Developmental changes in cardiac sarcoplasmic reticulum in sheep

Compartmentation of creatine kinases during perinatal development of mammalian heart

Maturation of the cardiac cell is characterized by increasing diversity of isozymic expression of creatine kinases. Expression of the M-CK isozyme always precedes that of mitochondrial isozyme (mi-CK), however the expression of an isoform does not inform about its localization or cellular function. The functional role of isozymes binding to sites of energy utilization and production characteristic of the adult myocardium can be evidenced by the functional coupling of M-CK to myofibrillar ATPase and mito-CK to translocase in Triton X-100 and saponin skinned fibers. Functional activity of M-CK and mito-CK were investigated during perinatal development. Both functional activities appear during late fetal life in species mature at birth like guinea pig, and in the first postnatal weeks in immature species like rat or rabbit. Thus, the functional activity of bound CK isozymes is not associated with birth per se but with the general process of cell maturation. Localization of CK in the cytosol appears optimal for the transfer of glycolytic production of ATP to sites of utilization in an immature heart. During cell maturation, the increasing contribution of oxidative phosphorylation to ATP production, the apparition and binding of mi-CK to mitochondria, the binding of M-CK to myofibrils, turn the cell in a compartmentalized system of energy production. This provides the cellular basis for energy transfer by the PCr-Cr-CK system between sites of ATP production and utilization. Compartmentation of both Ca handling and energy turnover leads to a highly structured cell organization and could be essential for the efficiency of heart function.

The role of the sarcoplasmic reticulum in various types of cardiomyocytes

The relative importance of the sarcoplasmic reticulum (SR) as a source of Ca2+ in the excitation-contraction coupling of mammalian myocytes was tested. Shortening and intracellular Ca2+ transients of electrically paced, isolated, adult rat myocytes were found to be absolutely dependent on the presence of a functional SR and were completely abolished by the SR Ca(2+)-ATPase inhibitors cyclopiazonic acid and thapsigargin or by the Ca(2+)-release channel opener ryanodine. Neonatal rat cardiomyocytes, on the other hand, elicited consistent intracellular Ca(2+)-transients even after complete functional inhibition of the SR. The transients, however, were markedly prolonged. Also isolated adult guinea pig myocytes maintained the ability to shorten after a complete inhibition of the SR Ca(2+)-ATPase by either thapsigargin or cyclopiazonic acid. The twitches and the intracellular Ca(2+)-transients, however, were considerably longer after inhibition of the SR Ca(2+)-ATPase. Different results were obtained after preincubation of the cells with 10 microM ryanodine to induce emptying of the SR Ca2+ pool. In this case, Ca2+ spikes and twitches were also markedly reduced in size, in addition to being prolonged. When a SR Ca(2+)-pump inhibitor was added to ryanodine-treated cells, the size of the Ca(2+)-transients and the capacity of the cells to shorten increased. Ryanodine leaves the activity of the Ca(2+)-pump of the SR intact and thus leads to an underestimation of the amount of excitatory Ca2+ flowing into the cell. The results show that, while the significance of the SR in regulating the Ca(2+)-transients and shortening of cardiomyocytes varies depending on the species and the stage of development, SR function is of paramount importance for the occurrence of rapid twitches.

Organization of calsequestrin-positive sarcoplasmic reticulum in rat cardiomyocytes in culture

The sarcoplasmic reticulum (SR) regulates the levels of cytoplasmic free Ca2+ ions in muscle cells. Calsequestrin is a major Ca(2+)-storing protein and is localized at special sites in the SR. To investigate the development of calsequestrin-positive SR and its interaction with the cytoskeleton, we examined the distribution of calsequestrin in cultured cardiomyocytes from newborn rats by immunofluorescence with anticalsequestrin and antitubulin antibodies and rhodamine-phalloidin. In frozen sections of neonatal rat heart, anticalsequestrin immunostaining was apparent as cross-striations at Z-lines. When newborn cardiomyocytes were isolated, calsequestrin-positive SR was disorganized and was apparent as small vesicles beneath the sarcolemma, whereas myofibrils accumulated in the center of the cells. As the cells spread in culture, calsequestrin-positive vesicles spread to the periphery of the cytoplasm, becoming associated with the developing myofibrils. In mature cells, calsequestrin was closely associated with myofibrils, showing cross-striations at the Z-lines. Double-labeling using anticalsequestrin and antitubulin antibodies demonstrated that the distribution of calsequestrin-positive structures was similar to that of the microtubular arrays. When the microtubules were depolymerized by nocodazole at an early stage, the extension of the SR to the cell periphery was inhibited. In mature cardiomyocytes, nocodazole appeared not to affect the distribution of the SR. These results indicate that the calsequestrin-positive SR in cardiomyocytes is organized at the proper sites of myofibrils during myofibrillogenesis and that the microtubules might serve as tracts for the transport of components of the SR.

