Glutathione alters calcium responsiveness of cardiac skinned fibers

The glutathione status of cardiac muscle, that is the ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG) changes in certain forms of cardiomyopathy and during reperfusion of ischemic heart. Here we show that it also affects the sensitivity of contractile proteins to calcium. GSH (4 mM) increased the pCa50 for force development in skinned fibers by 0.2 pCa units and increased force by 44% +/- 5.4% at pCa 5.6 whereas GSSG (4 mM) decreased it by 54% +/- 17.8% at pCa 5.6. Half maximal activations and inhibitions were seen with 4 mM GSH or GSSG, respectively. In contrast to GSH, the reducing agent dithiothreitol at 5 mM had no activating effect. Our results suggest that the loss of contractility observed after a reperfusion of the ischemic heart may, at least in part, be due to a decreased responsiveness of the contractile proteins due to an altered glutathione status.

Calcium sensitivity and unloaded shortening velocity of hypertrophied and non-hypertrophied skinned human atrial fibres

The mechanical properties of myocardium of different animals are modified by a chronic increase in haemodynamic load. In this study differences in calcium sensitivity and maximum unloaded shortening velocity of hypertrophic and non-hypertrophic chemically skinned human atrial fibres are characterized. Investigating right atria of 34 patients, possible correlations are studied between preoperative atrial pressure, degree of hypertrophy (estimated from the muscle fibre diameter), calcium responsiveness (pCa50 eliciting half-maximum contraction) and Vmax (unloaded shortening velocity). Hypertrophic fibres from atrial appendages of patients having an increased right atrial pressure (RAP 8.5 +/- 1.6 mm Hg) and suffering from mitral valve disease (stenosis and insufficiency combined) had a fibre diameter of 18.0 +/- 0.9 microns. They also had a higher calcium sensitivity (pCa50 5.65 +/- 0.08) and a lower unloaded shortening velocity (1.7 +/- 0.1 muscle lengths/s) than non-hypertrophic fibres from the appendages of patients with normal right atrial pressure (RAP 3.2 +/- 0.5 mm Hg) and coronary heart disease (CHD: pCa50 5.45 +/- 0.04; Vmax = 3.4 +/- 0.2 muscle lengths/s; fibre diameter 12.8 +/- 0.4 microns). Thus non-hypertrophic fibres from control CHD patients differed significantly (p less than 0.01) from hypertrophied atrial fibres of patients with mitral valve disease and with combined valve disease (MAV, pCa50 = 5.58 +/- 0.05, Vmax 2.0 +/- 0.3 muscle lengths/s, fibre diameter 14.6 +/- 0.9 microns) or aortic valve disease (stenosis combined with insufficiency, fibre diameter 14.8 +/- 1.4 microns, pCa50 5.56 +/- 0.03, Vmax 2.0 +/- 0.24 muscle length/s; RAP 11.0 +/- 2.6 mm Hg).(ABSTRACT TRUNCATED AT 250 WORDS)

An oil-well method for time-resolved microfluorescence assays

A simple fluorescence assay method has been devised for direct time-resolved recordings of NADH-coupled enzyme reactions in a microliter sample volume in an "oil-well". The apparatus allows practically instantaneous mixing and stirring of two droplets containing the enzyme and substrate, respectively, and which are viewed with a microscope spectrophotometer. This procedure can be used for instance for enzyme kinetic studies of ATPases on a microscale. Thus, by monitoring NADH-fluorescence changes we determined the time course of ATP splitting in nanogram quantities of myosin subfragment 1. In principle, the method can also be used to determine the time course of any other reaction causing fluorescence changes or luminescence changes.

