Decrease In Twitch Tension Research Articles

Decrease in sarcoplasmic reticulum calcium content, not myofilament function, contributes to muscle twitch force decline in isolated cardiac trabeculae.

We set out to determine the factors responsible for twitch force decline in isolated intact rat cardiac trabeculae. The contractile force of trabeculae declined over extended periods of isometric twitch contractions. The force-frequency relationship within the frequency range of 4-8 Hz, at 37 °C, became more positive and the frequency optimum shifted to higher rates with this decline in baseline twitch tensions. The post-rest potentiation (37 °C), a phenomenon highly dependent on calcium handling mechanisms, became more pronounced with decrease in twitch tensions. We show that the main abnormality during muscle run-down was not due to a deficit in the myofilaments; maximal tension achieved using a K(+) contracture protocol was either unaffected or only slightly decreased. Conversely, the sarcoplasmic reticulum (SR) calcium content, as assessed by rapid cooling contractures (from 27 to 0 °C), decreased, and had a close association with the declining twitch tensions (R(2) ~ 0.76). SR Ca(2+)-ATPase, relative to Na(+)/Ca(2+) exchanger activity, was not altered as there was no significant change in paired rapid cooling contracture ratios. Furthermore, confocal microscopy detected no abnormalities in the overall structure of the cardiomyocytes and t-tubules in the cardiac trabeculae (~23 °C). Overall, the data indicates that the primary mechanism responsible for force run-down in multi-cellular cardiac preparations is a decline in the SR calcium content and not the maximal tension generation capability of the myofilaments.

Immobilization with Atrophy Induces De Novo Expression of Neuronal Nicotinic α7 Acetylcholine Receptors in Muscle Contributing to Neurotransmission

Mature acetylcholine receptor (AChR) isoform normally mediates muscle contraction. The hypothesis that α7AChRs up-regulate during immobilization and contribute to neurotransmission was tested pharmacologically using specific blockers to mature (waglerin-1), immature (αA-OIVA), and α7AChRs (methyllycaconitine), and nonspecific muscle AChR antagonist, α-bungarotoxin. Mice were immobilized; contralateral limbs were controls. Fourteen days later, anesthetized mice were mechanically ventilated. Nerve-stimulated tibialis muscle contractions on both sides were recorded, and blockers enumerated above sequentially administered via jugular vein. Data are mean ± standard error. Immobilization (N = 7) induced tibialis muscle atrophy (40.6 ± 2.8 vs. 52.1 ± 2.0 mg; P < 0.01) and decrease of twitch tension (34.8 ± 1.1 vs. 42.9 ± 1.5 g; P < 0.01). Waglerin-1 (0.3 ± 0.05 μg/g) significantly (P = 0.001; N = 9) depressed twitch tension on contralateral (≥97%) versus immobilized side (approximately 45%). Additional waglerin-1 (total dose 1.06 ± 0.12 μg/g or approximately 15.0 × ED50 in normals) could not depress twitch of 80% or greater on immobilized side. Immature AChR blocker, αA-OIVA (17.0 ± 0.25 μg/g) did not change tension bilaterally. Administration of α-bungarotoxin (N = 4) or methyllycaconitine (N = 3) caused 96% or greater suppression of the remaining twitch tension on immobilized side. Methyllycaconitine, administered first (N = 3), caused equipotent inhibition by waglerin-1 on both sides. Protein expression of α7AChRs was significantly (N = 3; P < 0.01) increased on the immobilized side. Ineffectiveness of waglerin-1 suggests that the twitch tension during immobilization is maintained by receptors other than mature AChRs. Because αA-OIVA caused no neuromuscular changes, it can be concluded that immature AChRs contribute minimally to neurotransmission. During immobilization approximately 20% of twitch tension is maintained by up-regulation of α-bungarotoxin- and methyllycaconitine-sensitive α7AChRs.

