Sarcomere length-dependence of activity-dependent twitch potentiation in mouse skeletal muscle

BACKGROUND: It has been reported that potentiation of a skeletal muscle twitch response is proportional to muscle length with a negative slope during staircase, and a positive slope during posttetanic potentiation. This study was done to directly compare staircase and posttetanic responses with measurement of sarcomere length to compare their length-dependence. METHODS: Mouse extensor digitorum longus (EDL) muscles were dissected to small bundles of fibers, which permit measurement of sarcomere length (SL), by laser diffraction. In vitro fixed-end contractions of EDL fiber bundles were elicited at 22°C and 35°C at sarcomere lengths ranging from 2.35 μm to 3.85 μm. Twitch contractions were assessed before and after 1.5 s of 75 Hz stimulation at 22°C or during 10 s of 10 Hz stimulation at 22°C or 35°C. RESULTS: Staircase potentiation was greater at 35°C than 22°C, and the relative magnitude of the twitch contraction (P(t)*/P(t)) was proportional to sarcomere length with a negative slope, over the range 2.3 μm – 3.7 μm. Linear regression yielded the following: P(t)*/P(t )= -0.59·SL+3.27 (r(2 )= 0.74); P(t)*/P(t )= -0.39·SL+2.34 (r(2 )= 0.48); and P(t)*/P(t )= -0.50·SL+2.45 (r(2 )= 0.80) for staircase at 35°C, and 22°C and posttetanic response respectively. Posttetanic depression rather than potentiation was present at long SL. This indicates that there may be two processes operating in these muscles to modulate the force: one that enhances and a second that depresses the force. Either or both of these processes may have a length-dependence of its mechanism. CONCLUSION: There is no evidence that posttetanic potentiation is fundamentally different from staircase in these muscles

Pre-Power-Stroke Cross-Bridges Contribute to Force Transients during Imposed Shortening in Isolated Muscle Fibers

When skeletal muscles are activated and mechanically shortened, the force that is produced by the muscle fibers decreases in two phases, marked by two changes in slope (P1 and P2) that happen at specific lengths (L1 and L2). We tested the hypothesis that these force transients are determined by the amount of myosin cross-bridges attached to actin and by changes in cross-bridge strain due to a changing fraction of cross-bridges in the pre-power-stroke state. Three separate experiments were performed, using skinned muscle fibers that were isolated and subsequently (i) activated at different Ca2+ concentrations (pCa2+ 4.5, 5.0, 5.5, 6.0) (n = 13), (ii) activated in the presence of blebbistatin (n = 16), and (iii) activated in the presence of blebbistatin at varying velocities (n = 5). In all experiments, a ramp shortening was imposed (amplitude 10%Lo, velocity 1 Lo•sarcomere length (SL)•s−1), from an initial SL of 2.5 µm (except by the third group, in which velocities ranged from 0.125 to 2.0 Lo•s−1). The values of P1, P2, L1, and L2 did not change with Ca2+ concentrations. Blebbistatin decreased P1, and it did not alter P2, L1, and L2. We developed a mathematical cross-bridge model comprising a load-dependent power-stroke transition and a pre-power-stroke cross-bridge state. The P1 and P2 critical points as well as the critical lengths L1 and L2 were explained qualitatively by the model, and the effects of blebbistatin inhibition on P1 were also predicted. Furthermore, the results of the model suggest that the mechanism by which blebbistatin inhibits force is by interfering with the closing of the myosin upper binding cleft, biasing cross-bridges into a pre-power-stroke state

The effects of fatigue and oxidation on contractile function of intact muscle fibers and myofibrils isolated from the mouse diaphragm