Inhibition of TnI-TnC interaction and contraction of skinned muscle fibres by the synthetic peptide TnI [104-115]

Circular dichroism was used to study the induction of helix in TnC or TnI-TnC by the TnI peptide [104-115] at various Ca2+ concentrations. The increase in negative ellipticity and pCa2+ values for the peptide-TnC complex, indicates that binding of the peptide to TnC, induces a small helical conformational change in TnC. This results in an increase in the Ca2+ binding constant and the pCa50 value required to induce 50% of Ca2+ -dependent helix in TnC. The introduction of the peptide to a preformed mixture of TnI-TnC resulted in an increase in negative ellipticity and a decrease in the pCa50 and the apparent Ca2+ binding constant towards the values obtained for the TnI peptide-TnC complex and away from those of TnI-TnC. This demonstrates that the TnI peptide can successfully compete with TnI for TnC and thereby inhibit the TnI-TnC interaction. The addition of the TnI peptide to skinned rabbit psoas or porcine cardiac fibres resulted in the inhibition of the force development and a decrease in the pCa50 values required for 50% Ca2+ activation. The magnitude of the inhibition of tension development and the shift in the Ca2+ sensitivity for skinned cardiac muscle fibres was approximately half that observed with skeletal muscle fibres. In view of the CD findings, these skinned fibre results can be accounted for by the peptide inhibiting the TnI interaction with TnC. However, it is possible that the TnI peptide also has a direct inhibitory effect on TM-actin.(ABSTRACT TRUNCATED AT 250 WORDS)

A calmodulin-binding peptide relaxes skinned muscle from guinea-pig taenia coli

During smooth muscle activation the calcium calmodulin complex interacts with myosin light chain kinase (MLCK) whereby activating it. A synthetic peptide analogue (RS20) corresponding to the calmodulin recognition sequence of MLCK has been synthesized and previously found to inhibit the calmodulin stimulated light chain kinase activity. Here we studied the effect of this peptide on skinned fibers from guinea pig taenia coli. Maximal contractions induced by 30 microM Ca2+ at 0.1 microM calmodulin could be completely relaxed by the peptide at 1 microM. The inhibitory effect was accompanied by partial dephosphorylation only of the regulatory myosin light chain. Relaxation could be reversed by addition of calmodulin which also increased the extent of light chain phosphorylation. The calmodulin concentration required for reversing the inhibition depended on the concentration of the inhibitory peptide suggesting that the peptide competed with MLCK for the calmodulin binding site. As the calcium-calmodulin-peptide mixture constitutes a calmodulin buffer, our results suggest, that the peptide is a calmodulin antagonist unique in terms of its potency and that less than nanomolar concentrations of free calmodulin may be required for inducing smooth muscle contractions.

Myosin P-light chain isoenzymes in the human heart: evidence for diphosphorylation of the atrial P-LC form

We studied myosin light chains (LC) of human atrium and ventricle of normal and diseased individuals by a high-resolution 2-dimensional polyacrylamide gel electrophoresis (2D-PAGE) technique. Atrial LCs (ALC-1, ALC-2 (= P-LC)) revealed both higher molecular weights and lower isoelectric points (IEP) than their ventricular counterparts (VLC-1, VLC-2 (= P-LC)). Different P-LC forms with their distinct myosin isoenzymes have been designated as P-LC-polymorphism and myosin P-LC isoenzymes, respectively. In the dephosphorylated state two VLC-2 forms (VLC-2 and VLC-2*) with the same MW and different IEP, but only one ALC-2 form, were found. In the partially phosphorylated state ALC-2 appeared to be single- and double-phosphorylated (three spots in the 2D-PAGE), whereas the two VLC-2 forms appeared to be single-phosphorylated each (four spots in the 2D-PAGE). Phosphoryl-transfer from ATP to the P-LC forms was studied using skinned fibers incubated with MLCK (myosin light chain kinase) and (gamma-32P)ATP. Ventricular myosin P-LC isoenzyme pattern was usually the same in normal and diseased patients: the VLC-2 to VLC-2* ratio was approx. 70/30, but in one patient with valvular heart disease (VHD) the relation was 55/45 (shift to the VLC-2* form). In hypertrophied atria of VHD patients a shift of the myosin P-LC isoenzyme pattern to the VLC-2* form occurred, too.