Coupling of the Na+/Ca2+ exchanger, Na+/K+ pump and sarcoplasmic reticulum in smooth muscle

The Na+/Ca2+ exchanger, driven by a transmembrane Na+ gradient, plays a key role in regulating Ca2+ concentration in many cells. Although the exchanger influences Ca2+ concentration, its activity in smooth muscle appears to be closely coupled to Ca2+ availability from intracellular stores. This linkage might result if the exchanger were positioned close to Ca2+ storage sites within the sarcoplasmic reticulum. To test this hypothesis we have developed methods to assess the relative three-dimensional distribution of proteins involved in Na+/K+ pumping, Na+/Ca2+ exchange, Ca2+ storage within the sarcoplasmic reticulum, and attachment of contractile filaments to the membrane in smooth muscle. Here we report that the Na+/Ca2+ exchanger is largely co-distributed with the Na+/K+ pump on unique regions of the plasma membrane in register with, and close to, calsequestrin-containing regions of the sarcoplasmic reticulum in sites distinct from the sites where contractile filaments attach to the membrane. This molecular organization suggests that the plasma membrane is divided into at least two functional domains, and appear to provide a mechanism for the strong linkage seen in smooth muscle between Na+/K+ pumping and Na+/Ca2+ exchange, and between Na+/Ca2+ exchange and Ca2+ release from the sarcoplasmic reticulum.

Developmental differences in cardiac myocyte calcium homeostasis after steady-state potassium depolarization: mechanisms and implications for cardioplegia

We previously reported developmental differences in the response of cytosolic free calcium ([Cai]) to extracellular potassium ([Kex]) depolarization (cardioplegia) in juvenile (4 weeks post partum) and mature (12 to 18 months post partum) cardiac myocytes (rabbit). Our present study explored the physiologic basis for these observations. Single calcium-tolerant cardiac myocytes were isolated by sequential exposure to proteolytic agents, loaded with a fluorescent probe for calcium (fura-2) and [Cai] measured by standard fluorescence techniques. The response of [Cai] to [Kex] depolarization (30 mmol/L) was determined in the presence of varying levels of extracellular calcium [Caex], verapamil, ouabain, and amiloride. At nominal levels of [Caex] (0 mmol/L), no significant increases in [Cai] from the juvenile (2.1% +/- 3.3%) or mature (8.5% +/- 3.1%) myocytes were seen. At increasing [Caex], there was a progressive increase in the response of [Cai] to [Kex] depolarization in mature animals' cells (191.8% +/- 40.7%) but not in the juveniles' cells (28.0% +/- 11.5%). Exposure to verapamil resulted in an approximate 80% reduction in relative increase in [Cai] in the mature compared with 60% in the juvenile myocytes. Finally, ouabain exposure resulted in a significant increase in the relative change of [Cai] in juvenile cells (30% to 126.4% +/- 39.4%) but not in the adults' cells. This increase in the juvenile myocytes was blocked by amiloride. We conclude that increases in cardiac myocyte [Cai] after [Kex] depolarization occur predominantly through the calcium channel in the mature animal and through sodium-calcium exchange in the juvenile.

The MgATPase activity of rat cardiac sarcoplasmic reticulum is a function of the calcium ATPase protein