Train-of-Four and Tetanic Fade Are Not Always a Prejunctional Phenomenon as Evaluated by Toxins Having Highly Specific Pre- and Postjunctional Actions

Nerve-stimulated fade in muscle is generally accepted as a prejunctional phenomenon mediated by block of prejunctional acetylcholine receptors (AChRs) at the nerve terminal, whereas decrease of twitch tension is considered a postjunctional effect due to block of muscle AChRs. Using ligands with specific pre- or postjunctional effects only, we tested the hypothesis that fade is not necessarily a prejunctional phenomenon. Neuromuscular function in rats was evaluated after IM (2.5 U) or IV (12.0 U) injection of botulinum toxin (Botx), or IV (250 μg/kg) α-bungarotoxin (α-BTX) alone. The acute neuromuscular effects of IV 2 mg/kg dihydro-β-erythroidine (DHβE), alone and in combination with α-BTX, were also tested. Botx decreases vesicular release of ACh, and α-BTX binds to postjunctional nicotinic AChRs only, whereas DHβE binds specifically to prejunctional α3β2 AChRs only. In view of the lack of acute effects of Botx even at 2 hours after IV injection, its neuromuscular effects were also evaluated at 24 hours after IM injection (0.6 U) and compared with IM injection of α-BTX (25 μg/kg) or saline also given 24 hours earlier. The sciatic nerve-tibialis muscle preparation, during train-of-four and tetanic stimulation, was used to test neuromuscular effects in vivo. IV and IM Botx had no observable neuromuscular effects at 2 hours. IV α-BTX caused twitch depression within a few minutes, and significant fade (P = 0.002) at 75% of baseline twitch tension; these effects persisted until the end of the observation period of 2 hours. IV DHβE alone caused no significant change in single twitch (P = 0.899) or train-of-four ratio (P = 0.394), but significantly enhanced the fade of IV α-BTX (P = 0.001 at 75% of baseline twitch tension). IM Botx or α-BTX, at 24 hours after their injection, resulted in a significant decrease of single twitch and tetanic tensions (P < 0.0001), but Botx did not cause fade, whereas α-BTX caused significant (P < 0.0001) fade at 24 hours. The tibialis muscle weights and protein expression of α1 subunit of AChR (Western blots) did not differ between Botx, α-BTX and saline-injected groups at 24 hours but increased in denervated muscle (positive control). Botx-induced decreased ACh release in and of itself does not cause fade but does cause decrease of absolute tensions. Decrease of available (functional) postjunctional AChRs by α-BTX did induce fade. The prejunctional fade effects of DHβE on α3β2 AChRs become manifest only when the margin of safety was decreased by concomitant administration of α-BTX. Thus, fade during repetitive stimulation is not always a prejunctional phenomenon and may also reflect the decreased margin of safety of neurotransmission, which can be due to a pure postjunctional AChRs block or to a combination of both pre- and postjunctional AChRs block. Block of prejunctional α3β2 AChRs alone is not necessary and sufficient to cause fade.

A novel µ‐conopeptide, CnIIIC, exerts potent and preferential inhibition of NaV1.2/1.4 channels and blocks neuronal nicotinic acetylcholine receptors

The µ-conopeptide family is defined by its ability to block voltage-gated sodium channels (VGSCs), a property that can be used for the development of myorelaxants and analgesics. We characterized the pharmacology of a new µ-conopeptide (µ-CnIIIC) on a range of preparations and molecular targets to assess its potential as a myorelaxant. µ-CnIIIC was sequenced, synthesized and characterized by its direct block of elicited twitch tension in mouse skeletal muscle and action potentials in mouse sciatic and pike olfactory nerves. µ-CnIIIC was also studied on HEK-293 cells expressing various rodent VGSCs and also on voltage-gated potassium channels and nicotinic acetylcholine receptors (nAChRs) to assess cross-interactions. Nuclear magnetic resonance (NMR) experiments were carried out for structural data. Synthetic µ-CnIIIC decreased twitch tension in mouse hemidiaphragms (IC(50) = 150 nM), and displayed a higher blocking effect in mouse extensor digitorum longus muscles (IC = 46 nM), compared with µ-SIIIA, µ-SmIIIA and µ-PIIIA. µ-CnIIIC blocked Na(V)1.4 (IC(50) = 1.3 nM) and Na(V)1.2 channels in a long-lasting manner. Cardiac Na(V)1.5 and DRG-specific Na(V)1.8 channels were not blocked at 1 µM. µ-CnIIIC also blocked the α3β2 nAChR subtype (IC(50) = 450 nM) and, to a lesser extent, on the α7 and α4β2 subtypes. Structure determination of µ-CnIIIC revealed some similarities to α-conotoxins acting on nAChRs. µ-CnIIIC potently blocked VGSCs in skeletal muscle and nerve, and hence is applicable to myorelaxation. Its atypical pharmacological profile suggests some common structural features between VGSCs and nAChR channels.