Calcium-sensitivity modulation of cardiac myofibrillar proteins

Myocardial contractile force may be altered not only by changing intracellular free Ca2+ but also by increasing or decreasing the calcium responsiveness of the myofilaments. The latter effect may be due to interventions that decrease the calcium affinity of troponin C, such as intracellular acidosis or factors that act "downstream" of the calcium-troponin-C interaction in the process of muscle activation. These include inorganic phosphate, which accumulates under hypoxic conditions. Conversely, calcium responsiveness may be increased by drugs that probably increase the calcium affinity of troponin C, such as sulmazole, adibendan, pimobendan, and isomazole. These and related drugs also increase the force development of skinned trabeculae that are submaximally activated by Ca2+ stimulation in ATP salt solution. Thus, we confirmed earlier studies showing that adibendan increased submaximal Ca2+-activated force of pig-heart trabecula skinned by 0.5% lubrol-WX. Qualitatively similar results were obtained with pimobendan. However, calcium responsiveness may not only be modulated by factors influencing troponin, but also by modulation of crossbridge rate constants as well as by other factors that act downstream of troponin C. These include peptides derived from thin-filament proteins and subfragment S1 of myosin.

Gonadectomy and hormonal replacement changes systolic blood pressure and ventricular myosin isoenzyme pattern of spontaneously hypertensive rats

We investigated the influence of testosterone on systolic blood pressure, heart weight, body weight, and ventricular myosin isoenzyme pattern of male spontaneously hypertensive stroke-prone rats (SHRSP). In two different study series (study 1, postpubertal; study 2, prepubertal), SHRSP were gonadectomized (HG), gonadectomized and replaced with dihydrotestosterone (HG-T), and sham-operated (H). Blood pressure was significantly higher in HG-T animals in both study series. Only prepubertal gonadectomy (study 2, HG group) led to a significantly decreased blood pressure. Heart weight and body weight were significantly diminished in the HG group when compared to the H group in study 2. Dihydrotestosterone (HG-T group) reversed this effect. In both study series gonadectomy shifted the myosin isoenzyme pattern to the V3 form while testosterone replacement led to a myosin isoenzyme pattern in favor of the V1 form. We conclude that the ventricular myosin isoenzyme pattern is under the dominant control of androgens and dissociates the expression of myosin isoenzyme from both blood pressure and cardiac hypertrophy in spontaneously hypertensive rats.

Myosin composition and functional properties of smooth muscle from the uterus of pregnant and non-pregnant rats

The myosin heavy chain stoichiometry and the force-velocity relation have been determined in the myometrium of the non-pregnant and pregnant rat. The relative proportions of the slower migrating heavy chain (MHC1) greatly exceeded that of the faster migrating heavy chain (MHC2) as shown by electrophoresis on SDS 4%-polyacrylamide gels. The ratios of MHC1/MHC2 were 2.2/1 in the non-pregnant rats, 2.6/1 in the pregnant rat, and contrasted with 0.8/1 in the rat portal vein. This stoichiometry was unchanged by extracting the myosin from the smooth muscle as native myosin in a salt extract, as dissociated myosin using sodium dodecyl sulphate (SDS) or by isolating the native myosin first by a non-dissociating (pyrophosphate) electrophoresis step and subsequently analysing the protein bands on the SDS 4%-polyacrylamide gel. Although the unequal proportions of the heavy chains suggested the possibility that the native myosin molecule may be arranged as homodimeric heavy chains, no evidence for or against the existence of native myosin isoforms could be obtained by electrophoresing native myosin extracts on pyrophosphate-polyacrylamide gels. The force-velocity relations of the intact electrically stimulated myometrium from the non-pregnant and pregnant rats gave isometric force of 45 and 135 mN/mm2 and Vmax of 0.71 and 0.52 lengths/s (37 degrees C) when measured at 95% of optimal length, whereas in chemically skinned uterine strips at 22 degrees C Vmax was 0.09 and 0.13 lengths/s, respectively. The length-force relationship was of similar shape in the non-gravid and gravid skinned tissues. The energetic tension cost (ATP-turnover/active stress) in skinned fibres was also similar. The mechanical and metabolic characteristics of the gravid and non-gravid uterus found in the present study do not suggest an obvious difference in the intrinsic properties of the myosin, although significant functional alterations in the tissue appear during pregnancy. This corresponds to the lack of a difference in the pattern of the heavy chains.

