Decreased cardiac SERCA2 expression, SR Ca uptake, and contractile function in hypothyroidism are attenuated in SERCA2 overexpressing transgenic rats

The sarco/endoplasmic reticulum (SR) Ca(2+)-ATPase SERCA2a has a key role in controlling cardiac contraction and relaxation. In hypothyroidism, decreased expression of the thyroid hormone (TH)-responsive SERCA2 gene contributes to slowed SR Ca(2+) reuptake and relaxation. We investigated whether cardiac expression of a TH-insensitive SERCA2a cDNA minigene can rescue SR Ca(2+) handling and contractile function in female SERCA2a-transgenic rats (TG) with experimental hypothyroidism. Wild-type rats (WT) and TG were rendered hypothyroid by 6-N-propyl-2-thiouracil treatment for 6 wk; control rats received no treatment. In vivo measured left ventricular (LV) hemodynamic parameters were compared with SERCA2a expression and function in LV tissue. Hypothyroidism decreased LV peak systolic pressure, dP/dt(max), and dP/dt(min) in both WT and TG. However, loss of function was less in TG. Thus slowed relaxation in hypothyroidism was found to be 1.5-fold faster in TG compared with WT (P < 0.05). In parallel, a 1.4-fold higher V(max) value of homogenate SR Ca(2+) uptake was observed in hypothyroid TG (P < 0.05 vs. hypothyroid WT), and the hypothyroidism-caused decline of LV SERCA2a mRNA expression in TG by -24% was markedly less than the decrease of -49% in WT (P < 0.05). A linear relationship was observed between the SERCA2a/PLB mRNA ratio values and the V(max) values of SR Ca(2+) uptake when the respective data of all experimental groups were plotted together (r = 0.90). The data show that expression of the TH-insensitive SERCA2a minigene compensates for loss of expressional activity of the TH-responsive native SERCA2a gene in the female hypothyroid rat heart. However, SR Ca(2+) uptake and in vivo heart function were only partially rescued.     

In situ SR function in postinfarction myocytes.

Previous studies have shown lower systolic intracellular Ca(2+) concentrations ([Ca(2+)](i)) and reduced sarcoplasmic reticulum (SR)-releasable Ca(2+) contents in myocytes isolated from rat hearts 3 wk after moderate myocardial infarction (MI). Ca(2+) entry via L-type Ca(2+) channels was normal, but that via reverse Na(+)/Ca(2+) exchange was depressed in 3-wk MI myocytes. To elucidate mechanisms of reduced SR Ca(2+) contents in MI myocytes, we measured SR Ca(2+) uptake and SR Ca(2+) leak in situ, i.e., in intact cardiac myocytes. For sham and MI myocytes, we first demonstrated that caffeine application to release SR Ca(2+) and inhibit SR Ca(2+) uptake resulted in a 10-fold prolongation of half-time (t(1/2)) of [Ca(2+)](i) transient decline compared with that measured during a normal twitch. These observations indicate that early decline of the [Ca(2+)](i) transient during a twitch in rat myocytes was primarily mediated by SR Ca(2+)-ATPase and that the t(1/2) of [Ca(2+)](i) decline is a measure of SR Ca(2+) uptake in situ. At 5.0 mM extracellular Ca(2+), systolic [Ca(2+)](i) was significantly (P 相似文献   

Contribution of Ca(2+) transporters to relaxation in intact ventricular myocytes from developing rats

The relative contributions of Ca(2+) transporters to intracellular Ca(2+) concentration ([Ca(2+)](i)) decline associated with twitch relaxation were analyzed in intact ventricular myocytes from developing and adult rats. This was accomplished by estimation of individual integrated Ca(2+) fluxes with the use of kinetic parameters calculated from [Ca(2+)](i) measurements during twitches and caffeine-evoked contractures, and from myocardial passive Ca(2+) buffering data. Our main findings were the following: 1) twitch relaxation and [Ca(2+)](i) decline were significantly slower during the first postnatal week than in adults, 2) inhibition of sarcoplasmic reticulum (SR) Ca(2+) accumulation resulted in faster [Ca(2+)](i) decline in young cells than in adult cells, 3) the contributions of the SR Ca(2+) uptake and Na(+)/Ca(2+) exchange (NCX) to twitch relaxation increased from ~75 to 92%, and decreased from 24 to 5%, respectively, from birth to adulthood, and 4) Ca(2+) transport by the sarcolemmal Ca(2+)-ATPase was apparently increased in neonates. Our data indicate that despite a marked increase in NCX contribution to cell relaxation in immature rats, the SR Ca(2+)-ATPase appears to be the predominant transporter responsible for relaxation-associated [Ca(2+)](i) decline from birth to adulthood.     

