Rats genetically selected for low and high aerobic capacity exhibit altered soleus muscle myofilament functions.

Aerobic exercise capacity is critical to bodily health. As a model to investigate the mechanisms that determine health and disease, we employed low (LCR) and high (HCR) capacity running rat models selectively bred to concentrate the genes responsible for divergent aerobic running capacity. To investigate the skeletal muscle contribution to this innate difference in running capacity we employed an approach combining examination of the myofilament protein composition and contractile properties of the fast fiber extensor digitorum longus (EDL) and slow fiber soleus (SOL) muscles from LCR and HCR rats. Intact muscle force experiments demonstrate that SOL, but not EDL, muscles from LCR rats exhibit a three times greater decrease in fatigued force. To investigate the mechanism of this increased fatigability in the LCR SOL muscle, we determined the myofilament protein composition and functional properties. Force-Ca2+ measurements demonstrate decreased Ca2+ sensitivity of single skinned SOL muscle fibers from LCR compared with that of HCR rats. Segregating SOL fibers into fast and slow types demonstrates that the decreased Ca2+ sensitivity in LCR SOL results from a specific decrease in slow-type SOL fiber Ca2+ sensitivity such that it was similar to that of fast-type fibers. These results identify that the altered myofilament contractile properties of LCR SOL slow-type fibers result in a fast muscle type Ca2+ sensitivity and the LCR muscle phenotype. Overall our findings demonstrate alterations of the myofilament proteins could contribute to fatigability of the SOL muscle and the decreased innate aerobic running performance of LCR compared with HCR rats.

A comprehensive approach to enhancing sexual health education in the Case Western Reserve University School of Medicine.

We report on the Sexual Health Curriculum Enhancement project at Case Western Reserve University School of Medicine. Using a US dollars 100000 grant from Pfizer Pharmaceuticals, Inc., we have developed and are in the process of implementing a comprehensive, cross-disciplinary and innovative curriculum that is based on three primary objectives for teaching sexual health: attitude change, behavior change, and knowledge acquisition. Five general strategies to incorporate specific sexual health content into the medical school curriculum have been implemented: (1). Faculty Development; (2). Additional Didactics; (3). Cased-Based Learning; (4). Testing and Assessment; and (5). Electronic (Computer/Web-Based Enhancements).

Hypoxia/fatigue-induced degradation of troponin I and troponin C: new insights into physiologic muscle fatigue.

Conditions such as respiratory failure and cardiopulmonary arrest can expose the diaphragm to hypoxemia. In skeletal muscles, fatiguing stimulation renders muscles hypoxic, which has long been known to dramatically reduce muscle function. We have previously demonstrated that fatiguing stimulation under hypoxic conditions disrupts both the excitation-contraction coupling (ECC) process and the isometric contractile properties (ICP) in intact diaphragm muscle strips and the contractile properties of skinned fibers isolated from these muscles. Here we have analyzed the effects of intermittent fatiguing stimulation on specific muscle proteins in muscle strips from mouse diaphragms that have been exposed to hypoxia. We report for the first time that the effects of hypoxia-fatigue, namely to decrease maximal tetanic force, maximal calcium-activated force and calcium sensitivity of the mouse diaphragm muscle, are associated with the degradation of troponins TnI and TnC (Western blot analysis). The concentrations of TnT and actin did not change under these same conditions. Because troponins are integrally involved in regulating the interaction between actin and myosin during the cross-bridge cycle, the degradation of TnI and TnC may explain the effects of hypoxia-fatigue on the ICP. This interpretation is supported by the observations that extraction of troponins from control skinned fibers mimics the effects of hypoxia-fatigue on contractile function and that incorporation of native troponins into fibers isolated from hypoxic-fatigued muscles partially restores function.

Superoxide, hydroxyl radical, and hydrogen peroxide effects on single-diaphragm fiber contractile apparatus.

