Subcellular Ca2+ distribution with varying Ca2+ load in neonatal cardiac cell culture

Recent work in our laboratory has investigated and modeled subcellular calc ium compartmentation and Ca2+ movement under steady-state control condition s. This experimental study is directed to the further description and quant itation of cellular calcium compartmentation patterns and movements as corr elated with contraction in neonatal rat cardiac myocytes in culture under a variety of calcium loading conditions. Compartmental contents were assesse d after incubations in various [Ca2+](o), 0 Na+/1 mM Ca2+, and 10 mu M ouab ain/1.0 mM Ca2+ test solutions. The cellular components investigated includ e sarcolemmal bound, sarcoplasmic reticulum (SR), and mitochondrial calcium . The results indicate that I)sarcolemmal calcium binding is insensitive to changes in [Ca2+](o) in the range tested (0.25-6.0 mM) while highly sensit ive to changes in [Na+](i); 2) SR is sensitive to both changes in [Ca2+](o) and [Na+](i) and exhibits a maximum loading capacity of similar to 750 mu mol Ca2+/kg dw; 3) in the [Ca2+](o) range between 0.25 and 2.0 mM, contract ile amplitude is proportional to SR content; 4) the mitochondria comprise a high-capacity calcium-containing compartment that is sensitive to changes in [Ca2+](o) but does not reach saturation under the conditions tested (0.2 5-8.0 mM [Ca2+](o)); 5) SR calcium is divided into at least two functionall y discrete pools, one of which is available for release to the myofilaments during a normal/(Ca)-triggered contraction and other of which is caffeine releasable but unavailable for release to the myofilaments during a normal triggered release; and 6) mitochondrial calcium serves as a reservoir of ca lcium capable of replenishing and/or augmenting SR stores with anywhere fro m 10% to 50% of mitochondrial calcium cycling through SR calcium compartmen ts.