Contractile properties of the denervated external anal sphincter

The purpose of this study was to evaluate the effect of denervation on contractile properties of the external anal sphincter (EAS) of the female rat. Sham operation, pudendal nerve transection, pelvic neurectomy, or combined pudendal nerve transection/pelvic neurectomy was performed in young female rats. Contractile function of the EAS was determined after 2 weeks. Maximal force-generating capacity of the EAS was not impaired by bilateral pudendal denervation or pelvic neurectomy. Twitch tension, however, was decreased, and fatigability increased after pelvic neurectomy. Combined bilateral pudendal nerve-transection plus pelvic neurectomy resulted in compromised force-generating capacity, decreased twitch tension, and increased fatigability of the anal sphincter. Subtle changes in EAS function are detectable after pelvic neurectomy, but not pudendal denervation. In contrast, combination pudendal and pelvic neurectomy resulted in severe compromise of EAS function. These data suggest that EAS function is relatively preserved unless injury occurs to > 1 source of innervation.

Effects of caffeine at different temperatures on contractile properties of slow-twitch and fast-twitch rat muscles

The slow-twitch soleus muscle (SOL) exhibits decreased twitch tension (cold depression) in response to a decreased temperature, whereas the fast-twitch extensor digitorum longus (EDL) muscle shows enhanced twitch tension (cold potentiation). On the other hand, the slow-twitch SOL muscle is more sensitive to twitch potentiation and contractures evoked by caffeine than the fast-twitch EDL muscle. In order to reveal the effects of these counteracting conditions (temperature and caffeine), we have studied the combined effects of temperature changes on the potentiation effects of caffeine in modulating muscle contractions and contractures in both muscles. Isolated muscles, bathed in a Tyrode solution containing 0.1-60 mM caffeine, were stimulated directly and isometric single twitches, fused tetanic contractions and contractures were recorded at 35 degrees C and 20 degrees C. Our results showed that twitches and tetani of both SOL and EDL were potentiated and prolonged in the presence of 0.3-10 mM caffeine. Despite the cold depression, the extent of potentiation of the twitch tension by caffeine in the SOL muscle at 20 degrees C was by 10-15 % higher than that at 35 degrees C, while no significant difference was noted in the EDL muscle between both temperatures. Since the increase of twitch tension was significantly higher than potentiation of tetani in both muscles, the twitch-tetanus ratio was enhanced. Higher concentrations of caffeine induced contractures in both muscles; the contracture threshold was, however, lower in the SOL than in the EDL muscle at both temperatures. Furthermore, the maximal tension was achieved at lower caffeine concentrations in the SOL muscle at both 35 degrees C and 20 degrees C compared to the EDL muscle. These effects of caffeine were rapidly and completely reversed in both muscles when the test solution was replaced by the Tyrode solution. The results have indicated that the potentiation effect of caffeine is both time- and temperature-dependent process that is more pronounced in the slow-twitch SOL than in the fast-twitch EDL muscles.

Beta-adrenoceptor activation plays a role in the reverse rate-dependency of effective refractory period lengthening by dofetilide in the guinea-pig atrium, in vitro.