Serine phosphorylation of the sarcoplasmic reticulum Ca(2+)-ATPase in the intact beating rabbit heart.

Recent studies have demonstrated that Ca(2+)/calmodulin-dependent protein kinase phosphorylates the Ca(2+)-pumping ATPase of cardiac sarcoplasmic reticulum (SR) in vitro. Also, evidence from in vitro studies suggested that this phosphorylation, occurring at Ser(38), results in stimulation of Ca(2+) transport. In the present study, we investigated whether serine phosphorylation of the SR Ca(2+)-ATPase occurs in the intact functioning heart. Hearts removed from anesthetized rabbits were subjected to retrograde aortic perfusion of the coronary arteries with oxygenated mammalian Ringer solution containing (32)P(i) and contractions were monitored by recording systolic left ventricular pressure development. Following 45-50 min of (32)P perfusion, the hearts were freeze-clamped, SR isolated, and analyzed for protein phosphorylation. SDS-polyacrylamide gel electrophoresis and autoradiography showed phosphorylation of several peptides including the Ca(2+)-ATPase and Ca(2+) release channel (ryanodine receptor). The identity of Ca(2+)-ATPase as a phosphorylated substrate was confirmed by Western immunoblotting as well as immunoprecipitation using a cardiac SR Ca(2+)-ATPase-specific monoclonal antibody. The Ca(2+)-ATPase showed immunoreactivity with a phosphoserine monoclonal antibody indicating that the in situ phosphorylation occurred at the serine residue. Quantification of Ca(2+)-ATPase phosphorylation in situ yielded a value of 208 +/- 12 pmol (32)P/mg SR protein which corresponded to the phosphorylation of approximately 20% of the Ca(2+) pump units in the SR membrane. Since this phosphorylation occurred under basal conditions (i.e., in the absence of any inotropic intervention), a considerable steady-state pool of serine-phosphorylated Ca(2+)-ATPase likely exists in the normally beating heart. These findings demonstrate that serine phosphorylation of the Ca(2+)-ATPase is a physiological event which may be important in the regulation of SR function.     

Enhanced calcium mobilization in rat ventricular myocytes during the onset of pressure overload-induced hypertrophy

Early cardiovascular changes evoked by pressure overload (PO) may reveal adaptive strategies that allow immediate survival to the increased hemodynamic load. In this study, systolic and diastolic Ca(2+) cycling was analyzed in left ventricular rat myocytes before (day 2, PO-2d group) and after (day 7, PO-7d group) development of hypertrophy subsequent to aortic constriction, as well as in myocytes from time-matched sham-operated rats (sham group). Ca(2+) transient amplitude was significantly augmented in the PO-2d group. In the PO-7d group, intracellular Ca(2+) concentration ([Ca(2+)](i)) was reduced during diastole, and mechanical twitch relaxation (but not [Ca(2+)](i) decline) was slowed. In PO groups, fractional sarcoplasmic reticulum (SR) Ca(2+) release at a twitch, SR Ca(2+) content, SR Ca(2+) loss during diastole, and SR-dependent integrated Ca(2+) flux during twitch relaxation were significantly greater than in sham-operated groups, whereas the relaxation-associated Ca(2+) flux carried by the Na(+)/Ca(2+) exchanger was not significantly changed. In the PO-7d group, mRNA levels of cardiac isoforms of SR Ca(2+)-ATPase (SERCA2a), phospholamban, calsequestrin, ryanodine receptor, and NCX were not significantly altered, but the SERCA2a-to-phospholamban ratio was increased 2.5-fold. Moreover, greater sensitivity to the inotropic effects of the beta-adrenoceptor agonist isoproterenol was observed in the PO-7d group. The results indicate enhanced Ca(2+) cycling between SR and cytosol early after PO imposition, even before hypertrophy development. Increase in SR Ca(2+) uptake may contribute to enhancement of excitation-contraction coupling (augmented SR Ca(2+) content and release) and protection against arrhythmogenesis due to buildup of [Ca(2+)](i) during diastole.     

