomeric small heat shock protein complexes thus finding a safe haven during ischemia. After the ischemia is resolved, these proteins may be released and, with the help of HSP70, are shuttled to a productive refolding pathway resulting in proteins in their final folding state, which can assume their normal activity in cells recovered from ischemic injury.

Animals↗
PubMed1999

Calcium regulatory proteins and their alteration by transgenic approaches.

Abnormalities in calcium flux have been linked to abnormal contractile behavior of the heart in patients with congestive heart failure as well as in animal models. Decreased activity or levels of the calcium adenosine triphosphatase of the sarco(endo)plasmic reticulum (SERCA2) particularly have been known to cause a delay in calcium transients. The SERCA2 protein pumps 2 moles of calcium per mole of adenosine triphosphate (ATP) split from the cytoplasm into the sarcoplasmic reticulum, thus lowering the free cytoplasmic calcium concentration. It therefore is of interest to identify mechanisms by which SERCA activity could be increased in the heart. To determine influences of increased expression of the SERCA2 gene on calcium transient and contractile behavior, we constructed transgenic mice and rats expressing a SERCA2 transgene in their heart. In these animals, a 20% increase in SERCA levels occurs due to additional expression of the SERCA transgene. This leads to a corresponding increase in contractile activity as determined by the increase in left ventricular pressure measured as dP/dt(max) and decrease in diastolic ventricular pressure determined as dP/dt(min). Similarly, isolated cardiac myocytes obtained from the heart of transgenic mice showed an accelerated calcium transient and increased speed of shortening and relengthening as determined by edge detection. To determine if SERCA2 transgene expression could have a compensatory effect on the contractile behavior of the heart in transgenic mice expressing SERCA2, these mice were made hypothyroid, and papillary muscle function was determined. Contractile behavior of the papillary muscle of wild-type hypothyroid mice showed a significant increase in muscle relaxation time (RT50). In contrast, SERCA2 transgenic hypothyroid mice showed normal contractile behavior of papillary muscle. A compensatory effect of SERCA transgene expression was therefore demonstrated. In addition, we constructed transgenic rats expressing a SERCA2 transgene in which constriction of the ascending aorta induced cardiac hypertrophy and a delayed contraction of papillary muscle. In preliminary results, we found that SERCA2 transgenic rats submitted to ascending aortic constriction did not show the delayed relaxation of papillary muscle as was found in wild-type rats submitted to aortic constriction. In addition, adenoviral vectors expressing transgenes for calcium-handling proteins can be used to improve cardiac myocyte contraction. Adenoviruses expressing a SERCA transgene or a mutant phospholamban transgene exhibiting dominant negative action were used to infect isolated myocytes treated with a phorbol ester (phorbol 12-myristate 13-acetate), which delays the calcium transients. The calcium transients and contractile behavior of the isolated myocytes indicated that increased SERCA expression or increased expression of mutant phospholamban transgene led to increased SERCA2 activity, resulting in an increased contractile phenotype. Recent findings by other investigators also indicate that decreased SERCA2 activity can be increased under in vivo conditions using adenoviral vector-based SERCA2 expression. A gene therapy type of approach delivering increased amounts of SERCA or phospholamban mutants leading to increased SERCA activity should therefore be considered in the future.

Adenosine Triphosphate↗
PubMed1999

Phospholamban-to-SERCA2 ratio controls the force-frequency relationship.

The force-frequency relationship (FFR) describes the frequency-dependent potentiation of cardiac contractility. The interaction of the sarcoplasmic reticulum Ca2+-adenosinetriphosphatase (SERCA2) with its inhibitory protein phospholamban (PLB) might be involved in the control of the FFR. The FFR was analyzed in two systems in which the PLB-to-SERCA2 ratio was modulated. Adult rabbit cardiac myocytes were transduced with adenovirus encoding for SERCA2, PLB, and beta-galactosidase (control). After 3 days, the relative PLB/SERCA2 values were significantly different between groups (SERCA2, 0.5; control, 1.0; PLB, 4.5). SERCA2 overexpression shortened relaxation by 23% relative to control, whereas PLB prolonged relaxation by 39% and reduced contractility by 47% (0.1 Hz). When the stimulation frequency was increased to 1.5 Hz, myocyte contractility was increased by 30% in control myocytes. PLB-overexpressing myocytes showed an augmented positive FFR (+78%), whereas SERCA2-transduced myocytes displayed a negative FFR (-15%). A more negative FFR was also found in papillary muscles from SERCA2 transgenic mice. These findings demonstrate that the ratio of phospholamban to SERCA2 is an important component in the control of the FFR.

