The β-adrenergic receptor stimulation improves endurance in fast twitch muscles and these effects are sensitive to extracellular Ca²⁺ influx. Present study is aimed to determine the effects of adrenaline, with different concentrations of extracellular Ca²⁺\(\left( {{\text{Ca}}_{\text{ECF}}^{ 2+ } } \right)\), on the contractility and endurance of slow twitch muscles during high frequency stimulations (HFS). Isolated soleus of rabbit was electrically stimulated (strength; 50 Hz, duration; 0.5 ms) in the presence (Test) of adrenaline (1 × 10⁻⁷ mM) or without adrenaline (CTL). Fatigue was induced with HFS (80 Hz) for the duration of 20 s. Contractions were recorded through isometric transducer connected with Powerlab. Kreb’s buffer was used with three compositions: standard with 2.5 mM Ca²⁺ (Ca-S), Ca²⁺ free buffer (Ca-F) and buffer with raised Ca²⁺ i.e., 10 mM (Ca-R). Muscles endurance was assessed by measuring the decline in tetanic tension in the terms of percentage (%P_max) and rate of decline in tetanic tension (dP/dt). During 20 s, %P_max showed reduction of only 10% in Ca-S. This decline was enhanced in Ca-F (50%) and reduced in Ca-R (6%). Effect of adrenaline was observed only in Ca-F where %P_max was about 20% greater in Test than CTL. These effects were not observed in both Ca-S and Ca-R during 20 s. However, when duration of stimulation was increased to 120 or 150 s in Ca-S and Ca-R respectively, decline in %P_max was less in Test as compared to CTL. Thus, \({\text{Ca}}_{\text{ECF}}^{ 2+ }\) plays protective role against fatigue during continuous HFS in slow twitch muscles. In addition, adrenaline improves the muscles endurance during fatiguing contraction but these effects are not mediated through \({\text{Ca}}_{\text{ECF}}^{ 2+ }\) influx.

中文翻译：

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肾上腺素对不同钙浓度的离体哺乳动物比目鱼的收缩力和耐力的影响。

β-肾上腺素能受体刺激可改善快速抽搐肌肉的耐力，这些作用对细胞外Ca ^2+内流敏感。本研究旨在确定不同浓度的细胞外Ca ²⁺对肾上腺素的影响\（\ left（{{\ text {Ca}} _ {\ text {ECF}} ^ {2+}} \ right）\ ），关于高频刺激（HFS）期间慢抽搐肌肉的收缩力和耐力。在肾上腺素（1×10 ^-7）存在下（测试）电刺激兔子离体的比目鱼肌（强度； 50 Hz，持续时间； 0.5 ms） （mM）或不含肾上腺素（CTL）。HFS（80 Hz）引起疲劳持续20 s。通过与Powerlab连接的等距传感器记录收缩。克雷布氏缓冲液用于三种成分：含2.5 mM Ca ²⁺（Ca-S）的标准液，不含Ca ^2+的缓冲液（Ca-F）和含Ca ^2+的缓冲液，即10 mM（Ca-R）。通过以百分比（％P _max）和强直性张力的下降速率（dP / dt）为单位来测量强直性张力的下降来评估肌肉的耐力。在20 s内，％P _max在Ca-S中仅降低了10％。Ca-F（50％）升高，Ca-R（6％）降低。肾上腺素的作用仅在％P _max的Ca-F中观察到在测试中比CTL大20％。在20 s内，在Ca-S和Ca-R中均未观察到这些影响。但是，当刺激持续时间分别在Ca-S和Ca-R中增加到120或150 s时，与CTL相比，Test中的％P _max下降幅度较小。因此，\（{\ text {Ca}} _ {\ text {ECF}} ^ {2+} \）在缓慢抽搐的肌肉中连续HFS期间对疲劳起到保护作用。此外，肾上腺素可改善疲劳性疲劳期间的肌肉耐力，但这些作用不是通过\（{\ text {Ca}} _ {\ text {ECF}} ^ {2+} \）来介导的。