1. Blockers of the rapid component of the delayed rectifier potassium current (I(Kr)) prolong cardiac action potential duration (APD) and effective refractory period (ERP) in a reverse rate-dependent manner. Since activation of beta-adrenoceptors attenuates prolongation of APD evoked by I(Kr) blockers, rate-dependent neuronal noradrenaline liberation in the myocardium may contribute to the reverse rate-dependent nature of the effects of I(Kr) blockers. In order to test this hypothesis, we studied the effects of dofetilide, a pure I(Kr) blocker, on ERP after activation or blockade of beta-adrenoceptors and after catecholamine depletion in guinea-pig left atrial myocardium paced at 3, 2 and 1 Hz, in vitro. 2. Dofetilide (100 nM) lengthened ERP in a reverse rate-dependent manner in the left atrial myocardium of guinea-pigs. Strong activation of beta-adrenoceptors using 10 nM isoproterenol abolished the dofetilide-induced lengthening of ERP at all pacing rates. 3. Blockade of the beta-adrenoceptors with metoprolol (1 micro M), atenolol (3 micro M) or propranolol (300 nM) increased the dofetilide-evoked prolongation of ERP at 3 and 2 Hz, but not at 1 Hz. As a consequence, metoprolol attenuated while propranolol and atenolol fully eliminated the reverse rate-dependent nature of the dofetilide-induced ERP lengthening. In catecholamine-depleted atrial preparations of the guinea-pig (24 h pretreatment with 5 mg kg(-1) reserpine i.p.), the effect of dofetilide on ERP was not frequency dependent, and propranolol did not alter the effects of dofetilide. 4. In contrast to results obtained in guinea-pig atrial preparations, propranolol failed to change the reverse rate-dependent effect of dofetilide on ERP in the right ventricular papillary muscles of rabbits and guinea-pigs. 5. As an indication of the functional consequences of rate-dependent noradrenaline liberation, propranolol decreased twitch tension at 3 and 2 Hz but not at 1 Hz in the atrial myocardium of control guinea-pigs, whereas no such effect was detected in catecholamine-depleted atrial preparations. Propranolol failed to change contractility of ventricular myocardium in guinea-pigs and rabbits. 6. It is concluded that rate-dependent noradrenaline release and the ensuing beta-adrenoceptor activation contributed to the reverse rate-dependent nature of ERP prolongation caused by I(Kr) blockers in isolated guinea-pig atrial myocardium.

Lung volume reduction surgery does not improve diaphragmatic contractile properties or atrophy in hamsters with elastase-induced emphysema.

It is claimed that lung volume reduction surgery (LVRS) improves inspiratory muscle function. As diaphragm structure and function are not directly appraisable in patients, we studied the effects of LVRS on the diaphragm in vitro contractile properties and morphology in hamsters with elastase-induced emphysema. Four months after intratracheal instillation of elastase (40 U/100 g), hamsters underwent either bilateral LVRS (LVRS, n = 11) or a sham operation (SHAM, n = 8). Four animals died during the perioperative period in LVRS (n = 7). Hamsters instilled with saline served as control (CTL, n = 8). Animals were studied at the age of 9 mo. LVRS was associated with a significant 25% decrease in functional residual capacity compared to SHAM (p < 0.05). Compared with CTL, LVRS and SHAM showed a significant 18% and 14% reduction in diaphragm mass, respectively (p = 0.02). LVRS had a significantly decreased twitch tension compared to CTL and SHAM (p < 0.01). Both LVRS and SHAM showed increased resistance to muscle fatigue compared with CTL. The histochemical analysis revealed a significant shift from type IIx/b toward type IIa fibers in LVRS and SHAM compared with CTL. In conclusion, emphysema is associated with functional adaptations but LVRS does not appear to beneficially alter the diaphragm contractile and morphological characteristics in hamsters with elastase-induced emphysema.