Animals↗
PubMed1999

Heat shock proteins and protection against ischemic injury.

Heat shock proteins present a complex family of proteins exerting chaperone-like activities that are classified according to their molecular weight. We especially explored protective functions of inducible heat shock protein 70, the mitochondrial heat shock protein 60 and 10, and the small heat shock proteins HSP27 and alphaB-crystallin against ischemic, reoxygenation-mediated injury using transgenic animals and hearts under in vivo conditions and in isolated cardiac myocyte-derived cells using adenoviral vectors. We noted with great interest that differential protective effects are exerted by specific hsps. For example, alpha-B-crystallin and constitutive hsp70 markedly protect microtubular structure in cardiac myocytes from ischemia-induced injury. Inducible hsp70, hsp60 and hsp10 when coexpressed, and hsp27 and alphaB-crystallin have an overall protective effect against ischemic injury as determined by the release of enzymes like creatine kinase and LDH. We did not note inflammatory or immune responses elicited by the expression of hsps in transgenic animals and cardiac myocytes. The specific cell types in which hsps are expressed may contribute to the protective effect of hsps versus their inflammatory and immunogenic effects when expressed in other cell types.

Animals↗
PubMed1999

Altered cardiac phenotype in transgenic mice carrying the delta337 threonine thyroid hormone receptor beta mutant derived from the S family.

The heart has been recognized as a major target of thyroid hormone action. Our study investigates both the regulation of cardiac-specific genes and contractile behavior of the heart in the presence of a mutant thyroid hormone receptor beta1 (T3Rbeta1-delta337T) derived from the S kindred. The mutant receptor was originally identified in a patient with generalized resistance to thyroid hormone. Cardiac expression of the mutant receptor was achieved by a transgenic approach in mice. As the genes for myosin heavy chains (MHC alpha and MHC beta) and the cardiac sarcoplasmic reticulum Ca2+ adenosine triphosphatase (SERCA2) are known to be regulated by T3, their cardiac expression was analyzed. The messenger RNA levels for MHC alpha and SERCA2 were markedly down-regulated, MHC beta messenger RNA was up-regulated. Although T3 levels were normal in these animals, this pattern of cardiac gene expression mimics a hypothyroid phenotype. Cardiac muscle contraction was significantly prolonged in papillary muscles from transgenic mice. The electrocardiogram of transgenic mice showed a substantial prolongation of the QRS interval. Changes in cardiac gene expression, cardiac muscle contractility, and electrocardiogram are compatible with a hypothyroid cardiac phenotype despite normal T3 levels, indicating a dominant negative effect of the T3Rbeta mutant.

Animals↗
PubMed1999

Influence of phosphorylation and oligomerization on the protective role of the small heat shock protein 27 in rat adult cardiomyocytes.

Recent reports have demonstrated that the heat shock proteins (hsp) and in particular the hsp70 confer protection against cardiac ischemic damage. More recently, we have shown that increased expression of another heat shock protein, the hsp27, through an adenovirus vector system protects adult cardiomyocytes against ischemic injury. This small heat shock protein undergoes phosphorylation when the cell is under stress. This has led many to speculate that phosphorylation of hsp27 is required for the protective role this protein plays in the cell. In order to investigate this possibility, we have mutated the serines that are the sites of phosphorylation on the hsp27, to glycines or alanines. These nonphosphorylatable mutants of hsp27 were cloned into adenoviral vectors and used to infect adult rat cardiomyocytes to assess their ability in protecting against ischemic injury. In addition, we used a specific inhibitor of p38 MAP kinase that is a key member of the kinase pathway responsible for phosphorylating the hsp27. Our present results show that the nonphosphorylated hsp27 forms larger oligomeric complexes than the phosphorylated hsp27. Interestingly, phosphorylation of hsp27 seems not to play a role in its ability to protect adult rat cardiomyocytes against ischemic damage.

