Composite polymorphisms in the ryanodine receptor 2 gene associated with arrhythmogenic right ventricular cardiomyopathy

OBJECTIVE

Mutations in the cardiac ryanodine receptor (RYR2) gene have been reported to cause arrhythmogenic right ventricular cardiomyopathy (ARVC). The molecular mechanisms by which genetic modifications lead to ARVC are still not well understood.

METHODS

ARVC patients were screened for mutations in the RYR2 gene by denaturing HPLC and DNA sequencing. Single channel measurements were carried out with RyR2 channels purified from explanted hearts of ARVC patients.

RESULTS

None of the published RYR2 mutations were found in our ARVC-cohort. However, we identified two single nucleotide polymorphisms (SNPs) in exon 37 of the human RYR2 gene which lead to the amino acid exchanges G1885E and G1886S, respectively. Both SNPs together were found exclusively in 3 out of 85 ARVC patients in a composite heterozygous fashion (genotype T4). This genotype was associated with ARVC (p<0.05) but not with dilated cardiomyopathy (DCM, 79 patients) or none-failing controls (463 blood donors). However, either one of the two SNPs were identified in further 7 ARVC patients, in 11 DCM patients, and in 64 blood donors. The SNP leading to G1886S may create a protein kinase C phosphorylation site in the human RyR2. Single channel recordings at pCa4.3 revealed four conductance states for the RyR2 of genotype T4 and a single open state for the wild type RyR2. At pCa7.7, the lowest subconductance state of the RyR2 channel of genotype T4 persisted with a greatly enhanced open probability indicating a leaky channel.

CONCLUSION

The RyR2 channel leak under diastolic conditions could cause SR-Ca2+ depletion, concomitantly arrhythmogenesis and heart failure in a subgroup of ARVC patients of genotype T4. A change in the RyR2 subunit composition due to the combined expression of both SNPs alters the behaviour of the tetrameric channel complex.

Phosphorylation of human cardiac troponin I G203S and K206Q linked to familial hypertrophic cardiomyopathy affects actomyosin interaction in different ways

cAMP-dependent protein kinase (PKA)-dependent phosphorylation of the two serine residues in the amino terminal region unique to cardiac troponin I (cTnI) is known to cause two effects: (i) decrease of the maximum Ca2+-controlled thin filament-activated myosin S1-ATPase (actoS1-ATPase) activity and mean sliding velocity of reconstituted thin filaments; (ii) rightward shift of the Ca2+ activation curves of actoS1-ATPase activity, filament sliding velocity, and force generation. We have studied the influence of phosphorylation of human wild-type cTnI and of two mutant cTnI (G203S and K206Q) causing familial hypertrophic cardiomyopathy (fHCM) on the secondary structure by circular dichroism spectroscopy and on the Ca2+ regulation of actin-myosin interaction using actoS1-ATPase activity and in vitro motility assays. Both mutations slightly influence the backbone structure of cTnI but only the secondary structure of cTnI-G203S is also affected by bis-phosphorylation of cTnI. In functional studies, cTnI-G203S behaves similarly to wild-type cTnI, i.e. the mutation itself has no measurable effect and bis-phosphorylation alters the actoS1-ATPase activity and the in vitro thin filament motility in the same way as does bis-phosphorylation of wild-type cTnI. In contrast, the mutation K206Q leads to a considerable increase in the maximum actoS1-ATPase activity as well as filament motility compared to wild-type cTnI. Bis-phosphorylation of this mutant cTnI still suppresses the maximum actoS1-ATPase activity and filament sliding velocity but does no longer affect the Ca2+ sensitivity of these processes. Thus, these two fHCM-linked cTnI mutations, although reflecting similar pathological situations, exert different effects on the actomyosin system per se and in response to bis-phosphorylation of cTnI.