Role of magnesium in activation of smooth muscle

We studied the effects of Mg2+-free solutions on isometric force (F0) and unloaded shortening velocity (Vus) in contractions elicited by Ca2+ or by ATP after thiophosphorylation by adenosine 5'-O-(3-thiotriphosphate (ATP gamma S) in chemically skinned guinea pig taenia coli smooth muscle. In Mg2+-free solutions, increasing Ca2+ did not increase Fo above resting levels. At the peak of a control contraction elicited by Ca2+, transfer to Mg2+-free (but Ca2+-containing) solutions resulted in a rapid relaxation and concomitant dephosphorylation of myosin. After ATP gamma S, a contracture required neither Mg2+ nor Ca2+ in the solutions for control levels of Fo. Vus in the Mg2+-free solutions after ATP gamma S was approximately 50% of control and could be restored to near control levels by addition of Mg2+ but not Ca2+. After ATP gamma S, pretreatment with 4 mM EDTA and contracture in 0.1 mM EDTA-containing solutions decreased Fo to 70-80% of control and Vus to 50-60% of control. Our results suggest that the relatively high requirement for Mg2+ for contraction in skinned smooth muscle largely reflects the Mg2+ dependence of myosin kinase and not for actin-myosin interaction. The dependence of Fo on Mg2+ (in the presence of excess ATP) in taenia coli is less than that reported for skeletal muscle. Appreciable force can be maintained with no added Mg2+ in the presence of 4 mMEDTA, and thus it appears that ATP4- can be a substrate for contraction after ATP gamma S treatment. In addition, our data imply that any Ca2+-dependent regulatory mechanism that does not involve myosin phosphorylation/dephosphorylation, if present, requires Mg2+ for expression.

Alteration of calcium sensitivity of skinned frog skeletal muscle fibres by inositol triphosphate and calmodulin antagonists

We investigated the influence of inositol triphosphate (IP3), trifluoperazine (TFP), and perhexiline on the calcium sensitivity of freeze-dried frog semitendinosus muscle fibres. Further, the effect of IP3 on calcium release from the sarcoplasmic reticulum (SR) of frog semitendinosus fibres skinned by saponin was studied. IP3 decreased the calcium sensitivity of freeze-dried frog skeletal muscle fibres and failed to induce a calcium release from SR of saponin-skinned fibres. Freeze-dried frog skeletal muscle fibres were strongly sensitized for calcium by TFP and perhexiline.

Effects of a protein phosphatase inhibitor, okadaic acid, on membrane currents of isolated guinea-pig cardiac myocytes

The effects of a protein phosphatase inhibitor, okadaic acid (OA), were studied on membrane currents of isolated myocytes from guinea-pig cardiac ventricle. The whole-cell Ca2+ current (ICa) was recorded as peak inward current in response to test pulse to 0 mV. Extracellular application of OA (5-100 microM) produced an increase of ICa. The effect was markedly enhanced when the myocyte was pretreated with threshold concentrations of isoprenaline. ICa was increased from 11.3 +/- 0.8 microA cm-2 to 19.0 +/- 1.1 microA cm-2 (n = 4) by 5 microM-OA in the presence of 1 nM-isoprenaline. The delayed rectifier current was also slightly increased. Furthermore, the wash-out time of the beta-adrenergic increase of ICa was markedly prolonged by OA. The beta-adrenergic stimulation of cardiac Ca2+ current is thought to be mediated by cAMP-dependent phosphorylation. The present results strongly suggest that the effect of OA on ICa is related to inhibition of endogenous protein phosphatase activity which is responsible for the dephosphorylation process. By the isotope method, the inhibitory effect of OA on different types of phosphatase was compared. OA had a relatively high specificity to type 1-, and type 2A-phosphatases.