Animals↗
PubMed1998

Influences of increased expression of the Ca2+ ATPase of the sarcoplasmic reticulum by a transgenic approach on cardiac contractility.

Congestive heart failure is a significant clinical problem and leads to abnormalities in Ca2+ transients and to decreases in the level of the Ca2+ ATPase of the sarcoplasmic reticulum according to reports to some investigators. The Ca2+ ATPase of the sarcoplasmic reticulum (SERCA2) contributes in an important manner to diastolic Ca2+ lowering and relaxation of the heart. To determine the contractile alterations resulting from increased SERCA2 expression, we generated transgenic mice overexpressing a rat SERCA2 transgene. In these mice, SERCA2 mRNA was increased 2.6-fold, the relative synthesis rate of SERCA2 protein 1.8-fold, and SERCA2 protein levels 1.2-fold. Functional analysis of Ca2+ handling and contractile parameters in isolated cardiac myocytes indicated that the intracellular Ca2+ decline and myocyte relengthening were each accelerated by 22-23%. In addition, studies in isolated papillary muscles showed that the time to half-maximal post-rest potentiation was significantly shorter, hinting at an increased Ca2+ loading of the sarcoplasmic reticulum. Furthermore, in vivo cardiac functional studies demonstrated a significant accelerated contraction and relaxation in SERCA2 transgenic mice. We also cloned a SERCA2 transgene and mutants of the phospholamban gene into E1 deleted replication-deficient human adenovirus 5 viral vectors and infected cardiac myocytes. In the cardiac myocytes, endogenous SERCA2 levels were decreased by PMA treatment. Infection of such myocytes with a SERCA2 expressing adenovirus could reconstitute the Ca2+ transient, and augmented oxalate facilitated SERCA2 Ca2+ uptake. In addition, phospholamban mutants with changes of basic to acidic amino acids in the cytoplasmic domain increased SERCA2 activity by 30-35%. These findings, therefore, suggest that increased SERCA2 activity can be achieved by increasing SERCA2 levels or by expressing phospholamban mutants. Increased SERCA2 activity can lead to significant enhancements of Ca2+ transients and myocardial contractility.

Animals↗
PubMed1998

The Raf-MEK-ERK cascade represents a common pathway for alteration of intracellular calcium by Ras and protein kinase C in cardiac myocytes.

Ras and protein kinase C (PKC), which regulate the Raf-MEK-ERK cascade, may participate in the development of cardiac hypertrophy, a condition characterized by diminished and prolonged contractile calcium transients. To directly examine the influence of this pathway on intracellular calcium ([Ca2+]i), cardiac myocytes were cotransfected with effectors of this pathway and with green fluorescent protein, which allowed the living transfected myocytes to be identified and examined for [Ca2+]i via indo-1. Transfection with constitutively active Ras (Ha-RasV12) increased cell size, decreased expression of the myofibrils and the calcium-regulatory enzyme SERCA2, and reduced the magnitude and prolonged the decay phase of the contractile [Ca2+]i transients. Similar effects on [Ca2+]i were obtained with Ha-RasV12S35, a Ras mutant that selectively couples to Raf, and with constitutively active Raf. In contrast, Ha-RasV12C40, a Ras mutant that activates the phosphatidylinositol 3-kinase pathway, had a lesser effect. The PKC-activating phorbol ester, phorbol 12-myristate 13-acetate, also prolonged the contractile [Ca2+]i transients. Cotransfection with dnMEK inhibited the effects of Ha-RasV12, Raf, and phorbol 12-myristate 13-acetate on [Ca2+]i. The effects of Ha-RasV12 and Raf on [Ca2+]i were also counteracted by SERCA2 overexpression. Both Ras and PKC may thus regulate cardiac [Ca2+]i via the Raf-MEK-ERK cascade, and this pathway may represent a critical determinant of cardiac physiological function.

Animals↗
PubMed1998

Protection against myocardial dysfunction after a brief ischemic period in transgenic mice expressing inducible heat shock protein 70.