Defective intracellular Ca(2+) signaling contributes to cardiomyopathy in Type 1 diabetic rats

The goal of the study was to determine whether defects in intracellular Ca(2+) signaling contribute to cardiomyopathy in streptozotocin (STZ)-induced diabetic rats. Depression in cardiac systolic and diastolic function was traced from live diabetic rats to isolated individual myocytes. The depression in contraction and relaxation in myocytes was found in parallel with depression in the rise and decline of intracellular free Ca(2+) concentration ([Ca(2+)](i)). The sarcoplasmic reticulum (SR) Ca(2+) store and rates of Ca(2+) release and resequestration into SR were depressed in diabetic rat myocytes. The rate of Ca(2+) efflux via sarcolemmal Na(+)/Ca(2+) exchanger was also depressed. However, there was no change in the voltage-dependent L-type Ca(2+) channel current that triggers Ca(2+) release from the SR. The depression in SR function was associated with decreased SR Ca(2+)-ATPase and ryanodine receptor proteins and increased total and nonphosphorylated phospholamban proteins. The depression of Na(+)/Ca(2+) exchanger activity was associated with a decrease in its protein level. Thus it is concluded that defects in intracellular Ca(2+) signaling caused by alteration of expression and function of the proteins that regulate [Ca(2+)](i) contribute to cardiomyopathy in STZ-induced diabetic rats. The increase in phospholamban, decrease in Na(+)/Ca(2+) exchanger, and unchanged L-type Ca(2+) channel activity in this model of diabetic cardiomyopathy are distinct from other types of cardiomyopathy.     

Adenosine triphosphatases during maturation of cultured human skeletal muscle cells and in adult human muscle.

Na+/K(+)-ATPase, Mg(2+)-ATPase and sarcoplasmic reticulum (SR) Ca(2+)-ATPase are examined in cultured human skeletal muscle cells of different maturation grade and in human skeletal muscle. Na+/K(+)-ATPase is investigated by measuring ouabain binding and the activities of Na+/K(+)-ATPase and K(+)-dependent 3-O-methylfluorescein phosphatase (3-O-MFPase). SR Ca(2+)-ATPase is examined by ELISA, Ca(2+)-dependent phosphorylation and its activities on ATP and 3-O-methylfluorescein phosphate. Na+/K(+)-ATPase and SR Ca(2+)-ATPase are localized by immunocytochemistry. The activities of Na+/K(+)-ATPase and SR Ca(2+)-ATPase show a good correlation with the other assayed parameters of these ion pumps. All ATPase parameters investigated increase with the maturation grade of the cultured muscle cells. The number of ouabain-binding sites and the activities of Na+/K(+)-ATPase and K(+)-dependent 3-O-MFPase are significantly higher in cultured muscle cells than in muscle. The Mg(2+)-ATPase activity, the content of SR Ca(2+)-ATPase and the activities of SR Ca(2+)-ATPase and Ca(2+)-dependent 3-O-MFPase remain significantly lower in cultured cells than in muscle. The ouabain-binding constant and the molecular activities of Na+/K(+)-ATPase and SR Ca(2+)-ATPase are equal in muscle and cultured cells. During ageing of human muscle the activity as well as the concentration of SR Ca(2+)-ATPase decrease. Thus the changes of the activities of the ATPases are caused by variations of the number of their molecules. Na+/K(+)-ATPase is localized in the periphery of fast- and slow-twitch muscle fibers and at the sarcomeric I-band. SR Ca(2+)-ATPase is predominantly confined to the I-band, whereas fast-twitch fibers are much more immunoreactive than slow-twitch fibers. The presence of cross-striation for Na+/K(+)-ATPase and SR Ca(2+)-ATPase in highly matured cultured muscle cells indicate the development and subcellular organization of a transverse tubular system and SR, respectively, which resembles the in vivo situation.     