Differential regulation of Ca2+-dependent ATPase-activity in left ventricular myocardium during mechanical circulatory support

BACKGROUND

Myocardial recovery is observed in some end-stage heart failure patients after mechanical circulatory support. The sarcoplasmic reticulum Ca(2+)-adenosine triphosphatase (Ca2+-ATPase) activity is down-regulated in failing myocardium and contributes to heart failure-associated contraction/relaxation abnormalities. Regulation of Ca(2+)-ATPase after mechanical support was shown to be heterogeneous. Thus, we analyzed Ca(2+)-ATPase activity and protein expression in the paired myocardial samples of 21 patients supported by ventricular assist devices to identify factors that influence restoration of the Ca(2+)-transient after ventricular assist device support.

METHODS

We measured Ca(2+)-ATPase activity using a reduced nicotinamide-adenine dinucleotide-coupled reaction, determined sarcoplasmic reticulum Ca(2+)-dependent ATPase protein using Western blotting, and determined 4-hydroxyproline using amino-acid analysis.

RESULTS

The mean Ca(2+)-ATPase activity decreased at assist-device implantation and slightly increased at transplantation, but remained significantly lower than in non-failing donor hearts. However, individual responses were heterogeneous. Patients with older age, increased left ventricular diameter, and increased 4-hydroxyproline content showed down-regulation of Ca(2+)-ATPase activity, whereas we found up-regulation in patients with low values for these parameters after assist-device support.

CONCLUSIONS

Sarcoplasmic reticulum Ca(2+)-ATPase activity, which influences the myocardial Ca(2+)-transient, generally is not restored to normal values in assist-device-supported hearts, but depends on a combined score of the left ventricular end-diastolic diameter, degree of ventricular fibrosis, and age of the patient at the time of assist-device implantation.

Effects of phosphorylation and mutation R145G on human cardiac troponin I function

We have studied functional consequences of the mutations R145G, S22A, and S23A of human cardiac troponin I (cTnI) and of phosphorylation of two adjacent N-terminal serine residues in the wild-type cTnI and the mutated proteins. The mutation R145G has been linked to the development of familial hypertrophic cardiomyopathy. Cardiac troponin was reconstituted from recombinant human subunits including either wild-type or mutant cTnI and was used for reconstitution of thin filaments with skeletal muscle actin and tropomyosin. The Ca(2+)-dependent thin filament-activated myosin subfragment 1 ATPase (actoS1-ATPase) activity and the in vitro motility of these filaments driven by myosin were measured as a function of the cTnI phosphorylation state. Bisphosphorylation of wild-type cTnI decreases the Ca(2+) sensitivity of the actoS1-ATPase activity and the in vitro thin filament motility by about 0.15-0.21 pCa unit. The nonconservative replacement R145G in cTnI enhances the Ca(2+) sensitivity of the actoS1-ATPase activity by about 0.6 pCa unit independent of the phosphorylation state of cTnI. Furthermore, it mimics a strong suppressing effect on both the maximum actoS1-ATPase activity and the maximum in vitro filament sliding velocity which has been observed upon bisphosphorylation of wild-type cTnI. Bisphosphorylation of the mutant cTnI-R145G itself had no such suppressing effects anymore. Differential analysis of the effect of phosphorylation of each of the two serines, Ser23 in cTnI-S22A and Ser22 in cTnI-S23A, indicates that phosphorylation of Ser23 may already be sufficient for causing the reduction of maximum actoS1-ATPase activity and thin filament sliding velocity seen upon phosphorylation of both of these serines.

The time course of natriuretic hormones as plasma markers of myocardial recovery in heart transplant candidates during ventricular assist device support reveals differences among device types

BACKGROUND

The natriuretic hormones ANP and BNP are expressed differently in the myocardium. Both hormones have compensatory diuretic activity during heart failure. Mechanical stretch of the myocardial walls induces the expression of these hormones. In failing human myocardium, both ANP and BNP are transcribed in the ventricular myocardium in high amounts. We measured the plasma concentrations of ANP and BNP in patients supported by various ventricular assist devices (VADs) at various times. We analyzed the time courses of ANP and BNP to determine (1) the time scale of their down-regulation as a marker of putative myocardial recovery, (2) their steady-state levels under VAD support and (3) differences caused by various VAD devices.