Reactive oxygen species contribute to diaphragm dysfunction in certain pathophysiological conditions (i.e., sepsis and fatigue). However, the precise alterations induced by reactive oxygen species or the specific species that are responsible for the derangements in skeletal muscle function are incompletely understood. In this study, we evaluated the effect of the superoxide anion radical (O(2)(-).), hydroxyl radical (.OH), and hydrogen peroxide (H(2)O(2)) on maximum calcium-activated force (F(max)) and calcium sensitivity of the contractile apparatus in chemically skinned (Triton X-100) single rat diaphragm fibers. O(2)(-). was generated using the xanthine/xanthine oxidase system;.OH was generated using 1 mM FeCl(2), 1 mM ascorbate, and 1 mM H(2)O(2); and H(2)O(2) was added directly to the bathing medium. Exposure to O(2)(-). or.OH significantly decreased F(max) by 14.5% (P < 0.05) and 43.9% (P < 0. 005), respectively.OH had no effect on Ca(2+) sensitivity. Neither 10 nor 1,000 microM H(2)O(2) significantly altered F(max) or Ca(2+) sensitivity. We conclude that the diaphragm is susceptible to alterations induced by a direct effect of.OH and O(2)(-)., but not H(2)O(2), on the contractile proteins, which could, in part, be responsible for prolonged depression in contractility associated with respiratory muscle dysfunction in certain pathophysiological conditions.

Functional properties of skeletal muscle from transgenic animals with upregulated heat shock protein 70.

The influence of inducible heat stress proteins on protecting contracting skeletal muscle against fatigue-induced injury was investigated. A line of transgenic mice overexpressing the inducible form of the 72-kDa heat shock protein (HSP72) in skeletal muscles was used. We examined the relationship between muscle contractility and levels of the constitutive (HSC73) and inducible (HSP72) forms of the 72-kDa heat shock protein in intact, mouse extensor digitorum longus (EDL), soleus (SOL), and the diaphragm (DPH). In all transgenic muscles, HSP72 was expressed at higher levels compared with transgene-negative controls, where HSP72 was below the level of detection. At the same time, HSC73 levels were downregulated in all transgenic muscle types. Shipment-related stress caused an elevation in the levels of HSP72 in all muscles for 1 wk after arrival of the animals. We also found that, although no statistical differences in response to intermittent fatiguing stimulation in the contractile properties of intact transgene-positive muscles compared with their transgene-negative counterparts were observed, the response of intact transgene-positive EDL muscles to caffeine was enhanced. These findings demonstrate that elevated HSP72 does not protect EDL, SOL, or DPH muscles from the effects of intermittent fatiguing stimulation. However, HSP72 may influence the excitation-contraction coupling (ECC) process, either directly or indirectly, in EDL muscle. If the effects on ECC were indirect, then these results would suggest that manipulation of a specific gene might cause functional effects that seem independent of the manipulated gene/protein.

Increased susceptibility to fatigue of slow- and fast-twitch muscles from mice lacking the MG29 gene.

Mitsugumin 29 (MG29), a major protein component of the triad junction in skeletal muscle, has been identified to play roles in the formation of precise junctional membrane structures important for efficient signal conversion in excitation-contraction (E-C) coupling. We carried out several experiments to not only study the role of MG29 in normal muscle contraction but also to determine its role in muscle fatigue. We compared the in vitro contractile properties of three muscles types, extensor digitorum longus (EDL) (fast-twitch muscle), soleus (SOL) (slow-twitch muscle), and diaphragm (DPH) (mixed-fiber muscle), isolated from mice lacking the MG29 gene and wild-type mice prior to and after fatigue. Our results indicate that the mutant EDL and SOL muscles, but not DPH, are more susceptible to fatigue than the wild-type muscles. The mutant muscles not only fatigued to a greater extent but also recovered significantly less than the wild-type muscles. Following fatigue, the mutant EDL and SOL muscles produced lower twitch forces than the wild-type muscles; in addition, fatiguing produced a downward shift in the force-frequency relationship in the mutant mice compared with the wild-type controls. Our results indicate that fatiguing affects the E-C components of the mutant EDL and SOL muscles, and the effect of fatigue in these mutant muscles could be primarily due to an alteration in the intracellular Ca homeostasis.

Hypoxia and fatigue-induced modification of function and proteins in intact and skinned murine diaphragm muscle.

Fatigue studies of isolated, intact muscles typically utilize solutions saturated with O2. However, under in vivo fatiguing conditions, less oxygen is delivered to the muscles and they actually experience hypoxia. No studies to date have correlated the effects of acute hypoxia on the isometric contractile properties of intact muscles, skinned fibers isolated from the same muscles, and the cellular content of specific muscle proteins. Therefore, we have studied the effects of in vitro acute hypoxia on the fatigability of intact diaphragm muscle strips and on the isometric contractile properties of single Triton-skinned fibers isolated from control and hypoxic diaphragm muscles. We found that hypoxia and fatiguing stimulation per se affect the tetanic force of intact muscle strips without exhibiting any significant deleterious effects on the calcium-activated force of skinned muscle fibers dissected from the intact muscles. In contrast, fatiguing stimulation under hypoxic conditions decreased both the tetanic force of muscle strips and the calcium-activated force of skinned muscle fibers. Gel electrophoresis of muscles subjected to hypoxia and hypoxic-fatigue revealed that there is a significant reduction in three protein bands when compared to control muscles. Protein modification may be the underlying mechanism of muscle fatigue under physiologic conditions.