Involvement of L-type calcium channel and SERCA2a in myocardial dysfunction induced by obesity

Obesity has been shown to impair myocardial performance. Nevertheless, the mechanisms underlying the participation of calcium (Ca(2+) ) handling on cardiac dysfunction in obesity models remain unknown. L-type Ca(2+) channels and sarcoplasmic reticulum (SR) Ca(2+) -ATPase (SERCA2a), may contribute to the cardiac dysfunction induced by obesity. The purpose of this study was to investigate whether myocardial dysfunction in obese rats is related to decreased activity and/or expression of L-type Ca(2+) channels and SERCA2a. Male 30-day-old Wistar rats were fed standard (C) and alternately four palatable high-fat diets (Ob) for 15 weeks. Obesity was determined by adiposity index and comorbidities were evaluated. Myocardial function was evaluated in isolated left ventricle papillary muscles under basal conditions and after inotropic and lusitropic maneuvers. L-type Ca(2+) channels and SERCA2a activity were determined using specific blockers, while changes in the amount of channels were evaluated by Western blot analysis. Phospholamban (PLB) protein expression and the SERCA2a/PLB ratio were also determined. Compared with C rats, the Ob rats had increased body fat, adiposity index and several comorbidities. The Ob muscles developed similar baseline data, but myocardial responsiveness to post-rest contraction stimulus and increased extracellular Ca(2+) was compromised. The diltiazem promoted higher inhibition on developed tension in obese rats. In addition, there were no changes in the L-type Ca(2+) channel protein content and SERCA2a behavior (activity and expression). In conclusion, the myocardial dysfunction caused by obesity is related to L-type Ca(2+) channel activity impairment without significant changes in SERCA2a expression and function as well as L-type Ca(2+) protein levels.     

Inhibition of sarcoplasmic reticulum Ca(2+)-ATPase by 2,5-di(tert-butyl)-1,4-benzohydroquinone.

We characterized the interaction of 2,5-di(tert-butyl)-1,4-benzohydroquinone (tBuBHQ) with the sarcoplasmic reticulum (SR) Ca(2+)-ATPase from rabbit fast-twitch skeletal and canine cardiac muscles by examining the effect of this agent on the ATPase reaction. tBuBHQ at less than 10 microM inhibited ATP hydrolysis by both isoforms of Ca(2+)-ATPase by up to 80 and 90%, respectively. The half maximal inhibition of these enzymes was observed at about 1.5 microM tBuBHQ. Thus, this agent potently inhibits the fast-twitch skeletal and slow-twitch skeletal/cardiac isoforms of SR Ca(2+)-ATPase. tBuBHQ at 5-10 microM inhibited the rate of decomposition of the phosphoenzyme intermediate (EP), measured as a ratio between ATPase activity and the EP level in the steady state, by 35-40%. It also inhibited formation of EP by decreasing the rate of Ca2+ binding to the Ca(2+)-deficient, nonphosphorylated enzyme to about 1/8 of the control value. These results indicate that tBuBHQ has at least two sites of action in the reaction sequence for the SR Ca(2+)-ATPase.     

N-terminal chimeric constructs improve the expression of sarcoplasmic reticulum Ca(2+)-ATPase in yeast.

Wild-type and chimeric constructs comprising rabbit sarcoplasmic reticulum (SR) Ca(2+)-ATPase and the N-terminal cytoplasmic portion of yeast plasma membrane H(+)-ATPase were expressed in yeast under control of a heat-shock regulated promoter. The wild-type ATPase was found predominantly in endoplasmic reticulum (ER) membranes. Addition of the first 88 residues of H(+)-ATPase to the Ca(2+)-ATPase N-terminal end promoted a marked shift in the localization of chimeric H(+)/Ca(2+)-ATPase which accumulated in a light membrane fraction associated with yeast smooth ER. Furthermore, there was a three-fold increase in the overall level of expression of chimeric H(+)/Ca(2+)-ATPase. Similar results were obtained for a chimeric Ca(2+)-ATPase containing a hexahistidine sequence added to its N-terminal end. Both H(+)/Ca(2+)-ATPase and 6xHis-Ca(2+)-ATPase were functional as demonstrated by their ability to form a phosphorylated intermediate and undergo fast turnover. Conversely, a replacement chimera in which the N-terminal end of SR Ca(2+)-ATPase was replaced by the corresponding segment of H(+)-ATPase was not stably expressed in yeast membranes. These results indicate that the N-terminal segment of Ca(2+)-ATPase plays an important role in enzyme assembly and contains structural determinants necessary for ER retention of the ATPase.     