METHODS

We analyzed ANP and BNP using commercially available radioimmune assays. We analyzed the time courses of patients supported by Thoratec (THO) LVAD (n = 8), TCI Heartmate (TCI) (n = 6), Novacor (NOV) (n = 7), and Lionheart (LIO) (n = 3).

RESULTS

Patients supported with NOV and some patients with TCI showed down-regulation of BNP to a steady-state level at 30 to 50 days, following a single exponential decay. In contrast, patients supported by THO or LIO did not reveal a determined time course of the natriuretic hormones. Only a few patients reached normal plasma values during VAD support.

CONCLUSION

The time courses of ANP and BNP differ among VAD types because of construction and/or driving mode, which might be important when considering patients for weaning from VAD without heart transplant.

Cloning of a phospholipase C-delta 1 of rabbit skeletal muscle

The phospholipase C isoform responsible for the increase in the total myoplasmic inositol 1,4,5-trisphosphate concentration during tetanic contraction of isolated skeletal muscle and its mechanism of activation is not known. We have cloned and sequenced a phospholipase C cDNA of rabbit skeletal muscle coding for a protein of 745 amino acids with a molecular mass of 84,440 kDa. The deduced amino acid sequence exhibits the phospholipase C-specific domains X and Y which according to current knowledge very likely represent the catalytic centre of the enzyme. An overall sequence homology of 88% to the phospholipase C-delta 1 of rat brain suggests that the encoded protein represents a phospholipase C-delta 1 isoform of rabbit skeletal muscle. Northern blot analysis shows, that this phospholipase C-delta is dominantly expressed in skeletal muscle, less strongly in smooth muscle (uterus) and lung and weakly in heart, kidney and brain. In the N-terminal part of the primary structure a consensus sequence for a canonical EF-hand Ca2+ binding domain can be identified together with a short positively charged motif which recently has been suggested to be essential for the binding of phosphatidylinositol 4,5-bisphosphate. If these two domains which are unique for phospholipase C-delta are sufficient in establishing a mechanism for the activation of the enzyme, inositol 1,4,5-trisphosphate formation in skeletal muscle could be the consequence of an increase in myoplasmic Ca2+.

Pattern formation on cardiac troponin I by consecutive phosphorylation and dephosphorylation

Two serine residues located adjacently in the heart-specific N-terminus of cardiac troponin I can be phosphorylated in vivo. Both residues are sequentially phosphorylated and dephosphorylated by cAMP-dependent protein kinase (PKA) and protein phosphatase 2A (PP2A). The concentration changes of the different troponin I species have been determined separately for the phosphorylation and dephosphorylation reaction and approximated by time courses predicted by a reaction model. Dependent on the concentration ratio of active protein kinase/protein phosphatase, four different troponin I species can be generated; one nonphosphorylated, two monophosphorylated and one bisphosphorylated. This pattern generation will be observed in proteins phosphorylated and dephosphorylated by a single protein kinase and phosphatase on more than one site and is a new principle inherent in signal cascades.

Phosphoinositides in membranes that build up the triads of rabbit skeletal muscle

The total membrane concentrations of PtdIns, PtdIns4P, and PtdIns(4,5)P2 contribute to the functional capacity of the Ins(1,4,5)P3 signalling system which is operating in skeletal muscle but the function of which is still unknown. Total amounts of these phosphoinositides have been determined in purified membranes of transverse tubules (TT) and terminal cisternae (TC) of the sarcoplasmic reticulum (SR) of rabbit skeletal muscle. PtdIns and PtdIns4P have been detected in both membrane systems whereas PtdIns(4,5)P2 (290 mumol/mol phospholipid) is confined only to TT. A much greater pool of PtdIns(4,5)P2 seems, however, to be located in the sarcolemma away from the triadic junction.