Endotoxin administration alters the force vs. pCa relationship of skeletal muscle fibers.

Recent work indicates that endotoxemia elicits severe reductions in skeletal muscle force-generating capacity. The subcellular alterations responsible for these decrements have not, however, been fully characterized. One possibility is that the contractile proteins per se are altered in endotoxemia and another is that the mechanism by which these proteins are activated is affected. The purpose of the present study was to assess the effects of endotoxin administration on the contractile proteins by examining the maximum calcium-activated force (F(max)) and calcium sensitivity of single Triton-skinned fibers of diaphragm, soleus, and extensor digitorum longus (EDL) muscles taken from control and endotoxin-treated (8 mg/kg) rats. Fibers were mounted on a force transducer and sequentially activated by serial immersion in solutions of increasing Ca(2+) concentration (i.e., pCa 6.0 to pCa 5.0); force vs. pCa data were fit to the Hill equation. All fibers were typed at the conclusion of studies using gel electrophoresis. F(max), the calcium concentration required for half-maximal activation (Ca(50)), and the Hill coefficient were compared as a function of muscle and fiber type for the control and endotoxin-treated animals. Control group F(max) was similar for diaphragm, soleus, and EDL fibers, i.e., 112.34 +/- 2.64, 111.55 +/- 3.66, and 104.05 +/- 4.33 kPa, respectively. Endotoxin administration reduced the average F(max) for fibers from all three muscles to 80.25 +/- 2.30, 72.47 +/- 2.97, and 78.32 +/- 2.43 kPa, respectively (P < 0.001 for comparison of each to control). All fiber types in diaphragm, soleus, and EDL muscles manifested similar endotoxin-related reductions in F(max). The Ca(50) and the Hill coefficient for all fiber types and all muscles were unaffected by endotoxin administration. We speculate that these alterations in the intrinsic properties of the contractile proteins represent a major mechanism by which endotoxemia reduces muscle force-generating capacity.

Pinacidil suppresses contractility and preserves energy but glibenclamide has no effect during muscle fatigue.

The effects of 10 microM glibenclamide, an ATP-sensitive K(+) (K(ATP)) channel blocker, and 100 microM pinacidil, a channel opener, were studied to determine how the K(ATP) channel affects mouse extensor digitorum longus (EDL) and soleus muscle during fatigue. Fatigue was elicited with 200-ms-long tetanic contractions every second. Glibenclamide did not affect rate and extent of fatigue, force recovery, or (86)Rb(+) fractional loss. The only effects of glibenclamide during fatigue were: an increase in resting tension (EDL and soleus), a depolarization of the cell membrane, a prolongation of the repolarization phase of action potential, and a greater ATP depletion in soleus. Pinacidil, on the other hand, increased the rate but not the extent of fatigue, abolished the normal increase in resting tension during fatigue, enhanced force recovery, and increased (86)Rb(+) fractional loss in both the EDL and soleus. During fatigue, the decreases in ATP and phosphocreatine of soleus muscle were less in the presence of pinacidil. The glibenclamide effects suggest that fatigue, elicited with intermittent contractions, activates few K(ATP) channels that affect resting tension and membrane potentials but not tetanic force, whereas opening the channel with pinacidil causes a faster decrease in tetanic force, improves force recovery, and helps in preserving energy.

Bradykinin receptor localization and cell signaling pathways used by bradykinin in the regulation of gonadotropin-releasing hormone secretion.

