Cell volume control in phospholemman (PLM) knockout mice: do cardiac myocytes demonstrate a regulatory volume decrease and is this influenced by deletion of PLM?
Bell JR, Lloyd D, Curl CL, Delbridge LM, Shattock MJ
In addition to modulatory actions on Na/K-ATPase, phospholemman (PLM) has been proposed to play a role in cell volume regulation. Over-expression of PLM induces ionic conductances, with ‘PLM channels’ exhibiting selectivity for taurine. Osmotically challenging host cells over-expressing PLM increases taurine efflux and augments the cellular regulatory volume decrease (RVD) response, though a link between PLM and cell volume regulation has not been studied in the heart. We recently reported a depressed cardiac contractile function in PLM knockout mice in vivo, which was exacerbated in crystalloid-perfused isolated hearts, indicating these hearts were osmotically challenged. To address this, the present study investigated the role of PLM in osmoregulation in the heart. Isolated PLM wild-type and knockout hearts were perfused with a crystalloid buffer supplemented with mannitol in a bid to prevent perfusate-induced cell swelling and maintain function. Accordingly, and in contrast to wild-type controls, contractile function was improved in PLM knockout hearts with 30mM mannitol. To further investigate, isolated PLM wild-type and knockout cardiomyocytes were subjected to increasing hypo-osmotic challenges. Initial validation studies showed the IonOptix video edge-detection system to be simple and accurate ‘real time’ method for tracking cell width as a marker of cell size. Myocytes swelled equally in both genotypes indicating PLM, when expressed at physiological levels in cardiomyocytes, is not essential in limiting water accumulation in response to a hypo-osmotic challenge. Interestingly, freshly isolated adult cardiomyocytes consistently failed to mount RVDs in response to cell swelling, adding to conflicting reports in the literature. A proposed perturbation of the RVD response as a result of the cell isolation process was not restored, however, with short-term culture in either adult or neonatal cardiomyocytes.