Identification of a new 80 k isoform of phosphatidylinositol 4-phosphate 5-kinase from bovine brain

Phosphatidylinositol 4-phosphate 5-kinase is associated with bovine brain microsomes to an extent of approximately 65% of the total cellular enzyme activity. This membrane-associated kinase activity can be solubilized with Triton X-114. Following polyacrylamide gel electrophoresis in the presence of SDS the enzyme can be renaturated from gel slices in the presence of desoxycholate and Triton X-100. Catalytic activity appears at an apparent molecular weight of 80 k. Analysis of the reaction product formed by the 80 k protein reveals the existence of a 5-phosphotransferase, identifying the 80 k polypeptide as a new phosphatidylinositol 4-phosphate 5-kinase isoform.

Excitation-contraction coupling in a pre-vertebrate twitch muscle: the myotomes of Branchiostoma lanceolatum

The segmented trunk muscle (myotome muscle) of the lancelet (Branchiostoma lanceolatum), a pre-vertebrate chordate, was studied in order to gain information regarding the evolution of excitation-contraction (EC) coupling. Myotome membrane vesicles could be separated on isopycnic sucrose gradients into two main fractions, probably comprising solitary microsomes and diads of plasma membrane and sarcoplasmic reticulum, respectively. Both fractions bound the dihydropyridine PN 200/110 and the phenylalkylamine (-)D888 (devapamil) while specific ryanodine binding was observed in the diad preparation only. Pharmacological effects on Ca2+ currents measured under voltage-clamp conditions in single myotome fibers included a weak block by the dihydropyridine nifedipine and a shift of the voltage dependences of inactivation and restoration to more negative potentials by (-)D888. After blocking the Ca2+ current by cadmium in voltage-clamped single fibers, the contractile response persisted and a rapid intramembrane charge movement could be demonstrated. Both responses exhibited a voltage sensitivity very similar to the one of the voltage-activated Ca2+ channels. Our biochemical and electrophysiological results indicate that the EC coupling mechanism of the protochordate myotome cell is similar to that of the vertebrate skeletal muscle fiber: Intracellular Ca2+ release, presumably taking place via the ryanodine receptor complex, is under control of the cell membrane potential. The sarcolemmal Ca2+ channels might serve as voltage sensors for this process.

Compartmentalized ATP synthesis in skeletal muscle triads

Isolated skeletal muscle triads contain a compartmentalized glycolytic reaction sequence catalyzed by aldolase, triosephosphate isomerase, glyceraldehyde-3-phosphate dehydrogenase, and phosphoglycerate kinase. These enzymes express activity in the structure-associated state leading to synthesis of ATP in the triadic junction upon supply of glyceraldehyde 3-phosphate or fructose 1,6-bisphosphate. ATP formation occurs transiently and appears to be kinetically compartmentalized, i.e., the synthesized ATP is not in equilibrium with the bulk ATP. The apparent rate constants of the aldolase and the glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase reaction are significantly increased when fructose 1,6-bisphosphate instead of glyceraldehyde 3-phosphate is employed as substrate. The observations suggest that fructose 1,6-bisphosphate is especially effectively channelled into the junctional gap. The amplitude of the ATP transient is decreasing with increasing free [Ca2+] in the range of 1 nM to 30 microM. In the presence of fluoride, the ATP transient is significantly enhanced and its declining phase is substantially retarded. This observation suggests utilization of endogenously synthesized ATP in part by structure associated protein kinases and phosphatases which is confirmed by the detection of phosphorylated triadic proteins after gel electrophoresis and autoradiography. Endogenous protein kinases phosphorylate proteins of apparent Mr 450,000, 180,000, 160,000, 145,000, 135,000, 90,000, 54,000, 51,000, and 20,000, respectively. Some of these phosphorylated polypeptides are in the Mr range of known phosphoproteins involved in excitation-contraction coupling of skeletal muscle, which might give a first hint at the functional importance of the sequential glycolytic reactions compartmentalized in triads.