Water Deprivation Reveals Early Neuromyopathy In Diabetic Mice

The effects of water deprivation on peripheral nerve and muscle function were investigated in flexor digitorum superficialis muscle of control and diabetic mice. Twenty mice (30 g average body weight) were injected once with streptozotocin solution (200 mg/kg) to induce experimental diabetes and another 20 mice of similar body weight served as controls. Two weeks later, comparative analyses of in situ muscle isometric contractile characteristics were performed by direct muscle stimulation and indirect nerve stimulation (at 1, 5 and 30 Hz) in urethane-anesthetized (2 mg/g, i.p.) control and diabetic mice. One day prior to the experiments, 10 control and 10 diabetic mice were deprived of water. The study contained four groups: hydrated (H) control, dehydrated (DH) control, H diabetic and DH diabetic. There were no significant differences in synaptic delay or twitch tension between H control and DH control. Comparing H control and H diabetic groups, no differences were noticed in synaptic delay or twitch tension; except at 30 Hz where twitch tension was reduced in H diabetic mice. Significant differences were observed when comparing DH control and DH diabetic mice. DH diabetic showed a significant increase in synaptic delay (from 7.4 to 9.3 ms) and a significant decrease in twitch tension evoked either by indirect nerve or by direct muscle stimulation (from 4.4 g to 1.9 g and from 4.4 g to 2.3 g, respectively). These results revealed that water deprivation enhances diabetes effects at the neuromuscular junction and at the muscle leading to further complication of neuromyopathy.

THE INFLUENCE OF NITRIC OXIDE ON IN-VIVO HUMAN SKELETAL MUSCLE PROPERTIES.

We have investigated the action of exogenous nitric oxide (NO) on the strength and contractile properties of human skeletal muscle working in vivo. Maximum isometric voluntary contraction force (MVC) of the quadriceps was measured and superimposed electrical stimulation was used to estimate the level of activation and 'true maximum force' (TMF). Force-frequency relationships were determined to assess changes in contractile properties of the muscle. Subjects in the experimental group (E, n=10) were measured before and during two separate periods of treatment with different doses of glyceryl trinitrate, a NO donor, delivering 100 (GTN100) or 200 (GTN200) microg h-1 as a trans-dermal patch. A control group (C, n=6) was measured during two similar periods whilst taking an oral placebo. There was a significant increase in strength with GTN200 (MVC: +5. 15%; TMF: +3.87%). There was no change in the strength of group C. There was a trend towards reduced forces at submaximal frequencies with GTN administration but the most notable change was a decline in twitch force (approximately 12%, P < 0.05) with GTN100 treatment and this remained depressed throughout the study. No changes were seen in the contractile properties of the control group C. The present results show that GTN treatment increased maximum voluntary strength but decreased twitch tension. The time course and dose-response characteristics indicate that these are two separate actions of NO on human muscle working in vivo.

Sepsis attenuates the intensity of the neuromuscular blocking effect of d-tubocurarine and the antagonistic actions of neostigmine and edrophonium accompanying depression of muscle contractility of the diaphragm.

Prolonged effects of non-depolarizing muscle relaxants in septic patients have been reported, although the influence of sepsis on neuromuscular transmission has not yet been clarified satisfactorily. These studies were intended to elucidate the influence of sepsis on neuromuscular transmission and on the action of drugs being utilized for regulation of muscle tone (a neuromuscular blocker and anti-cholinesterase (anti-ChE) drugs). The effect of d-tubocurarine (dTc) on neuromuscular transmission and the antagonistic action of anti-ChE drugs (neostigmine and edrophonium) on dTc-induced twitch depression were estimated using sham control and septic rat nerve-hemidiaphragm preparations in vitro. Isometric twitch tension elicited by indirect (phrenic nerve) or direct (muscle) stimulation at 0.1 Hz was evaluated. Sepsis induced by panperitonitis attenuated the twitch tension elicited by indirect or direct stimulation (P < 0.01) without obvious morphological inflammatory damage to the diaphragm. dTc dose-dependently decreased twitch tension elicited by indirect stimulation (P < 0.01) less intensely in the septic group than in the sham group (P < 0.01). The antagonistic actions of the anti-ChE drugs on dTc (1 microM)-induced twitch depression were less intense in the septic group (P < 0.01 or 0.05). These results demonstrate that sepsis in the acute phase attenuates the effects of a neuromuscular blocker and anti-ChE drugs and depresses muscle contractility simultaneously.

Neuromuscular blocking actions of the aminoglycoside antibiotics sisomicin and micronomicin in the rabbit.

