Calcium Stored in the Sarcoplasmic Reticulum Acts as a Safety Mechanism in Fish Heart

Abstract

Cardiac contraction is mediated by a transient increase in calcium ([Ca]) inside the cytoplasm. In mammalian cardiomyocytes most of the calcium for cardiac contraction is from the calcium stored in the sarcoplasmic reticulum (SR). In fish cardiomyocytes, the relative contribution of calcium coming from the extracellular space versus SR is not clear. This study addresses this point. Trout ventricular myocytes were enzymatically isolated. Calcium current (ICa) was recorded using the patch clamp technique. Using different calcium buffers, inactivation of ICa was used as an index of SR calcium release. [Ca] was also recorded using fura-2 AM. With a slow Ca buffer (EGTA 2mM in the pipette solution), ICa inactivated slowly: the time to reach 37% of peak current (T37) was 27.1±1.8 ms (n=37). With a fast Ca buffer (BAPTA 10 mM), ICa decay was similar to the decay in the presence of EGTA (T37: 30.3±2.4 ms, n=20). In contrast, during beta-adrenergic stimulation, inactivation of ICa in the presence of EGTA (T37: 11.6±1.7 ms, n =18) was significantly faster than in presence of BAPTA (T37: 27.3±1.6 ms, n=12), indicating the presence of SR calcium release. [Ca] during electrical field stimulation was significantly larger during beta-adrenergic stimulation compare to control condition (Fura-2, F340/380= 0.026±0.005 RU versus 0.046±0.010 RU, n=10 and 10 respectively). Caffeine-induced transients (an index of SR calcium load) indicate that a small, but significant, increase in [Ca] in the cytoplasm was present after beta-adrenergic stimulation (F340/380= 0.080±0.022 RU control versus 0.112±0.028 RU beta-adrenergic stimulation, n=10 and 10 respectively). At rest, fish cardiomyocytes are not using calcium stored in the SR. However, during a stress calcium from the SR plays an important role in cardiac contraction.Supported by the Wellcome Trust.