Sprint training normalizes Ca(2+) transients and SR function in postinfarction rat myocytes.

Previous studies have shown that myocytes isolated from sedentary (Sed) rat hearts 3 wk after myocardial infarction (MI) undergo hypertrophy, exhibit altered intracellular Ca(2+) concentration ([Ca(2+)](i)) dynamics and abnormal contraction, and impaired sarcoplasmic reticulum (SR) function manifested as prolonged half-time of [Ca(2+)](i) decline. Because exercise training elicits positive adaptations in cardiac contractile function and myocardial Ca(2+) regulation, the present study examined whether 6-8 wk of high-intensity sprint training (HIST) would restore [Ca(2+)](i) dynamics and SR function in MI myocytes toward normal. In MI rats, HIST ameliorated myocyte hypertrophy as indicated by significant (P 相似文献   

Reversible inhibition of the calcium-pumping ATPase in native cardiac sarcoplasmic reticulum by a calmodulin-binding peptide. Evidence for calmodulin-dependent regulation of the V(max) of calcium transport

Calmodulin (CaM) and Ca(2+)/CaM-dependent protein kinase II (CaM kinase) are tightly associated with cardiac sarcoplasmic reticulum (SR) and are implicated in the regulation of transmembrane Ca(2+) cycling. In order to assess the importance of membrane-associated CaM in modulating the Ca(2+) pump (Ca(2+)-ATPase) function of SR, the present study investigated the effects of a synthetic, high affinity CaM-binding peptide (CaM BP; amino acid sequence, LKWKKLLKLLKKLLKLG) on the ATP-energized Ca(2+) uptake, Ca(2+)-stimulated ATP hydrolysis, and CaM kinase-mediated protein phosphorylation in rabbit cardiac SR vesicles. The results revealed a strong concentration-dependent inhibitory action of CaM BP on Ca(2+) uptake and Ca(2+)-ATPase activities of SR (50% inhibition at approximately 2-3 microM CaM BP). The inhibition, which followed the association of CaM BP with its SR target(s), was of rapid onset (manifested within 30 s) and was accompanied by a decrease in V(max) of Ca(2+) uptake, unaltered K(0.5) for Ca(2+) activation of Ca(2+) transport, and a 10-fold decrease in the apparent affinity of the Ca(2+)-ATPase for its substrate, ATP. Thus, the mechanism of inhibition involved alterations at the catalytic site but not the Ca(2+)-binding sites of the Ca(2+)-ATPase. Endogenous CaM kinase-mediated phosphorylation of Ca(2+)-ATPase, phospholamban, and ryanodine receptor-Ca(2+) release channel was also strongly inhibited by CaM BP. The inhibitory action of CaM BP on SR Ca(2+) pump function and protein phosphorylation was fully reversed by exogenous CaM (1-3 microM). A peptide inhibitor of CaM kinase markedly attenuated the ability of CaM to reverse CaM BP-mediated inhibition of Ca(2+) transport. These findings suggest a critical role for membrane-bound CaM in controlling the velocity of Ca(2+) pumping in native cardiac SR. Consistent with its ability to inhibit SR Ca(2+) pump function, CaM BP (1-2.5 microM) caused marked depression of contractility and diastolic dysfunction in isolated perfused, spontaneously beating rabbit heart preparations. Full or partial recovery of contractile function occurred gradually following withdrawal of CaM BP from the perfusate, presumably due to slow dissociation of CaM BP from its target sites promoted by endogenous cytosolic CaM.     

Differences in mechanisms of SR dysfunction in ischemic vs. idiopathic dilated cardiomyopathy