The neuromuscular blocking actions of sisomicin sulfate (SISO), micronomicin sulfate (MCR) and d-tubocurarine (dTc) were studied in 20 rabbits anesthetized with halothane. The i.v. administration of SISO 20-40 mg/kg, MCR 40-80 mg/kg or dTc 0.1-0.3 mg/kg resulted in dose-dependent decreases in twitch tension. The ED50s for SISO, MCR and dTc were 23.5, 58.2 and 0.2 mg/kg, respectively. SISO- and MCR-induced neuromuscular blockade was partially antagonized by neostigmine or by calcium.

Participation of PKC in modulation of the excitation-contraction process of chick embryo cardiac muscle.

Phorbol esters are activators of protein kinase C (PKC) and have been widely used to study the function of this enzyme. We investigated the effect of phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), on tension responses and free cytosolic Ca in small strips of chick embryo ventricle and on the 45Ca influx in primary cultures of chick embryo ventricular cells. TPA (10(-8) M) decreased twitch tension about 40% when driven at 1 Hz. The free cytosolic Ca measured with fura-PE3 was rapidly increased in 110 mM KCl solution and this high K-induced increase was slightly inhibited by TPA (10(-8) M), by 12%. The resting 45Ca uptake in 5 mM KCl was slightly increased by a low concentration of TPA (10(-9) M), but high K-induced 45Ca uptake was decreased by TPA in a dose-dependent manner, but to 40% at the utmost. 4 alpha-Phorbol, an inactive phorbol ester, did not inhibit 45Ca uptake. These results indicate that TPA decreased the depolarization induced transsarcolemmal Ca uptake and thus the excitation-contraction process was attenuated, but to 40% at most. PKC may participate in the modulation of myocardial Ca homeostasis and contractile state.

Cardiac force and high-energy phosphates under metabolic inhibition in four ectothermic vertebrates.

Isometric twitch tension of ventricular preparations stimulated at 0.2 Hz fell over 30 min of anoxia by a fraction decreasing in the order rainbow trout, cod, eel, and freshwater turtle. Drops in the estimated cytoplasmic energy state were related to larger tension losses for trout than for the other species, possibly due to larger changes in free phosphate. Anoxic energy degradation was slower for turtle than for the other species. Anoxia combined with glycolytic inhibition (1 mmol/l iodoacetate) enhanced the decrease in twitch tension for a drop in energy state and enlarged the increase in ADP/ATP relative to that in creatine/phosphocreatine to an extent inversely related to the creatine kinase activity. Furthermore, it increased resting tension to an extent possibly related to myosin-adenosinetriphosphatase (ATPase) activity and lowered the content of phosphorylated adenylates in trout and turtle myocardium. The results indicate that species differences in performance of the metabolically challenged myocardium depend on energy-degrading processes, e.g., myosin-ATPase activity, phosphate release, creatine kinase activity, and efflux/degradation of ADP and AMP, and that glycolysis offers protection due to its cytoplasmic localization.

Mechanism of theophylline-induced inotropic effects on foreshortened canine diaphragm

The mechanisms of theophylline-induced inotropic effects at shorter diaphragm length have not yet been explored. We wondered whether the greater inotropic effects of the drug at shorter diaphragm length might result from an effect on intracellular calcium level. Forty pairs of diaphragm bundles were stimulated at 70% of optimal length in the presence of either verapamil (10(-5)M), calcium-free Krebs solution (buffered or not with 2 mM ethylene glycol tetra-acetic acid (EGTA)) or ryanodine (10(-6) M). Theophylline (1 mM) was subsequently added to one muscle bundle and, after 15 min, twitches were repeated. The twitch potentiation induced by theophylline (37 +/- 21%) was unaffected by verapamil (43 +/- 26%), or zero calcium (39 +/- 18%) and virtually unchanged when the latter was buffered with EGTA. By contrast, theophylline failed to increase twitch tension after pretreatment with ryanodine, a blocker of the calcium release by the sarcoplasmic reticulum. This decreased twitch tension in control (-5 +/- 11%) and experimental (-14 +/- 12%) bundles and prolonged half-relaxation time as a result of impaired sarcoplasmic reticulum calcium reuptake. We conclude that the inotropic effects of theophylline on twitch tension in foreshortened canine diaphragm bundles were not related to sarcoplasmic reticulum. This is consistent with an action of theophylline on the sarcoplasmic reticulum.