Masses of inositol phosphates in resting and tetanically stimulated vertebrate skeletal muscles

The masses of inositol phosphates have been determined in isolated skeletal muscles from Xenopus laevis (sartorius, tibialis anterior and iliofibularis) and rat (gastrocnemius and soleus) which were quick-frozen in the resting state and at different stages of an isometric (Xenopus) or isotonic (rat) tetanus. The isomeric spectrum of inositol phosphates detected was similar to that in other tissues and cell types. The total sarcoplasmic concentrations of the isomers Ins-(1,4,5,6)P4/Ins(3,4,5,6)P4 (0.2-0.9 microM), Ins(1,3,4,6)P4 (not detectable), Ins(1,3,4,5,6)P5 (about 1 microM) and InsP6 (3.2-4.6 microM) were lower than in other cell types. Variations in these concentrations were due to the muscle type rather than to the donor species. The putative second messenger Ins(1,4,5)P3, as well as its dephosphorylation product Ins(1,4)P2, were present at surprisingly high total myoplasmic resting concentrations, ranging from 1.2 to 2.5 microM and 3.5 to 6.9 microM respectively. Upon tetanic stimulation these two inositol phosphates in particular exhibited significantly increased total sarcoplasmic concentrations, up to 4.2 microM and 11.3 microM respectively, with a time scale of seconds. From the initial rate of increase in the total sarcoplasmic concentrations of Ins(1,4,5)P3 and its rapidly formed metabolic products, a minimal phosphoinositidase C (PIC) activity in tetanically activated Xenopus skeletal muscle of about 1.7-2.6 microM/s can be estimated. This PIC activity observed in vivo seems to be far too low to account for a functional role for Ins(1,4,5)P3 as a chemical transmitter in the fast excitation-contraction coupling (ECC) process in skeletal muscle. The presence of Ins(1,3,4,5)P4 in all muscle types is indicative of a Ca(2+)-activated Ins(1,4,5)P3 3-kinase activity. The rapid transient increases in Ins(1,3,4)P3 and Ins(1,3)P2 in isometrically contracting Xenopus muscles suggest that corresponding Ins(1,3,4,5)P4 phosphatases are operating in skeletal muscle as well. In all muscles investigated except rat soleus, the fructose 1,6-bisphosphate [Fru(1,6)P2] concentration increased substantially during a tetanus, up to about 2 mM. This increase is correlated with a simultaneous decrease in phosphocreatine, whereas the energy charge of the muscles was essentially unaffected by the applied tetani. The time course of the rise in Fru(1,6)P2 was used to model changes in the free concentrations of high-affinity aldolase-binding inositol phosphates during the course of a tetanus. These calculations demonstrate that the free concentration of Ins(1,4,5)P3 and other aldolase-bound inositol phosphates can increase much faster and to a larger extent than the corresponding total concentrations as a result of their competitive displacement from aldolase-binding sites by the rapidly rising concentration of Fru(1,6)P2.

Relation of phosphatidylinositol metabolism to glycolytic pathway in skeletal muscle membranes

Skeletal muscle triads are possessing the whole set of enzymes of the phosphatidylinositol (PI)-linked signal generating pathway, PI-kinase, PI(4)P-kinase, and PI(4,5)P2-phospholipase C (PLC). The activities of these enzymes are comparable to those found in other cell types for which a functional role of the PI-pathway in intracellular signal transduction has been established. For skeletal muscle an unequivocal function and an initiating signal for Ins(1,4,5)P3-liberation is still unknown. However, the observed Ca-dependency of PLC activity suggests that here Ins(1,4,5)P3 production is a consequence rather than a cause of increasing cytosolic Ca2+. Recently, the glycolytic enzyme aldolase, whose activity can be modulated by inositol polyphosphates, has been localized in the triadic structure. The enzyme which has a high affinity to Ins(1,4)P2, Ins(1,4,5)P3 and Ins(1,3,4,5)P4, seems to be compartmentalized to the junctional foot structure from which it is released upon binding of these molecules. This phenomenon could reflect a capability for regulation of the glycolytic flux even for aldolase, especially if a non steady-state situation in the junctional gap is considered. Meanwhile we have accumulated evidence for the operation of a partial glycolytic sequence in the junctional region established by the enzymes aldolase, glyceraldehyde-3-P (GAP) dehydrogenase and phosphoglycerate kinase. This system is able to produce ATP upon oxidation of GAP and could be, because of the inositol polyphosphate-sensing abilities of aldolase, a target for the membrane associated PI-pathway. The ATP production is however transient which indicates the coupling to an ATP hydrolyzing reaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Inositol polyphosphate-mediated repartitioning of aldolase in skeletal muscle triads and myofibrils