Magnesium-dependent ATPase (MgATPase) activity is associated with many E1-E2 or P-type transport ATPases including the sarcoplasmic reticulum (SR) calcium ATPase. The SR isolated from rat heart has a MgATPase activity which is 6-12 times faster than the MgATPase activity of the SR isolated from dog heart. To determine the origin of the high MgATPase activity of rat heart SR, we compared and contrasted cardiac SR isolated from both species. The preparations were similar in the following ways: (i) contamination by other organelles; (ii) the comigration of MgATPase activity with calcium-dependent ATPase (CaATPase) activity through a sucrose gradient; (iii) a similar ATPase activity sensitivity to pH and ATP concentration; (iv) the high and similar of sensitivity of ATPase activity to detergent; and (v) a similar protein profile. In both preparations, a single protein in the 105,000-Da region of polyacrylamide gels was phosphorylated by ATP, and the phosphorylated species was an acylphosphate formed in the presence and absence of calcium. Dimethyl sulfoxide, which slows acylphosphoenzyme breakdown, markedly inhibited both CaATPase and MgATPase activities of both preparations but not other enzyme activities. Importantly, the specific inhibitor of the SR calcium pump, thapsigargin, completely inhibited the CaATPase activity with an I50 of 6-7 nM; however, a higher concentration (I50 of 2 microM) was required to inhibit the MgATPase activity of the rat cardiac SR. These results provide evidence that the MgATPase activity of rat cardiac SR is part of the enzyme cycle of the calcium ATPase protein.

Sequential expression during postnatal development of specific markers of junctional and free sarcoplasmic reticulum in chicken pectoralis muscle

Skeletal muscle sarcoplasmic reticulum comprises two distinct membrane domains, i.e., the Ca(2+)-pump membrane, corresponding mainly to longitudinal tubules, and the junctional membrane of the terminal cisternae containing the ryanodine receptor/Ca(2+)-release channel. Additional minor proteins previously shown in rabbit fast-twitch skeletal muscle to fractionate selectively to each membrane domain comprise 160- and 53-kDa glycoproteins and 170-kDa low-density lipoprotein (LDL)-binding protein, respectively (Damiani and Margreth, 1991, Biochem. J. 277, 825-832). We report evidence in chicken pectoralis, a predominantly fast muscle, on two closely immunologically related glycoproteins, a minor component of 130-kDa and a major 53-kDa protein. In contrast to the seemingly highly conserved structure of this protein, our results show marked differences in mobilities for chicken 125I-LDL that were detected as a 130- to 116-kDa protein doublet after sodium dodecyl sulfate-polyacrylamide gel electrophoresis, although being otherwise indistinguishable from rabbit 170-kDa protein in LDL-binding characteristics, as well as for preferential association to junctional terminal cisternae. Chicken Ca(2+)-ATPase, although being extensively homologous to rabbit Ca(2+)-ATPase, is shown to be less active and to differ slightly in electrophoretic properties. We have investigated the time course of expression of the specific protein components of longitudinal and of junctional sarcoplasmic reticulum in chick pectoralis muscle from late embryonic development up to 2 months after hatching. Coincident with the posthatching increase in membrane density of high-affinity [3H]ryanodine-binding sites in muscle, both calsequestrin and the species-specific LDL-binding protein(s) are detected in increasing amounts, using ligand blot techniques. In contrast, the appearance and steady accumulation in muscle of Ca(2+)-ATPase, like the time-correlated increase of sarcoplasmic reticulum glycoproteins, are relatively delayed, the most striking changes occurring from 1 week after hatching onward. The sequential expression in chick developing muscle of proteins selectively associated with the junctional terminal cisternae and with longitudinal sarcoplasmic reticulum, respectively, argues for a similar morphogenetic program in avian and mammalian species and, to account for that, for the existence of common epigenetic differentiating influences on the expression of sarcoplasmic reticulum protein genes.

Force-frequency relationship, contraction duration and recirculating fraction of calcium in postnatally developing rat heart ventricles: correlation with heart rate

Heart rates (HR) of awake unrestrained animals, isometric contraction duration and force-frequency relationship of ventricular tissue were determined in adult and postnatally developing rats. Resting HR was lowest in newborns (256 beats min-1), reached maximum at the age of 2.5 weeks (506 beats min-1) and then declined to the level of adult rats (381 beats min-1). Duration of isometric contraction correlated negatively with HR. Time to peak tension (TPT) was 185 ms in newborns but fell rapidly during the first days of post-natal life. Minimum was attained at the age of 2.5 weeks (TPT = 98 ms), followed by a slight prolongation towards adulthood. Recirculating fraction of activator Ca2+ increased parallel with HR, being 6% in newborns, 33% in 11-day-old pre-weanlings, and 87% in adult rats. Similar developmental pattern of the parameters suggests that a post-natal increase in HR and a shortening of contraction duration are closely associated with a shift from extracellular to intracellular source of activator Ca2+. Force-frequency curves were similar at different developmental stages and consisted of three phases; a negative staircase between 0.05 and 1.0 Hz, a positive staircase between 1.0 and 4.0 Hz, and a secondary decline above 4.0 Hz. In adult rats the positive force staircase was weak or absent. Furthermore, our results show that negative staircase is not only a property of adult rat heart but is present, and even more pronounced, in pre-weanling and weanling rat heart. Therefore negative staircase is not solely explained by quantitative changes in the contribution of sarcoplasmic reticulum (SR) to contractile activation, but rather by the mechanisms which regulate loading and/or release of sarcoplasmic reticular Ca2+.