Increased calcium sensitivity of chemically skinned human atria by myosin light chain kinase

We investigated the influence of myosin P-LC phosphorylation catalysed by calcium/calmodulin-dependent myosin light chain kinase (MLCK) on the tension-pCa relation of chemically skinned human atrial fibres. MLCK-induced increased myosin P-LC phosphorylation sensitized human atrial skinned fibres for calcium by 0.11 pCa-units in patients with valvular heart disease, and by 0.05 to 0.07 pCa-units in patients with coronary heart disease. The MLCK effect could be antagonized by a light chain phosphatase. The protein phosphatase ocadaic acid (OA) had no influence on the tension-pCa relation of skinned human atrial fibres and had no potentiating effect together with MLCK. The MLCK preparation used in this study was from bovine ventricle and revealed a KM of 1.8 x 10(-5) M and a Vmax of 822 nmol Pi/min/mg using purified bovine ventricular myosin-LCs as substrate.

Different phosphorylation patterns of cardiac myosin light chains using ATP and ATP gamma S as substrates

Controversial views have been reported regarding the role of myosin light chain phosphorylation in the regulation of cardiac contractility (for review see. In the past, adenosine 5'-(-thio)triphosphate) (ATP gamma S) instead of ATP has frequently been used to study mechanical and biochemical consequences of myosin P-light chain (P-LC, LC-2) phosphorylation since thiophosphorylated sites are not significantly attacked by phosphatases. Unlike thiophosphorylation phosphorylation of myosin by myosin light chain kinase did neither decrease maximal (unloaded) shortening velocity of cardiac skinned fibres nor ATPase activity of cardiac myofibrils. We have accordingly investigated the phosphorylation pattern of purified cardiac myosin light chains using radioactive labeled ATP gamma S and ATP. We found that both the 28 kDa myosin light chain (LC-1) and the 18 kDa myosin light chain (LC-2, P-LC) were phosphorylated when ATP gamma S was present. In the presence of ATP, however, only LC-2 was found to be phosphorylated.

Effects of okadaic acid on isometric tension and myosin phosphorylation of chemically skinned guinea-pig taenia coli

1. In guinea-pig taenia coli skinned with Triton X-100, the marine sponge toxin okadaic acid (OA; 0.1-10 microM) produced a dose-dependent enhancement of isometric tension in the presence of low concentrations (0.1-1 microM) of Ca2+. 2. The Ca2+-tension relation of the skinned taenia showed a high co-operativity (Hill coefficient, h = 5) in the presence of 0.2 microM-calmodulin. The concentration of Ca2+ required to obtain half-maximal tension (ED50) was 1.8 microM. OA (5 microM) reduced the co-operativity (h = 2.3) and increased the Ca2+ sensitivity (ED50 = 0.92 microM-Ca2+). OA further increased the tension produced with 30 microM-Ca2+, while it failed to produce any mechanical effect in Ca2+-free solution. When the calmodulin concentration was increased the Ca2+ sensitivity increased as well, but the co-operativity was not affected both in the absence and in the presence of OA. 3. The level of myosin phosphorylation was analysed by two-dimensional gel electrophoresis. OA produced an increase in phosphorylated light chains and a concomitant decrease in unphosphorylated light chains. The effect was completely reversed when OA was washed out. 4. In solutions containing more than 1 microM-Ca2+, a third protein band appeared on the gels next to the bands of light chains. OA markedly increased the third band which disappeared when OA and Ca2+ were simultaneously removed. 5. OA reversibly slowed down both relaxation and dephosphorylation induced by Ca2+ removal following activation with 30 microM-Ca2+. Complete relaxation did not occur in the presence of more than 1 microM-OA. The concentration of OA required to produce a 50% reduction (ID50) of the relaxation rate was 78 nM. 6. The phosphatase activity in the taenia extract was inhibited by OA (1-10 microM) in a dose-dependent manner. The inhibition was well described as a mixed noncompetitive inhibition, and the dose-inhibition relation was shifted to the right when the concentration of substrate (phosphorylated light chains) was increased. The lower and upper limits of the change of ID50 produced by changing the substrate concentration were estimated to be 10 and 165 nM-OA, respectively. 7. These results strongly suggest that the tension enhancement and the slow-down of relaxation are both causally related to inhibition of myosin phosphatase activity by OA.