The effects of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), which has been hypothesized to be a chemical transmitter in excitation-contraction coupling in skeletal muscle, on aldolase bound to isolated triad junctions were investigated. Fructose-1,6-bisphosphate aldolase was identified as the major specific binding protein for the Ins(1,4,5)P3 analogue glycolaldehyde (2)-1-phospho-D-myo-inositol 4,5-bisphosphate which can form covalent bonds with protein amino groups by reduction of the Schiff's base intermediate with [3H]NaCNBH3. This analogue, Ins(1,4,5) P3, and the inositol polyphosphates inositol 1,3,4,5-tetrakisphosphate and inositol 1,4-bisphosphate were nearly equipotent in selectively releasing membrane bound aldolase with a K0.5 of about 3 microM. The rank order of the K0.5 values was identical to the KI values for inhibition of aldolase. Aldolase was also released by its substrate fructose 1,6-bisphosphate and by 2,3-bisphosphoglycerate. Ins(1,4,5)P3-induced aldolase release did not disrupt the triad junction; glyceraldehyde-3-phosphate dehydrogenase, a known junctional constituent, was displaced only at much higher Ins(1,4,5)P3 concentrations. Ins(1,4,5)P3 was as effective as fructose 1,6-bisphosphate in releasing aldolase from myofibrils. A finite number of binding sites for aldolase exist on triads (Bmax = 43-47 pmol of tetrameric aldolase exist on triads (Bmax = 43-47 pmol of tetrameric aldolase/mg of triad protein, KD = 23 nM). The junctional foot protein was implicated as an aldolase binding site by affinity chromatography with the junctional foot protein immobilized on Sepharose 4B. The potential consequences of aldolase being bound in the gap between the terminal cisternae and the transverse tubule to inositol polyphosphate and glycolytic metabolism in that local region are discussed.

Effects of guanidinium on EC coupling and tension generation in frog skeletal muscle

The effect of the chaotropic cation guanidinium on tension generation was investigated in voltage-clamped intact and mechanically skinned muscle fibres of the frog. When sodium was replaced by guanidinium in the solution a 20-mV shift of the sigmoidal activation curve towards less negative potentials was recorded. A similar shift in the voltage dependence of mechanical inactivation did not occur. The plateau phase of contractures activated by long-lasting depolarizations was significantly shortened in the presence of 77.5 mM guanidinium. In a second set of experiments, charge displacement currents were measured using the cut fibre preparation. Apparently, guanidinium had no effect on the voltage dependence of intramembrane charge movement. On the other hand, this cation caused a distinct increase in the amount of charge necessary to reach the contraction threshold at rheobase voltage from 12.4nC microF-1 to 23.4nC microF-1. Experiments on skinned fibres containing an operating sarcoplasmic reticulum demonstrated that 5 mM guanidinium diminished caffeine-induced tension development and substantially delayed the onset of the contractile response. However, guanidinium did not impair calcium-induced tension development of the contractile apparatus. These results suggest that the inhibitory action of guanidinium on excitation-contraction coupling is due to a depression of calcium release from the sarcoplasmic reticulum.

Localization of phosphorylase kinase subunits at the sarcoplasmic reticulum of rabbit skeletal muscle by monoclonal and polyclonal antibodies