Regulation of sarcoplasmic reticulum gene expression during cardiac and skeletal muscle development

The expression of major sarcoplasmic reticulum proteins during cardiac and fast-twitch skeletal muscle development was examined using gene-specific probes. Through the use of S1 nuclease mapping, Northern blot, and RNA slot-blot analysis, sarcoplasmic reticulum proteins were shown to exhibit both narrow tissue specificity and plasticity in their expression during muscle development. In fast-twitch skeletal muscle, the cardiac/slow-twitch isoforms of Ca(2+)-ATPase and calsequestrin were detected at high levels in fetal stages but were gradually replaced by fast-twitch isoforms in adult muscle. In contrast, cardiac muscle expressed exclusively cardiac/slow-twitch isoforms of Ca(2+)-ATPase and calsequestrin at all stages. Both fast-twitch and slow-twitch skeletal muscle expressed the same skeletal muscle ryanodine receptor isoform, whereas cardiac muscle expressed a cardiac isoform. Phospholamban expression was restricted to cardiac and slow-twitch skeletal muscle and did not appear in developing fast-twitch skeletal muscle. During in vitro myogenesis of C2C12 cells, the mRNA transcripts encoding sarcoplasmic reticulum proteins were found to be coordinately induced in synchrony with that of contractile protein mRNA. The myogenic factor "myogenin" induced sarcoplasmic reticulum gene transcripts along with contractile protein mRNAs in nonmyogenic cells. These data suggest that the induction of both sarcoplasmic reticulum and contractile protein gene families is under the control of a common myogenic differentiation program.

Expression of sarcoplasmic reticulum Ca(2+)-ATPase and calsequestrin genes in rat heart during ontogenic development and aging

Little is known concerning the molecular mechanisms responsible for changes in sarcoplasmic reticulum (SR) function during ontogenic development and aging except that the amount of SR Ca(2+)-ATPase mRNA varies in these conditions. The aim of the present work was to determine whether SR maturation requires expression of specific isoforms and synchronous accumulation of mRNAs encoding proteins located in SR. Thus, we have studied expression of SR Ca(2+)-ATPase and calsequestrin genes in the rat at different developmental stages from 14 fetal days to 24 months of age. Analysis of alternative splicing of the major Ca(2+)-ATPase gene expressed in heart by nuclease S1 mapping led us to conclude that the Ca(2+)-ATPase gene expressed in heart was not differentially spliced during ontogenic development and senescence. A single calsequestrin mRNA isoform was also detected in rat heart whatever the developmental stage. The amount of specific mRNA was then measured by dot blot and normalized to 18S ribosomal RNA or to myosin heavy chain mRNA. The amount of Ca(2+)-ATPase mRNA relative to 18S RNA increases substantially at the end of fetal life and in the early postnatal period (9.5 +/- 0.5% in the 14-15 day fetus versus 99 +/- 7% in the 4-day-old rat). A stable high level is observed during adulthood. In aged rats (24 months), Ca(2+)-ATPase mRNA represents only 44.6% the amount observed in young adults (1-2 months).(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of the major protein phosphatases in mammalian cardiac muscle which dephosphorylate phospholamban