Skinned fibers of human atrium and ventricle: myosin isoenzymes and contractility

Different myosin isoenzymes of pig and human atrium and ventricle and rat ventricle were characterized by two approaches: pyrophosphate polyacrylamide gel electrophoresis (PP-PAGE) and analysis of the myosin P light chains by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). We further investigated the relation between atrial and ventricular myosin isoenzymes of human, pig, and rat, and the maximum (unloaded) shortening velocity (Vmax) and the Ca2+ sensitivity of chemically skinned fibers of the same species. The myosin isoenzymes of both human and pig atrium comigrated in the PP-PAGE with rat V2 isomyosin, whereas the ventricle of human and pig comigrated with rat V3. In both human and pig ventricle, a myosin P light chain polymorphism exists (two phosphorylatable P light chains with the same molecular weight but different isoelectric points). In contrast, we found no P light chain polymorphism in the atrium of human and pig and in the ventricle of rat (one phosphorylatable P light chain only). A correlation exists between Vmax, Ca2+ sensitivity, and atrium- and ventricle-specific myosin isoenzymes of human and pig. Vmax was determined by the slack-test method. Plots of delta l versus delta t of atrial and ventricular skinned fibers were well fitted by a single straight line up to delta l = 15% and delta l = 13%, respectively. Vmax of skinned ventricular fibers was lower than Vmax of skinned atrial fibers in both human and pig. Ca2+ sensitivity of skinned fibers of ventricle, however, was higher than Ca2+ sensitivity of atrial skinned fibers in both human and pig.

In vitro investigations on a new positive inotropic and vasodilating agent (BM 14.478) that increases myocardial cyclic AMP content and myofibrillar calcium sensitivity

BM 14.478 (7,7-dimethyl-2-(4-pyridyl)-6,7-dihydro-3H,5H pyrrolo[2,3-f]benz-imidazol-6-one) was investigated in several in vitro experiments to elucidate its positive inotropic and vasodilating efficacy and its mode of action. A direct positive inotropic action was achieved in papillary muscles (10(-6) to 5 X 10(-4) M) and electrically driven atria (10(-8) to 5 X 10(-4) M) from guinea pig hearts. The effect was not affected by propranolol, cimetidine, or tetrodotoxin, but diminished by carbachol. The effect of isoprenaline was amplified by threshold concentrations of BM 14.478 (10(-6) M). There was only a slight intrinsic chronotropic activity in spontaneously beating guinea pig atria. Atrial cyclic AMP (cAMP) was increased from 1.46 +/- 0.06 to 1.97 +/- 0.03 pmol/mg wet wt, at 3 X 10(-6) M. This might be due to an inhibition of cardiac phosphodiesterase(s) (PDE). IC50 of bovine PDE was 7.2 X 10(-5) M (5.4 X 10(-5) M to 9.7 X 10(-5) M). BM 14.478 shortened the duration of transmembrane action potential (90% repol.) by 8% and increased the Vmax of slow action potentials by 32% at 3 X 10(-4) M. In skinned porcine heart muscle fibers an increase in calcium-activated force up to 43 +/- 7% was observed (10(-7) to 10(-4) M). Rat aortas were relaxed by about 75% maximally (10(-7) to 10(-4) M). It is concluded that BM 14.478 is a potent inotropic drug which acts via an increase in myocardial cAMP content and in calcium sensitivity of contractile proteins.