Molecular structures related to phosphorylase kinase have been localized by light and electron microscopy in tissue sections of rabbit skeletal muscle employing polyclonal antibodies directed against the holoenzyme as well as monoclonal antibodies specific for its alpha-, beta- or gamma-subunits. In frozen sections of prefixed muscle fibres both known major regions of glycogen deposition, the intermyofibrillar space and the perinuclear area, are stained predominantly. In sections of unfixed muscle in which cytosolic phosphorylase kinase was removed by extensive washes prior to immunostaining the immunolabel is mainly associated with the sarcoplasmic reticulum (SR). This membrane location is further confirmed by immunoblot analysis of proteins solubilized from isolated SR with Triton X-114. Employing monoclonal antibodies two membrane proteins are identified as the alpha- and beta-subunits of phosphorylase kinase by Western blots. Immunoprecipitates reveal also the gamma-subunit; the delta-subunit, i.e., calmodulin, is enriched with the solubilized enzyme. It proves that a SR membrane associated form of holophosphorylase kinase exists in muscle. Functionally, this kinase might be involved in phosphorylation of phosphatidylinositol present on the SR Ca2+ transport ATPase and thereby might play a role in regulation of Ca2+ transport.

Activation of the contractile apparatus of skinned fibres of frog by the divalent cations barium, cadmium and nickel

The contractile apparatus of mechanically skinned muscle fibres of frog can be reversibly activated by Ba2+ and Cd2+. The maximum force induced by both Ba2+ and Cd2+ is the same as that induced by Ca2+ and Sr2+. The ionic concentrations of the divalent cations required to induce 50% of the maximum activated force at 1 mM-Mg2+, pH 7.10, 22 degrees C and 250 mM ionic strength are about 8 X 10(-7) M for Ca2+, 5 X 10(-6) M for Cd2+, 2.6 X 10(-5) M for Sr2+ and 7 X 10(-4) M for Ba2+. Exposure of the skinned fibre to relatively low Ni2+ concentrations (between 10(-6) and 10(-5) M) resulted in a transient force response accompanied by an irreversible change in the ability of the preparation to develop force. The Ba2+- and Cd2+-activation curves are considerably flatter than the corresponding curves for Ca2+ and Sr2+. An increase in Mg2+ concentration from 1 to 3 mM decreased the sensitivity of the contractile apparatus to Ba2+ by a factor of about 1.5 without affecting the maximum force response. The Ca2+-activation curve was modified in the presence of subthreshold concentrations of Ba2+ and the results indicate that Ba2+ could have both an activating and an inhibitory action on the Ca2+-activated force. A kinetic model which can quantitatively explain the results for activation of contraction by Ba2+ and Ca2+, is described.

Inositol 1,4,5-trisphosphate enhances Ca2+-sensitivity of the contractile mechanism of chemically skinned rabbit skeletal muscle fibres

The possibility that inositol 1,4,5-trisphosphate may also act on subcellular structures different from membraneous compartments has been examined using chemically skinned skeletal muscle fibres. At about 1 to 25 microM IP3 reversibly enhanced isometric steady-state force production of these preparations at free Ca2+ concentrations corresponding to submaximum activation in a concentration-dependent manner. The maximum Ca2+-induced tension was not altered by IP3. These results show that IP3 can modulate the apparent Ca2+-sensitivity of the contractile mechanism. They suggest a new modulatory function of IP3 in skeletal muscle.

Monoclonal antibodies to rabbit skeletal muscle phosphorylase kinase. Probes for studies of subunit function

Monoclonal antibodies to rabbit skeletal muscle phosphorylase kinase were produced by the conventional hybridoma cell technique. 90 out of 600 hybridomas were found to produce phosphorylase kinase binding antibodies from which only five secreted also phosphorylase kinase activity affecting antibodies. Three of them were cloned; two hybridomas resisted all cloning efforts. Employing immunoblot technique all monoclonal antibodies show cross-reactivity with the alpha, beta, and gamma subunits of phosphorylase kinase indicating that similar, if not identical, epitopes are present on these three subunits. No cross-reactivity with delta is observed. Monoclonal antibodies secreted by two clones which bind to the alpha subunit stimulate the Ca2+-independent A0 activity of phosphorylase kinase more than 30-fold, whereas all other monoclonal antibodies obtained are ineffective in this respect. Monoclonal antibodies binding to the beta subunit inhibit the Ca2+-dependent activities significantly. Antibody produced by one hybridoma binds to the alpha, beta, and gamma subunits with approximately the same affinity. Based on the dual function of calmodulin in phosphorylase kinase (Hessová, Z., Varsányi, M., and Heilmeyer, L.M.G., Jr. (1985) Eur. J. Biochem. 146, 107-115) we conclude that binding of anti-alpha monoclonal antibodies to a regulatory domain in the alpha subunit results in an uncoupling of the inhibitory function of the Ca2+-free delta from the holoenzyme which leads to a concomitant increase in A0 activity. Furthermore, binding of anti-beta monoclonal antibodies to the beta subunit prevents a signal transfer from the Ca2+-saturated delta to the catalytic site of the holoenzyme which inhibits the Ca2+-dependent activities.