Abstract The goal of this study was to investigate the effects of repetitive stimulation and the oxidant H2O2 on fatigue of diaphragm intact fibers and in myofibrils measured with different Ca2+ concentrations. Intact fibers were isolated from mice diaphragm, and twitch and tetanic contractions (500 ms duration) were performed at different frequencies of stimulation ranging from 15 Hz to 150 Hz to establish a force-frequency relation before and after a fatigue and recovery protocol, without or after a treatment with H2O2. Fatigue was induced with isometric contractions (500 ms, 40 Hz) evoked every 0.8 seconds, with a total of 625 tetani. After the fatigue, the force recovery was followed by invoking tetanic contractions (500 ms, 40 Hz) every 1 min, with a total duration of 30 min. Individual myofibrils were also isolated from the mouse diaphragm and were tested for isometric contractions before and after treatment with H2O2 and NAC. In a second series of experiments, myofibrils were activated at different pCa (pCa = −log10 [Ca2+]), before and after H2O2 treatment. After 15 minutes of H2O2 treatment, the myofibrillar force was decreased to 54 ± 12% of its control, maximal value, and a result that was reversed by NAC treatment. The force was also decreased after myofibrils were treated with H2O2 and activated in pCa ranging between 4.5 and 5.7. These results suggest that fatigue in diaphragm intact fibers and at the myofibrils level is caused partially by oxidation of the contractile proteins that may be responsible for changing the force in various levels of Ca2+ activation

BRONCOESPASMO INDUZIDO POR EXERCÍCIO

O broncoespasmo induzido por exercício (BIE), ou asma induzida por exercício (AIE), é uma obstrução das vias aéreas após vários minutos de exercício. Esta síndrome está presente em 90% dos indivíduos asmáticos, em 30 a 40% dos alérgicos não asmáticos e em 3 a 10% dos atletas de várias modalidades, interferindo diretamente na prática esportiva destes. As hipóteses da perda de calor e água das vias respiratórias, da deficiência simpatoadrenal e da hiperemia reativa, as quais tentam explicar o mecanismo de desencadeamento do BIE, possuem pontos ainda não esclarecidos. Na relação asma/exercício, as particularidades tais como, tipo, intensidade, duração e forma de execução do exercício, são colocadas ora como provocadoras, ora como fatores preventivos do BIE, criando uma situação paradoxal a qual merece futuras pesquisas face às dúvidas que ainda permanecem nesta área

Blood doping in sports

“Doping” sangüíneo é a infusão ou reinfiisão de sangue no organismo de um atleta. Seu princípio fisiológico é que após a flebotomia o organismo do atleta é exposto a uma hipoxia, que induz uma eritrocitemia, gerando, assim, a produção de novos eritrócitos. Após a infusão ou reinfiisão sangüínea têm-se como conseqüência uma elevada concentração de hemoglobina plasmática seguida de uma melhora da “performance” Este procedimento pode ser realizado utilizando-se o sangue da mesma pessoa (reinfusãoautóloga) ou de outra pessoa (infusão-heteróloga). A eritrocitemia pode ser induzida também através da injeção de eritropoietina recombinante humana (rhEPO). Este hormônio age sobre as colônias formadoras de unidades de eritróides desencadeando a formação de novos eritrócitos. Espera-se que indivíduos tratados com rhEPO melhorem sua capacidade de realizar exercícios físicos prolongados. Existe a suspeita de que alguns atletas das provas de resistência aeróbia têm se beneficiado deste recurso. Apesar de estarem sendo pesquisadas algumas formas de detecção, não existem, ainda, métodos analíticos de detecção desta forma de “doping” que possam ser confiáveisBlood doping is the infusion or reinfusion of blood in the athlete’s body. Its physiological basis is that after bloodletting the athlete’s organism is exposed to a hypoxic condition that induces erythrocythaemia, and consequently new erythrocytes are produced. After blood infusion or reinfusion, a high concentration of plasmatic hemoglobin takes place followed by an increase in performance. This procedure may be done by using either the subject’s own stored blood (autologous reinfiision) or blood donated by another person (heterologous infusion). Moreover, erythrocythaemia may be induced by injection of recombinant human erythropoietin (rhEPO). This hormone operates on colony forming erythroid units inducing new erythrocytes formation. It is expected that the capacity to perform endurance exercise of subjects treated with rhEPO increases. Blood doping has aroused interest in the sports community because it cannot be detected in standard doping control tests. Although some methods of detection have been proposed, a reliable analytic method to detect blood doping has not been discovered ye

ERITROPOIETINA E EXERCÍCIO FÍSICO

O objetivo deste trabalho é abordar, através de revisão de literatura, a relação da EPO com o exercício físico. Para tal, será apresentada, num primeiro momento, a ação da EPO no organismo. Em seguida, será discutida a relação da EPO com o exercício físico, com base nos estudos realizados nessa área