The protein phosphatases which dephosphorylate native, sarcoplasmic reticulum (SR)-associated phospholamban were studied in cardiac muscle extracts and in a Triton fraction prepared by detergent extraction of myofibrils, the latter fraction containing 70-80% of the SR-associated proteins present in the tissue. At physiological concentrations of free Mg2+ (1 mM), protein phosphatase 1 (PP1) accounted for approximately 70% of the total phospholamban phosphatase activity in these fractions towards either Ser-16 (the residue labelled by cAMP-dependent protein kinase, PK-A) or Thr-17 (the residue phosphorylated by an SR-associated Ca2+/calmodulin-dependent protein kinase). Protein phosphatase 2A (PP2A) and protein phosphatase 2C (PP2C) accounted for the remainder of the activity. A major form of cardiac PP1, present in comparable amounts in both the extract and Triton fraction, was similar, if not identical, to skeletal muscle protein phosphatase 1G (PP1G), which is composed of the PP1 catalytic (C) subunit complexed to a G subunit of approximately 160 kDa, responsible for targeting PP1 to both the SR and glycogen particles of skeletal muscle. This conclusion was based on immunoblotting experiments using antibody to the G subunit, ability to bind to glycogen and the release of PP1 activity from glycogen upon incubation with PK-A and MgATP. PP1 accounted for approximately 90% of the phospholamban (Ser-16 or Thr-17) phosphatase activity in the material sedimented by centrifugation at 45,000 x g, a fraction prepared from cardiac extracts which is enriched in SR membranes. The G subunit in this fraction could be solubilised by Triton X-100, but not with 0.5 M NaCl or digestion with alpha-amylase, indicating that it is bound to membranes and not to glycogen. By analogy with the situation in skeletal muscle, the PK-A catalysed phosphorylation of the G subunit, with ensuing release of the C subunit from the SR, may prevent PP1 from dephosphorylating SR-bound substrates and represent one of the mechanisms by which adrenalin increases the phosphorylation of cardiac phospholamban (Ser-16 and Thr-17) in vivo. Hearts left in situ post mortem lose 85-95% of their PP1 activity within 20-30 min. This remarkable disappearance of PP1 may partly explain why the importance of this enzyme in cardiac muscle metabolism has not been recognized previously.

Developmental changes in cardiac myocyte calcium regulation

Developmental changes in the contributions of transsarcolemmal Ca2+ influx and Ca2+ release from intracellular storage sites to myocardial contraction were evaluated in isolated ventricular myocytes from neonatal (aged 1-7 days) and adult (aged 8-10 weeks) New Zealand White rabbits. Contractions ceased in one beat when extracellular Ca2+ was decreased from 1mM to micromolar levels using a rapid perfusion technique. On reperfusion with 1 mM Ca2+, recovery of control contraction amplitude occurred after significantly fewer beats in neonatal myocytes compared with adult myocytes, and after 1 minute compared with 5 minutes of reduced Ca2+. After 15 minutes of perfusion with either 1 or 10 microM ryanodine, contraction amplitude decreased in both age groups, but the decrease was significantly greater in adults than in neonates. These experiments indicate that isolated ventricular myocytes may be used in the study of developmental changes in intracellular Ca2+ regulation. Results suggest that cardiac contraction in neonates is relatively more dependent on transsarcolemmal Ca2+ influx. Furthermore, although Ca2+ release from intracellular storage sites is present in both neonates and adults, its role in cardiac contraction is more significant in adults.

The cardiac sarcoplasmic reticulum calcium-release channel: modulation of ryanodine binding and single-channel activity

[3H]Ryanodine binding to a preparation of isolated cardiac sarcoplasmic reticulum has been investigated. A method is reported which produces a very high level of specific binding. Scatchard analysis of binding up to 50 nM ryanodine yields data which infer a single class of binding sites with a Kd of 1.4 nM and a Bmax of 9.7 pmol/mg protein. Micromolar calcium is the principal activating ligand and its effects on binding are modulated by ligands which similarly affect the activity of single calcium-release channels incorporated into artificial planar phospholipid bilayers. The benzimidazole drug, sulmazole, is able to stimulate ryanodine binding in the presence of sub-activating calcium concentrations. Ryanodine binds to the native channel only when it is in its open state and stimulation of maximal ryanodine binding is achieved by ligands which are insufficient to produce full single-channel activation. A model is proposed which relates the modulation of ryanodine binding to the behaviour of single channels.

Development of the myocardial contractile system

Recent studies regarding developmental changes in the myocardial contractile system from fetal, newborn, and adult animals are reviewed. From the data obtained so far, we conclude that in the early fetus myocardial contraction is mainly dependent on Ca which enters via the sarcolemma. Ca release from the sarcoplasmic reticulum is minimal. The role of the sarcoplasmic reticulum as a source of contractile Ca increases and the role of Ca influx across the sarcolemma in contractile system decreases with development.