We examined 1) contractile properties and the intracellular Ca(2+) concentration ([Ca(2+)](i)) transient in cardiac myocytes and 2) sarcoplasmic reticulum (SR) Ca(2+) uptake and release function in myocardium from patients with end-stage heart failure caused by ischemic (ICM) vs. idiopathic dilated cardiomyopathy (DCM). The amplitude of cell motion was decreased 43 +/- 6% in ICM and 68 +/- 7% in DCM compared with that in normal organ donors (DN). Time to peak of shortening was increased 43 +/- 15% in DCM, but not in ICM. Prolongation of the relaxation time was more predominant in ICM. In DCM the systolic [Ca(2+)](i) was decreased 27 +/- 9% and diastolic [Ca(2+)](i) was increased 36 +/- 11%. In ICM the diastolic [Ca(2+)](i) was increased 59 +/- 12% but the systolic [Ca(2+)](i) was unchanged. A significant decrease of the ATP-dependent SR Ca(2+) uptake rate associated with the reduction of the SR Ca(2+)-ATPase protein level was found in ICM. In contrast, the significant decrease in SR Ca(2+) release rate was distinct in DCM. The large amount of Ca(2+) retained in the SR associated with a significant decrease in the maximum reaction velocity and increase in the Michaelis-Menten constant in the caffeine concentration-response curve suggests a fundamental abnormality in the SR Ca(2+) release channel gating property in DCM. We conclude that potentially important differences exist in the intracellular Ca(2+) homeostasis and excitation-contraction coupling in ICM vs. DCM. The SR Ca(2+) release dysfunction may play an important pathogenetic role in the abnormal Ca(2+) homeostasis in DCM, and the SR Ca(2+) uptake dysfunction may be responsible for the contractile dysfunction in ICM.     

硒对心肌质膜(Ca~(2+)·Mg~(2+))-ATPase和肌浆网Ca~(2+)-ATPase活力的影响

本文以豚鼠和大白鼠心肌肌浆网膜(SR)Ca~(2+)-ATPase的活力,心肌质膜(SL)(Ca~(2+)Mg~(2+))-ATPase的活力和电子显微镜的方法探索克山病病区粮中低硒与心肌细胞钙转运调控的共系,实验结果为硒对克山病有预防作用的观点提供了新的理论依据,并进一步支持了“克山病是一种心肌线粒体病”的观点。     

Chronic and acute exercise do not alter Ca2+ regulatory systems and ectonucleotidase activities in rat heart.

The purpose of this investigation was to examine the effects of chronic and acute exercise on the main components involved in excitation-contraction coupling and relaxation in rat heart. Sixty male Wistar rats were divided into a sedentary (S) and three 12-wk treadmill-trained groups (T-1, moderate intensity; T-2, high intensity; T-3, interval running). After 12-wk, 15 rats from the S group and 15 rats from the T-2 group were subjected to a single treadmill-exercise session until exhaustion before being killed at 0, 24, or 48 h (acute exercise). The remaining animals were killed 48 h after the last standard exercise session (chronic exercise). The efficacy of the training programs was confirmed by an increase in treadmill endurance time and in skeletal muscle citrate synthase activity. None of the exercise programs modified heart weight or cardiac oxidative capacity. [(3)H]PN200-110 and [(3)H]ryanodine binding to cardiac homogenates indicated that the density of L-type and sarcoplasmic reticulum (SR) Ca(2+) channels was the same in S and trained rats. The SR Ca(2+)-ATPase activity was also unmodified. Finally, the activities of the ectoenzymes Mg(2+)-ATPase and 5'-nucleotidase, which are involved in degradation of extracellular nucleotides, were not affected by either of the running programs. After the acute exercise session, no changes were detected in either of the tested parameters in heart homogenates of S and T-2 animals. We conclude that neither treadmill-exercise training for 12 wk nor exhaustive exercise alters the density of Ca(2+) channels involved in excitation-contraction coupling or the SR Ca(2+)-ATPase and the ectonucleotidase activities in rat heart.     

Sarcoplasmic reticulum Ca2+ transport and gene expression in congestive heart failure are modified by imidapril treatment

This study was designed to test the hypothesis that blockade of the renin-angiotensin system improves cardiac function in congestive heart failure by preventing changes in gene expression of sarcoplasmic reticulum (SR) proteins. We employed rats with myocardial infarction (MI) to examine effects of an angiotensin-converting enzyme inhibitor, imidapril, on SR Ca(2+) transport, protein content, and gene expression. Imidapril (1 mg.kg(-1).day(-1)) was given for 4 wk starting 3 wk after coronary artery occlusion. Infarcted rats exhibited a fourfold increase in left ventricular end-diastolic pressure, whereas rates of pressure development and decay were decreased by 60 and 55%, respectively. SR Ca(2+) uptake and Ca(2+) pump ATPase, as well as Ca(2+) release and ryanodine receptor binding activities, were depressed in the failing hearts; protein content and mRNA levels for Ca(2+) pump ATPase, phospholamban, and ryanodine receptor were also decreased by approximately 55-65%. Imidapril treatment of infarcted animals improved cardiac performance and attenuated alterations in SR Ca(2+) pump and Ca(2+) release activities. Changes in protein content and mRNA levels for SR Ca(2+) pump ATPase, phospholamban, and ryanodine receptor were also prevented by imidapril treatment. Beneficial effects of imidapril on cardiac function and SR Ca(2+) transport were not only seen at different intervals of MI but were also simulated by another angiotensin-converting enzyme inhibitor, enalapril, and an ANG II receptor antagonist, losartan. These results suggest that blockade of the renin-angiotensin system may increase the abundance of mRNA for SR proteins and, thus, may prevent the depression in SR Ca(2+) transport and improve cardiac function in congestive heart failure due to MI.     

