Acute effects of cobalt on calcium handling in cardiac myocytes

Abstract
Introduction In recent years, Cobalt has been used as a component in metal-on-metal (MoM) bearings for orthopaedic knee and hip replacements, due to abnormal wear and corrosion they can result in the release of cobalt ions (Co2+). Chronic cobalt exposure has been shown to cause cardiac contractile dysfunction, but the cellular mechanisms underlying these cardiotoxic effects are not completely understood. This study aims to investigate the acute effects of cobalt on cardiomyocytes, focusing on calcium handling and its impact on the oxidation of Ca2+/calmodulin-dependent protein kinase II (CaMKII).

Methods Male Sprague Dawley rat hearts were perfused with collagenase type I and protease, in order to isolate cardiac myocytes. Isolated cardiomyocytes were exposed to various CoCl2 concentrations, including control (no cobalt), 1μM, 10μM, and 100μM, for 5 min and 1 h. To measure cellular Ca2+, cells were leaded with Cal520 AM for 20 mins. A spinning disk confocal microscope was used to measure calcium at 200Hz. Myocytes were electrically paced at 1Hz at room temperature and the effects of cobalt on cardiomyocyte Ca2+ transient amplitude, Ca2+ spark amplitude, and Ca2+ spark frequency were assessed. Furthermore, western blotting was employed to quantify the expression of oxidated-CAMKII in isolated cardiomyocytes.

Results Calcium transients refer to the increase and subsequent decrease in global intracellular calcium concentration that occurs during each heartbeat. Ca2+ transient amplitude in cardiac myocytes were significantly reduced following exposure to 10μM of Cobalt for 5 minutes as well as 1 hour of 1μM and 10μM of cobalt when compared with control (p < 0.05). These effects were not observed in the groups exposed to 100μM of CoCl2 at any time point (figure 1A).

Following acute (5 mins and 1 hr) cobalt treatment, we quantified spontaneous SR calcium release events visualized as Ca2+ sparks. Ca2+ spark amplitude remained constant across cobalt concentration when cardiomyocytes were exposed for 5 minutes. However, at 1 hour the spark amplitude was reduced following exposure to 10μM and 10μM of cobalt (p < 0.05) (figure 1B).

Due to the changes observed in calcium handling in cardiomyocytes, we investigated whether cobalt treatments affected oxidative activation of CaMKII in cardiomyocytes. During pathology, CaMKII elevated phosphorylation of RyR2 has been observed, which has been linked to abnormal SR Ca2+ leak and delayed afterdepolarizations (Uchinoumi et al., 2016). However, we did not observe any significant difference in phosphorylation between the control and any of the treatment groups (figure 2).

Conclusions This study demonstrates that acute cobalt exposure in cardiomyocytes results in calcium handling dysregulation; however, there is not enough evidence to confirm that this occurs due to CaMKII activation. Further research is necessary to further understand the underlying mechanisms of action of cobalt on these cells.