[Calcium regulation of muscle contraction: the molecular regulation mechanisms of contracility]

The role of calcium in muscle activation is described with particular emphasis on interactions occurring between the regulatory proteins troponin-C, troponin-I, actin and myosin. These interactions depend on calcium, but in turn they also influence the calcium binding to troponin-C. They may be affected by short "competitive" peptides whose sequences are similar to the amino-acid sequences on the protein-interaction sites. They may be also affected by certain drugs (cardiotonic calcium sensitizers) that influence the calcium sensitivity of myofilaments.

A myosin phosphatase modulates contractility in skinned smooth muscle

The influence of a purified holoenzyme form of polycation-modulable (PCM-) myosin phosphatase on Ca2+-dependent actin-myosin interactions was studied in detergent-skinned smooth muscle fibers from chicken gizzard. The concentration of Ca2+ required for half maximal isometric contraction (A0.5; 0.26 microM) of fibers incubated in the absence of phosphatase was increased 2-fold when PCM-phosphatase (13 U/ml) was included in the medium. Removal of the phosphatase restored A0.5 to control level showing that the enzyme-mediated decrease in Ca2+-sensitivity was reversible. Two-dimensional electrophoresis of fiber homogenates revealed that PCM-phosphatase decreased Ca2+-sensitivity for phosphorylation of the regulatory myosin light chains in parallel fashion. Ca2+-dependent increases in isometric force were directly correlated to increases in the extent of light chain phosphorylation up to about 0.35 mol PO4/mol light chain; further increases in phosphorylation were not associated with further increases in force. Addition of PCM-phosphatase to fibers which had been contracted with a suboptimal concentration of Ca2+ (0.35 microM) resulted in rapid relaxation. Unloaded shortening velocity, reflecting cross-bridge cycling rate, was reduced by 92% in the presence of PCM-phosphatase and light chain phosphorylation was decreased by 50%. These data show that both tension and unloaded shortening velocity may be related to Ca2+-dependent phosphorylation of the light chains. The results indicate that the level of phosphorylation attained in the fiber preparations studied probably reflects the ratio of myosin kinase to phosphatase activities. Since protein phosphatases are regulated enzymes the results also suggest that modulation of phosphatase activity may participate in control of smooth muscle contractility.

Effect of sulmazole and pimobendan on contractility of skinned fibres from frog skeletal muscle

The influence of two benzimidazole derivatives, 2-[(2-methoxy-4-methylsulfinyl)phenyl]-1H-imidazo [4,5-b] pyridine (sulmazole, AR-L 115 BS) and 4,5-dihydro-6-[2-(4-methoxyphenyl)-1H-benzimidazol-5-yl]-5-methyl- 3(2H)- pyridazinone (pimobendan, UD-CG 115 BS) on the Ca2+-dependent isometric tension of skinned frog skeletal muscle fibres (tibialis anterior of Rana esculenta) was investigated. Both drugs sensitized the myofibrils of skeletal muscle for Ca2+: at a drug concentration of 2 X 10(-4) mol/l, sulmazole and pimobendan increased the isometric tension obtained at a Ca2+ concentration of 10(-6) mol/l by 21.7% and 49%, respectively.