肾上腺髓质素对大鼠损伤性心肌肌浆网功能的改善

通过观察下述五个指标,评价肾上腺髓质素(adrenomedullin,Adm)对大鼠损伤性心肌肌浆网功能的改善程度左心室压力最大变化速率(±dp/dtmax)、肌浆网钙摄取和释放及钙泵活性.皮下注射异丙肾上腺素(isoproterenol,ISO,69μmol/kg体重)制备大鼠心肌损伤坏死模型.摘取心脏后用Adm灌流,观察左心室压力最大变化速率(±dp/dtmax);制备并提纯心肌肌浆网(sarcoplasmicreticulum,SR)膜,测定SRCa2+摄取和释放速率、SR钙泵活性和钙通道蛋白～3H-ryanodine受体的最大结合量.结果发现,5×10-5mol/LAdm灌流能使ISO损伤的大鼠心脏左室±dp/dtmax分别增加16.9%(2?135±281vs1?980±302)和29.2%(1?375±267vs1?064±355,均P<0.05);SRCa2+摄取和释放率分别增加23.0%(15.0±1.4vs12.2±1.2)和43.5%(6.6±1.0vs4.6±0.6,均P<0.01);SRCa2+-ATPase活性和～3H-ryanodine受体最大结合量(Bmax)分别增加24.2%(P<0.01)和42.2%(P<0.05).提示Adm对ISO诱导的大鼠心肌损伤具有保护作用,其机制可能与Adm增加SRCa2+-ATPase活性、增加～3H-ryanodine所致SRCa2+摄取和释放升高有关.外源性给予Adm对损伤心肌可能具有临床治疗作用.     

Effect of milrinone on left ventricular relaxation and Ca(2+) uptake function of cardiac sarcoplasmic reticulum

Milrinone, a phosphodiesterase 3 (PDE3) inhibitor, is known to enhance left ventricular (LV) contractility by an inhibition of the breakdown of cAMP through the mechanism inhibiting PDE3. However, it is unclear whether milrinone also exerts positive lusitropy, like dobutamine. Here, we assessed the effects of milrinone on in vivo LV relaxation, as well as the Ca(2+)-ATPase activity and the Ca(2+) uptake function of the cardiac sarcoplasmic reticulum (SR), compared with the effect of dobutamine on those functions. After dobutamine (3 microg x kg(-1) x min(-1)) was administered, the peak value of the first derivative of LV pressure (+dP/dt) increased by 46%, whereas the time constant (tau) of LV pressure decay decreased by 6.9%, respectively. After milrinone (10 microg/kg) was administered, the peak +dP/dt increased to a similar extent as dobutamine (46%), whereas tau decreased much more than dobutamine (19.9%; P < 0.05). In LV crude homogenate, the thapsigargin-sensitive, Ca(2+)-ATPase activity-cAMP relationships was significantly less increased by milrinone compared with dobutamine (P < 0.05), indicating the higher sensitivity of the SR Ca(2+)-ATPase activity on cAMP by milrinone than by dobutamine. In the SR vesicles purified from LV muscles, the addition of cAMP increased the SR Ca(2+) uptake in a dose-dependent fashion, and the PDE3 inhibitors (milrinone and cGMP) significantly augmented this response (P < 0.05). Hence, milrinone substantially improved LV relaxation in association with an acceleration of the SR Ca(2+)-ATPase activity and the SR Ca(2+) uptake. This acceleration might be due to an inhibition of the membrane-bound PDE3 in the SR, leading to a local elevation of cAMP.