In a previous publication we provided evidence of a novel neuronal pathway for the control of GnRH secretion by bradykinin. The action of bradykinin appeared to be exerted through the bradykinin B2 receptor. In this study we demonstrated that the bradykinin B2 receptor is densely localized in the arcuate nucleus, median eminence, organum vasculosum of the lamina terminalis, and preoptic area, regions known to be critical for the control of GnRH secretion. To determine the mechanism of action of bradykinin in stimulating GnRH release, we used immortalized GnRH (GT1-7) cells in vitro. Bradykinin stimulation of GnRH secretion from GT1-7 cells appears to involve activation of the phospholipase C signaling pathway and mobilization of extracellular and intracellular calcium stores. Evidence to support this contention was derived from the observations that incubation of the phospholipase C inhibitor, U-73122 with bradykinin, blocked the ability of bradykinin to stimulate release from GT1-7 cells. This effect was specific, as a nitric oxide synthase inhibitor and a cyclooxygenase inhibitor were found to have no effect on bradykinin-induced GnRH secretion, suggesting that nitric oxide and PGs do not mediate bradykinin effects. Pertussis toxin also had no effect on bradykinin action. This suggests that the bradykinin B2 receptor may be coupled to a pertussis toxin-insensitive G protein in GT1-7 cells. With respect to calcium involvement in bradykinin action, fura-2 calcium indicator studies revealed that bradykinin can rapidly increase intracellular Ca2+ levels in GT1-7 cells. A role for intracellular Ca2+ in bradykinin action was further suggested by the finding that an intracellular calcium chelator, 1,2-bis(O-aminophenoxy)]ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester, significantly attenuated the effects of bradykinin on GnRH release. The elevation of intracellular calcium by bradykinin appears to be due to mobilization of calcium from the endoplasmic reticulum, as incubation of the Ca2+-adenosine triphosphatase inhibitor thapsigarin, which depletes endoplasmic reticulum Ca2+ stores, significantly attenuated bradykinin action on GnRH release. Extracellular calcium may also be involved in bradykinin action, as the L-type Ca2+ channel blockers verapamil and nifedipine had no effect on bradykinin-induced GnRH release, whereas the nonselective Ca2+ channel blocker, nickel chloride, attenuated bradykinin-induced GnRH release. Taken as a whole, these studies demonstrate that the bradykinin B2 receptor is densely localized in key hypothalamic nuclei responsible for regulation of GnRH release, and that the mechanism of bradykinin stimulation of GnRH secretion involves activation of the phospholipase C signaling pathway, with a critical role implicated for calcium in bradykinin action in GT1-7 cells.

Peroxynitrite induces contractile dysfunction and lipid peroxidation in the diaphragm.

Peroxynitrite may be generated in and around muscles in several pathophysiological conditions (e.g., sepsis) and may induce muscle dysfunction in these disease states. The effect of peroxynitrite on muscle force generation has not been directly assessed. The purpose of the present study was to assess the effects of peroxynitrite administration on diaphragmatic force-generating capacity in 1) intact diaphragm muscle fiber bundles (to model the effects produced by exposure of muscles to extracellular peroxynitrite) and 2) single skinned diaphragm muscle fibers (to model the effects of intracellular peroxynitrite on contractile protein function) by examining the effects of both peroxynitrite and a peroxynitrite-generating solution, 3-morpholinosydnonimine, on force vs. pCa characteristics. In intact diaphragm preparations, peroxynitrite reduced diaphragm force generation and increased muscle levels of 4-hydroxynonenal (an index of lipid peroxidation). In skinned fibers, both peroxynitrite and 3-morpholinosydnonimine reduced maximum calcium-activated force. These data indicate that peroxynitrite is capable of producing significant diaphragmatic contractile dysfunction. We speculate that peroxynitrite-mediated alterations may be responsible for much of the muscle dysfunction seen in pathophysiological conditions such as sepsis.

Excessive microtubules are not responsible for depressed force per cross-bridge in cardiac neural-crest-ablated embryonic chick hearts.

Ablation of the cardiac neural crest (CNCA) in embryonic chicks results in a high incidence of persistent truncus arteriosus, a congenital heart defect associated with decreased myocardial contractility. Using left ventricular trabeculae from chicks at embryonic day (ED) 15, we have previously shown that the twitch force of intact preparations is significantly reduced whereas the maximal calcium-activated force of skinned preparations is not significantly different in CNCA and sham-operated animals. We also previously found that the ventricular content of myosin, as well as of actin and tropomyosin, was nearly doubled in ED 15 hearts after CNCA. Since the number of cross-bridges is proportional to the myosin concentration, these data suggest that the force exerted per cross-bridge is decreased in CNCA hearts. We investigated the possibility that the decrease in force per cross-bridge is caused by inhibition of the contractile apparatus by excessive microtubules. To the contrary, we found that the total beta-tubulin content and the fraction of beta-tubulin polymerized in microtubules measured by Western blotting was the same in ventricular muscle strips from CNCA and sham-operated embryos. Furthermore, exposure to microtubule-destabilizing agents did not improve the force-producing capability of the contractile apparatus in CNCA embryos. We conclude that depression of force per cross-bridge in hearts from CNCA embryos is not due to an excess of microtubules.