Relaxation of chemically skinned guinea pig taenia coli smooth muscle from rigor by photolytic release of adenosine-5'-triphosphate

The mechanical events following release of ATP from P3-1-(2-nitro)phenylethyladenosine-5'-triphosphate (caged-ATP) in skinned guinea pig taenia coli smooth muscle in rigor were investigated. A rigor force of about 25-35% of the maximal active force was obtained by removing ATP at the plateau of a maximal active contraction. In the rigor solution free-Mg2+ was 2 mM, ionic strength 90 mM and pH 7.0. When caged-ATP (12.5 mM) was diffused into the preparation there was no change in the rigor force. Photolytic production of about 2 mM ATP was achieved with a xenon flash lamp. Following illumination, force decreased with an approximate initial rate constant of 0.7 s-1. The rate of relaxation was increased in the presence of inorganic phosphate (at 3 mM: 1.3 s-1; 10 mM: 2.2 s-1). At higher Mg2+ concentrations the rate of relaxation was slower (5 mM: 0.2 s-1) and at lower concentrations the rate was faster (0.5 mM: 1.2 s-1). An increased rate of relaxation was observed when ionic strength was increased to 150 mM (2.2 s-1). Phosphate increased the rate of relaxation at the different levels of Mg2+ (0.5-10 mM) and ionic strength (90, 150 mM). In preparations shortened (by 1-3%) to give reduced rigor force, a small transient increase in tension was recorded after ATP release. In comparison to the rates of ATP-induced dissociation of actomyosin in solution, reported in the literature, the rate of relaxation from rigor is slower. This may reflect a slow rigor cross-bridge dissociation or mechanical interactions not associated with cross-bridges in the muscle fibre.(ABSTRACT TRUNCATED AT 250 WORDS)

Smooth muscle myosin phosphatase inhibition and force enhancement by black sponge toxin

Smooth muscle contraction depends on the state of myosin phosphorylation and hence on the balance of myosin light chain kinase and phosphatase activity. Effects of okadaic acid isolated from black sponge on both enzyme activities and contractility were studied in chemically skinned fibers from guinea pig taenia coli. The toxin strongly inhibits myosin phosphatase and enhances tension development.

Ca2+-sensitizing effect of BM 14.478 on skinned cardiac muscle fibres of guinea-pig papillary muscle

A concentration-dependent increase in force development was obtained with BM 14.478 (10(-9)-5 X 10(-4) M) in skinned fibres of guinea-pig papillary muscles. Guinea-pig papillary muscles are standard preparations for evaluating inotropic effects and they were also used in the present case for evaluating the positive inotropic effect of BM 14.478. We therefore conclude that a marked calcium-sensitizing effect contributes to the positive inotropic effect obtained with BM 14.478 even at very low concentrations.

The effect of inorganic phosphate on the ATP hydrolysis rate and the tension transients in chemically skinned rabbit psoas fibers

The role of orthophosphate ions (Pi) in crossbridge kinetics was investigated by parallel measurements of the ATP hydrolysis rate and tension transients in maximally activated, chemically skinned rabbit psoas fibers. The hydrolysis rate of the standard activation at 20 degrees C was measured at 1.25 nmole X s-1 X m-1 X fiber-1, which corresponds to the hydrolysis of 3 moles ATP per mole of myosin head per second. The isometric tension, stiffness extrapolated to the infinite frequency, and the ATPase rate progressively decreased when increasing concentrations of Pi (0-16 mM) were added to the activating saline. The decrease was greatest with tension, followed by stiffness and the ATPase rate. Both the apparent rate constant and the magnitude parameters of exponential process (B) increased with Pi concentration resulting in a significant increase in the oscillatory power output. The effects of Pi on processes (A) and (C) were only marginal. When fibers were oscillated at 1 Hz [close to the characteristic frequency of process (A)], no significant increase in the ATP hydrolysis rate was observed. However, a small increase was noticed at 10 Hz [1%, process (B)], and at 100 Hz [6%, process (C)]. We interpret these results in terms of a crossbridge scheme which adds a branch pathway to the conventional hydrolysis cycle. In the proposed scheme, the number of crossbridges entering the branch pathway increases at higher Pi concentrations and in the presence of imposed oscillations at the proper frequency.