Brief ischemic periods lead to myocardial dysfunction without myocardial infarction. It has been shown that expression of inducible HSP70 in hearts of transgenic mice leads to decreased infarct size, but it remains unclear if HSP70 can also protect against myocardial dysfunction after brief ischemia. To investigate this question, we developed a mouse model in which regional myocardial function can be measured before and after a temporary ischemic event in vivo. In addition, myocardial function was determined after brief episodes of global ischemia in an isolated Langendorff heart. HSP70-positive mice and transgene negative littermates underwent 8 min of regional myocardial ischemia created by occlusion of the left descending coronary artery, followed by 60 min of reperfusion. This procedure did not result in a myocardial infarction. Regional epicardial strain was used as a sensitive indicator for changes in myocardial function after cardiac ischemia. Maximum principal strain was significantly greater in HSP70-positive mice with 88+/-6% of preischemic values vs. 58+/-6% in transgene-negative mice (P < 0.05). Similarly, in isolated Langendorff perfused hearts of HSP70-positive and transgene-negative littermates exposed to 10 min of global ischemia and 90 min of reperfusion, HSP70 transgenic hearts showed a better-preserved ventricular peak systolic pressure. Thus, we conclude that expression of HSP70 protects against postischemic myocardial dysfunction as shown by better preserved myocardial function.

Animals↗
PubMed1998

Constitutive and inducible hsp70s are involved in oxidative resistance evoked by heat shock or ethanol.

Improved cardiac post-ischemic recovery after whole-body hyperthermia is correlated with an increased expression of the heat shock proteins (hsps). The inducible hsp70 (hsp70i) has a known cardioprotective effect against ischemia/reperfusion injury. Here, we studied whether other hsps are also involved in cardioprotection. Using rat heart-derived H9c2 myocytes, we observed that preheating at 43 degreesC for 20 min conferred resistance to hydrogen peroxide (H2O2). The resistance to mild H2O2 toxicity (3-5 micro mol/10(7) cells) appeared early and persisted, whereas the resistance to moderate H2O2 toxicity (6-9 micro mol/10(7) cells) was detectable only at 20-44 h post heat shock. No resistance was observed at higher doses of hydrogen peroxide (10-12 micro mol/10(7) cells), indicating that severe toxicity exceeds the capacity of the induced protective mechanism. Coincidentally, this thermal regimen elicited a rapid and prolonged increase in the cellular level of hsp70i, and a delayed and transient induction of the constitutive hsp70 (hsp70c). Nuclear translocations of hsp70i and hsp70c also occurred upon heat stimulation. A homogeneous distribution of the accumulated hsp70i and hsp70c throughout the nuclei and cytoplasm paralleled the development of heat-induced resistance to moderate H2O2 challenge. Application of another hsp inducer, ethyl alcohol, evoked a similar pattern of H2O2 resistance, and hsp induction and distribution. Our results suggest that induction and subcellular distribution of hsp70s contribute importantly to cellular antioxidant defenses, and that a co-operation between hsp70i and hsp70c may improve cardiac preservation during oxidative insult.

Animals↗
PubMed1998

Sarcoplasmic reticulum function in murine ventricular myocytes overexpressing SR CaATPase.

To examine the effects of the overexpression of sarcoplasmic reticulum (SR) CaATPase on function of the SR and Ca2+ homeostasis, we measured [Ca2+]i transients (fluo-3), and L-type Ca2+ currents (ICa.L), Na/Ca exchanger currents (INa/Ca), and SR Ca2+ content with voltage clamp in ventricular myocytes isolated from wild type (WT) mice and transgenic (SRTG) mice. The amplitude of [Ca2+]i transients was insignificantly increased in SRTG myocytes, while the diastolic [Ca2+]i tended to be lower. The initial and terminal declines of [Ca2+]i transients were significantly accelerated in SRTG myocytes, implying a functional upregulation of the SR CaATPase. We examined the functional contribution of only the SR CaATPase to the initial and the terminal phase of the decline of [Ca2+]i, by abruptly inhibiting Na/Ca exchange with a rapid switcher device. The rate of [Ca2+] decline mediated by the SR CaATPase was increased by 40% in SRTG compared with WT myocytes. The function of the L-type Ca2+ channel was unchanged in SRTG myocytes, while INa/Ca density was slightly (10%) decreased. Measured SR Ca2+ content was significantly increased by 29% in SRTG myocytes. Thus, overexpression of SR CaATPase markedly accelerates the decline of [Ca2+]i transients, and induces in increase in SR Ca2+ content, with some downregulation of the Na/Ca exchanger.

Animals↗