Energy metabolism and adenine nucleotide degradation in twitch-stimulated rat hindlimb during ischemia-reperfusion.

The purpose of this study was to characterize twitch tension and energy metabolism in ischemic, stimulated rat hindlimb to determine its suitability as a rapid time course model of ischemia-reperfusion injury. After 15 min equilibration, rat hindlimbs were stimulated (1-Hz twitches, 0.2 ms pulse duration, 15 V) for 5 min (control, n = 8). This twitch protocol was maintained throughout the ischemic and reperfusion periods. The control period was followed by 5, 20, or 40 min of ischemia (ligation of femoral artery and vein) or 40 min of ischemia with 0, 5, or 20 min of reperfusion (removal of ligature). The soleus [89% slow oxidative (SO)] and the white gastrocnemius [WG; 91% fast glycolytic (FG)] were analyzed for phosphocreatine (PCr), adenine nucleotides, glycogen, and glycolytic intermediates. Ischemia was characterized by progressive decreases in twitch tension, high-energy phosphagens, total adenine nucleotides (TAN), and glycogen. Also, energy metabolism was altered at a greater rate in WG than in soleus. Reperfusion resulted in a recovery in PCr and lactate, with little change in ATP, TAN, or glycogen. The inability to resynthesize adenine nucleotides and glycogen during reperfusion is characteristic of damaged skeletal muscle. The extent of the metabolic alterations in SO and FG muscles during twitch stimulation was comparable with previously reported noncontracting ischemia protocols of 2-4 and 4-7 h in length, respectively. The present study demonstrates that twitch stimulation of ischemic skeletal muscle is a useful model for inducing rapid metabolic changes and an ischemic insult comparable to prolonged noncontracting ischemia-reperfusion models.

Galanin stimulates phosphatidylinositol turnover in cardiac tissue of the mudpuppy

A galanin-like peptide has been found in the parasympathetic fibers innervating the mudpuppy myocardium and direct application of galanin produces hyperpolarization of atrial myocytes and a decrease in twitch tension. In the present study, atrial and ventricular strips were incubated with galanin and then evaluated for changes in either phosphatidylinositol turnover or cyclic nucleotide levels. Galanin caused a significant and concentration-dependent increase in phosphatidylinositol turnover in both atrial and ventricular tissue. However, galanin had no effect on cyclic AMP or cyclic GMP levels in either basal or adrenergically-stimulated preparations. These results suggest that the galanin-induced cardio-inhibition in the mudpuppy may be mediated, at least in part, by a change in phosphatidylinositol turnover.

Calcium current and tension generation in immature mammalian myocardium: Effects of diltiazem

Single sucrose gap and isolated myocyte voltage-clamp techniques were used to study the effects of diltiazem on calcium current ( I Ca) and tension generation in isolated ventricular myocytes and right ventricular papillary muscles from neonatal New Zealand White rabbits. Diltiazem was shown to significantly shorten the duration of isolated myocyte action potentials with no effect on overshoot potential or resting membrane potential. Diltiazem blocked but did not completely abolish I Ca in these neonatal cells. Addition of diltiazem to the solution bathing papillary muscles resulted in a similar reduction in action potential duration accompanied by a reduction in twitch tension. When the duration of depolarization was controlled employing the single sucrose gap voltage clamp, the decrease in tension development caused by diltiazem was abolished despite a significant decrease in twitch tension in the same muscles. In another series of experiments it was demonstrated that the magnitude of developed tension in neonatal papillary muscles is dependent upon the duration of depolarization. Taken together, the results of this investigation suggest that in neonatal myocardium when I Ca is blocked by diltiazem, the resulting reduction in developed tension is caused in part by reduction of action potential duration. The calcium carried into the neonatal heart cell by I Ca does not appear to be the only source of extracellular Ca 2+ for excitation-contraction coupling. Finally, the action potential appears to act as a gate for calcium movement into the neonatal heart cell.

Mechanism of potassium efflux and action potential shortening during ischaemia in isolated mammalian cardiac muscle.