Ion-specific protein destabilization of the contractile proteins of cardiac muscle fibers.

We investigated the inhibitory effects of increased salt concentrations on maximal calcium-activated force (Fmax) of rabbit cardiac papillary muscle bundles skinned with Triton X-100. While other studies have reported a lack of ion-specific effects on Fmax of cardiac muscle, we clearly demonstrated the presence of such effects when a wider variety of salts was investigated. In addition, like skeletal muscle, cardiac muscle was found to be sensitive to ionic strength and not to ionic equivalence. In support of our hypothesis that the ion-specific effects are due to protein destabilization, we found that a protein stabilizer (trimethylamine N-oxide, TMAO) completely abolished the ion-specific effects on Fmax. The ion-specific effect is probably due to binding of ions to the contractile proteins. The general ionic effect is most likely due to electrostatic shielding that remains in the presence of TMAO. Neither 300 mM sucrose nor TMAO significantly altered Fmax at physiological ionic strength indicating that the ion-specific depression of Fmax was not due to a colligative/osmotic effect. Furthermore, adding sucrose to solutions with a supraphysiological ionic strength caused a further decrease in Fmax indicating that certain osmolytes can alter Fmax if the contractile proteins are initially destabilized.

Alterations of myocardial contraction associated with a structural heart defect in embryonic chicks.

Ablation of cardiac neural crest at stages 8-10 produces a structural heart defect (persistent truncus arteriosus, PTA) in embryonic chicks. PTA is associated with decreased myocardial contractility, as indicated by decreased left ventricular ejection fraction. We compared the force of small ventricular strips from normal and defective chick hearts. In intact muscle, ablation of the neural crest leads to a 30-50% decrease in twitch force at any level of extracellular Ca2+ (0.45-20 mM) at embryonic days (ED) 7 and 15, relative to sham-operated controls. These differences could reflect defects at the level of the contractile apparatus and/or in the excitation-contraction coupling process. To distinguish changes of the contractile apparatus, we used detergent skinned preparations. The maximal Ca(2+)-activated force (Fmax) at ED15 was not significantly different between control and experimental embryos. At ED 7, however, Fmax was reduced by 36% in experimental preparations. Electron-micrographs showed that the organization and orientation of the myofibrils was similar in experimental and control ventricles. At ED 14, however, the average myofibrillar diameter was significantly increased in experimental ventricles. The content of the major myofibrillar proteins (myosin heavy chain, actin, and tropomyosin), determined from polyacrylamide gel electrophoresis and Coomassie Blue staining, normalized to total protein, was not statistically different in experimental and control ventricles at ED7. At ED15, however, content of these proteins was doubled in experimental ventricles. These data suggest a possible defect of the contractile apparatus at both ED 7 and 15, since the ratio of Fmax/myosin is reduced in the experimental hearts.

Muscle fatigue and induction of stress protein genes: a dual function of reactive oxygen species?

Definitive characterization of the mechanisms of skeletal muscle fatigue is still an area of active investigation. One emerging theory concerns a role for the reactive oxygen species (ROS) produced primarily as a consequence of elevated rates of mitochondrial respiration. It has been theorized that the long-lasting effects of low-frequency fatigue (LFF) can be attributed to disruption of some stage of the excitation contraction coupling (ECC) process. Recent evidence suggests that ROS likely denature one or more proteins directly associated with the sarcoplasmic reticulum (SR) Ca2+ release mechanism. Given the potential of ROS to damage intracellular proteins during subsequent bouts of muscle contractions, the capacity of preexisting antioxidant pathways may be complemented by the synthesis of inducible heat-stress proteins (HSPs). HSPs collectively function to maintain cellular protein conformation during stressful proteotoxic insults. The goal of this article is to illustrate how recent findings suggest a dual role of ROS generated during muscle contractions.

Effect of cardiac neural crest ablation on contractile force and calcium uptake and release in chick heart.

Cardiac neural crest ablation (CNCA) in the chick embryo at stages 8-10 results in reduced contractility of the heart that can be observed as early as stage 14. We found that intact trabeculae from embryonic day (E) 15 experimental animals after CNCA display an approximately 50% decrease in twitch force relative to sham-operated E15 control animals. In control and CNCA trabeculae skinned in Triton X-100 and bathed in our standard solutions, neither maximum Ca(2+)-activated force nor Ca2+ sensitivity of the contractile apparatus was significantly different. CNCA resulted in a marked reduction in the magnitude of the Ca2+ transient in trabeculae, estimated using fura 2 acetoxymethyl ester. CNCA had no effect on the half-time of Ca2+ loading by the sarcoplasmic reticulum (SR) of saponin skinned trabeculae at fixed Ca2+. However, it slightly reduced the Ca2+ sensitivity of Ca2+ uptake by the SR. Its most dramatic effect was to essentially abolish Ca(2+)-induced Ca2+ release from the SR. These effects on Ca2+ metabolism explain, in part, the decrease in the intracellular Ca2+ transient and myocardial contractility observed with CNCA.

