Hypotonically induced calcium increase and regulatory volume decrease in newborn rat cardiomyocytes

Abstract

 The effect of cell swelling on intracellular calcium concentration ([Ca²⁺]_i) was studied in newborn rat cardiomyocytes. Hypotonic cell swelling induced a fast and transient [Ca²⁺]_i increase (hypotonically induced calcium increase, HICI; 388±47 nM, n=14). HICI was not inhibited by cyclopiazonic acid (CPA), an inhibitor of sarcoplasmic Ca²⁺-ATPase, nor ryanodine (an inhibitor of calcium-induced calcium release), whereas it was abolished (11±19 nM, n=5) in the absence of external calcium. Thus, HICI appeared to depend exclusively on entry of external calcium. Gadolinium ion (Gd³⁺), a generic inhibitor of stretch-activated cation channels (SACs), was unable to affect HICI (353±79 nM, n=6). Similarly, HICI was unaffected by internal Na⁺ depletion and external Na⁺ omission. These results suggest that neither Gd³⁺-sensitive SACs nor Na⁺-Ca²⁺ exchange is responsible for HICI. Conversely, HICI was inhibited by diltiazem (42±4 nM, n=3) and by membrane predepolarization (40±18 nM, n=5), suggesting an involvement of L-type voltage-activated calciumchannels. Cardiomyocyte swelling was followed by a regulatory volume decrease (RVD). The putative role of HICI in volume regulation was studied by removal of external calcium. This procedure significantly slowed RVD but did not abolish it. In conclusion, newborn rat cardiomyocytes exhibit an external-calcium-dependent HICI which contributes partially to the RVD.