1. Ischaemia was simulated in the isolated sheep cardiac Purkinje fibre and guinea-pig papillary muscle by immersing the preparations in paraffin oil. Ion-selective microelectrodes recorded potassium (Ks+) and pH (pHs) in the thin film of Tyrode solution trapped at the fibre surface while other microelectrodes recorded intracellular pH (pHi), membrane potential and action potentials (AP) (evoked by field stimulation), or membrane current (two-microelectrode voltage clamp in shortened Purkinje fibres). Twitch tension was also monitored. The paraffin oil model reproduced the salient characteristics of myocardial ischaemia, i.e. a decrease of twitch tension; a decrease of pHi and pHs; a rise in Ks+ (by 2-3 mM); a depolarization of diastolic membrane potential; considerable shortening of the AP (up to 30% within 4 min). 2. The sulphonylurea compounds, glibenclamide (200 microM) and tolbutamide (1 mM), known inhibitors of the KATP channel, completely blocked the ischaemic rise of Ks+ and prevented AP shortening. Ischaemic tension decline was notably less pronounced in the presence of sulphonylureas. 3. The ischaemic increase of slope conductance (Purkinje fibre) was prevented by 1 mM-tolbutamide and 200 microM-glibenclamide. 4. Sulphonylureas did not affect resting membrane potential, the AP or the current-voltage relationship under non-ischaemic conditions (this also indicates that ischaemic Ks+ accumulation is not fuelled by the background K+ current [iK1] which was shown, as expected, to be Ba2+ sensitive). 5. In a normally perfused preparation, reducing intracellular ATP by inhibiting glycolysis with 2-deoxyglucose (DOG) produced a similar AP shortening plus a membrane hyperpolarization, both of which were inhibited by tolbutamide or glibenclamide. The AP shortening was not related uniquely to the fall of pHi observed under these conditions since experimentally reducing pHi (by reducing pHo in the absence of DOG) lengthened rather than shortened the AP. 6. The possibility that the ischaemic rise in Ks+ might be the cause of AP shortening was excluded by the observation that, in a normally perfused Purkinje fibre, experimentally reducing pHi (by an amount similar to that seen during ischaemia) completely neutralized the AP-shortening effect of an elevated Ko+ (from 4.5 to 6.5 mM). Furthermore, the sulphonylurea-sensitive AP shortening seen during DOG treatment could not have been associated with a Ks+ rise since, in these particular experiments, the fibres were well perfused and diastolic membrane potential hyperpolarized.(ABSTRACT TRUNCATED AT 400 WORDS)

The Biphasic Effect of Amrinone on Tension Development in Newborn Mammalian Myocardium

The effect of the bipyridine compound, amrinone, on tension generation in neonatal and adult myocardium was studied over the concentration range 30-500 micrograms/mL. Increasing concentrations of amrinone caused a monotonic increase in twitch tension and the rate of tension development in adult papillary muscles. In contrast, lower concentrations of amrinone (30 and 100 micrograms/mL) caused a decrease in twitch tension and dP/dt in newborn papillary muscles, whereas 500 micrograms/mL amrinone caused a significant increase in both parameters in the younger age group. Lactic acid, used to dissolve amrinone, was shown to have no effect on tension development. Half relaxation time was decreased in adult preparations at all concentrations of amrinone. In comparison, the decrease in half relaxation time produced by amrinone in the newborn was significant only at a concentration of 500 micrograms/mL. Action potential duration in the newborn was significantly shortened by 30 micrograms/mL amrinone. In voltage clamp experiments, 30 micrograms/mL amrinone was shown to have no effect on tension accompanying two second voltage clamp steps to the plateau potential in newborn myocardium. Developed tension at 400 ms into the clamp step, final tension, and the ratio of early peak tension to final tension were all unchanged by the low concentration of amrinone. In contrast, 500 micrograms/mL amrinone in the newborn increased tension at 400 ms and final tension but had no effect on the ratio of early peak tension to final tension. These results suggest that the negative inotropic effect of lower concentrations of amrinone on neonatal myocardium is the result of changes in action potential configuration and not a true alteration in basic mechanisms of intracellular Ca2+ regulation.(ABSTRACT TRUNCATED AT 250 WORDS)