Increased superoxide production during fatigue in the perfused rat diaphragm.

This study test the hypothesis that a temporal relationship exists between the production of superoxide anion (O2-) and the contractile activity of perfused rat diaphragm. O2- levels were determined minute to minute by measuring the reduction of cytochrome c in the perfusate as the diaphragms were subjected to various levels of contractile activity. After equilibrating at low contractile rates (one 500 ms 80 Hz train/min), diaphragms were fatigued by increasing their contractile activity for 5 min (one 500 ms 80 Hz train/s) and then allowed to recover for 30 min (one 500 ms 80 Hz train/min). During equilibration, diaphragms did not produce O2- above the background level measured in the presence of superoxide dismutase (SOD). Within the first minute of fatigue-inducing stimulation, however, the rate of O2- production increased to 0.70 +/- 0.17 nmol/min and remained elevated until the recovery period when production returned towards baseline. SOD blocked this stimulation-related increase of O2-. Tension (+/-SOD) fell to 12% of the control value during the fatigue-inducing stimulation. During recovery the contractile response returned to 51% of control, indicating long-lasting effects on the contractile machinery. SOD did not limit fatigue or improve recovery, probably because it is a large protein that cannot cross cell membranes and protect the cells by scavenging O2- at its site of production.

Hydrogen peroxide disrupts Ca2+ release from the sarcoplasmic reticulum of rat skeletal muscle fibers.

Reactive oxygen species such as superoxide (O2-) and H2O2 are produced at low levels in resting muscles and at substantially higher levels in exercising muscles. Increased respiratory activity with exercise leads to O2- production by the NADPH oxidase reaction and the subsequent generation of H2O2 from O2- by spontaneous dismutation or by the superoxide dismutase reaction. The long-lasting (24-h) depression of contractile function after exercise has been linked to damage of one or more proteins important in the excitation-contraction coupling process. We studied mechanically and chemically skinned fibers from the extensor digitorum longus muscle of the rat to evaluate the effects of a 5-min exposure to 1.0 mM H2O2 on muscle function. We found that H2O2 had no effect on the isometric force-producing properties of the contractile apparatus or on Ca2+ uptake by the sarcoplasmic reticulum. It did, however, significantly affect Ca2+ release from the sarcoplasmic reticulum. Maximum depolarization-induced Ca2+ release was inhibited, and the sensitivity to depolarization was decreased. Ca(2+)-induced release was completely blocked. We conclude that elevated levels of H2O2 with exercise are capable of damaging one or more proteins of the excitation-contraction coupling process to produce a disruption in function that can account, at least in part, for the long-lasting effects of fatiguing stimulation.

Influence of physiological L(+)-lactate concentrations on contractility of skinned striated muscle fibers of rabbit.

These experiments investigated the effects of physiological concentrations of L(+)-lactate on the contractility of chemically skinned rabbit fast-twitch psoas, slow-twitch soleus, and cardiac muscles at pH 7.L(+)-Lactate depressed maximal calcium-activated force (Fmax) of all muscles studied within the range of 5-20 (slow-twitch muscle) or 5-25 mM (fast-twitch and cardiac muscles). Fmax of fast-twitch fibers was inhibited to the greatest degree (9% in K2 creatine phosphate solutions). In all of these muscle types, Fmax returned to control levels as L(+)-lactate was increased to 30-50 mM. Substitution of neither D-lactate nor propionate for L(+)-lactate significantly altered Fmax. In addition, with the exception of fast-twitch muscle (where the Hill coefficient decreased), L(+)-lactate concentrations, which maximally inhibited Fmax, did not affect the force vs. pCa relationship of muscles tested. These results demonstrate that L(+)-lactate significantly contributes to the depression of muscle function noted during lactic acidosis, directly inhibiting Fmax of the contractile apparatus. This contribution is maximal in fast-twitch muscle where L(+)-lactate is responsible for as much as one-third of the depressant effect on Fmax of the contractile apparatus noted during lactic acidosis.