Electromechanical coupling III. Estimation of the Ca storage capacity of the SR by analysing the time course of caffeine-induced tension transients of skinned muscle fibres

The Ca uptake of the SR of a skinned fibre, which was studied with an indirect method, shows a biphasic time course. A total Ca concentration between 8 and 23 mm was accumulated in the SR by an initial fast process (tau approximately equal to 4.7 s). This Ca-uptake activity of the SR is high enough to bring about relaxation of twitch contraction in vivo. The magnitude of a subsequent slow process (tau approximately equal to 10(3) s) indicates that the total Ca storage capacity of the SR of skeletal muscle is in the order of 60-75 mm.

Sarcomere length effects on the Sr2+- and Ca2+-activation curves in skinned frog muscle fibres

1. A procedure recently described to produce rapid changes in [Ca2+] and [Sr2+] within the whole cross-section of skinned muscle preparations (Moisescu, D.G. (1976) Nature 262, 610--613, and Moisescu, D.G. and Thieleczek, R. (1978) J. Physiol. 275, 241--262) has enabled us to obtain whole Ca2+- or Sr2+-activation curves at different sacromere lengths with the same preparation. 2. The maximal isometric force response was found to be very similar in Ca2+-and Sr2+-buffered solutions for otherwise identical conditions. 3. The change in sarcomere length between approx. 2.2 and 2.6 micron reversibly shifted both the Ca2+- and the Sr2+-activation curves by approx. 0.1 log units towards lower concentrations of the activator, without affecting their shape. However, the change in sarcomere length in the range above 2.6 micron did not have an effect upon the relative isometric force response-pCa (and -pSr) relationship. 4. All the Ca2+- and Sr2+-activation curves present a similar steepness and indicate that the relative isometric force increases from approx. 10 to 90% if the concentration of the activator is increased 3-fold. 5. The half time for force development in these experiments did not appear to be influenced by the length of the sarcomeres. 6. A potentiometric method for determining the apparent affinity constants of Ca2+, Mg2+ and Sr2+ to EGTA and ATP under various conditions is described.

Calcium and strontium concentration changes within skinned muscle preparations following a change in the external bathing solution

1. A method for producing rapid [Ca2+] and [Sr2+] changes in the frog skinned muscle fibre preparation while maintaining constant all other cationic concentrations (Moisescu, 1976a, b) is described and analysed in detail. 2. Different experiments, some of them involving the Ca2+-sensitive photoprotein aequorin, as well as theoretical considerations, indicate that with this method one can produce a Ca2+ (or Sr2+) concentration change within 0.1--0.15 sec in a whole preparation having a diameter of 50 micrometer. 3. The rate of force development was similar to that observed in vivo. 4. The radial diffusion coefficient of EGTA in relaxed myofibrillar preparations was measured and found to be 4.6 x 10(-6) cm2sec-1 at 20 degrees C. 5. The sarcoplasmic reticulum in myofibrillar bundles was found to be active with respect to both Ca2+ and Sr2+ in the solutions used ([Mg2+] 1 mM; [Na] 30 mM; [K] 140-170 mM; [Cl] less than or equal to 20 mM; pH 7.10). 6. The amount of Ca released by caffeine from internal stores (previously loaded with Ca) can raise the total Ca concentration in the muscle fibre preparation by at least 1.8 mM. 7. The presence of 10 mM-caffeine in all bathing solutions reduced drastically the ability of the sarcoplasmic reticulum to accumulate both Ca and Sr.