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

Recent work in our laboratory has investigated and modeled subcellular calcium compartmentation and Ca2+ movement under steady-state control conditions. This experimental study is directed to the further description and quantitation of cellular calcium compartmentation patterns and movements as correlated with contraction in neonatal rat cardiac myocytes in culture under a variety of calcium loading conditions. Compartmental contents were assessed after incubations in various [Ca2+]o, 0 Na+/1 mM Ca2+, and 10 microM ouabain/1.0 mM Ca2+ test solutions. The cellular components investigated include sarcolemmal bound, sarcoplasmic reticulum (SR), and mitochondrial calcium. The results indicate that 1) sarcolemmal calcium binding is insensitive to changes in [Ca2+]o in the range tested (0.25-6.0 mM) while highly sensitive 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 approximately 750 micromol Ca2+/kg dw; 3) in the [Ca2+]o range between 0.25 and 2.0 mM, contractile 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 (0.25-8.0 mM [Ca2+]o); 5) SR calcium is divided into at least two functionally discrete pools, one of which is available for release to the myofilaments during a normal ICa-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 calcium capable of replenishing and/or augmenting SR stores with anywhere from 10% to 50% of mitochondrial calcium cycling through SR calcium compartments.