Effects of temperature and calcium availability on cardiac contractility in Synbranchus marmoratus, a neotropical teleost

An isometric muscle preparation was used to investigate the importance of the ventricular sarcoplasmic reticulum (SR) and extracellular Ca²⁺ (1.25 up to 11.25 mM) to force generation at 25 °C (acclimation temperature), 15 and 35 °C. The post-rest tension and force–frequency relationship were conducted with and without 10 μM ryanodine in the bathing medium. Increments in extracellular Ca²⁺ resulted in increases in twitch force development only at 35 °C. A significant post-rest potentiation was recorded for the control preparations at 25 °C (100% to 119.8 ± 4.1%). However, this post-rest potentiation was inhibited by ryanodine only at 25 °C (100% to 97.6 ± 1.5%). At 35 °C, force remained unchanged in the control preparations, but a significant post-rest decay was recorded in the presence of ryanodine (100% to 76.6 ± 4.6%) while at 15 °C, ryanodine was not able to preventing the post-rest potentiation observed in the control preparations. The increases in the imposed contraction frequency caused a decline of the force at 25 and 35 °C and ryanodine decreased significantly peak tension at both temperatures. The findings suggest a high or medium calcium turnover, possibly related to the presence of a functional SR, whose functionality is diminished when temperature is decreased.