Critical remarks on the Ca-flux determinations in smooth and cardiac muscle

The commonly used methods of determining Ca fluxes in smooth and cardiac muscle are scrutinized for their appropriateness. These methods include an incubation of the tissue in 45Ca, followed by a washout period either in ice-cold EGTA-containing Ca-free solutions or in lanthanum-containing Ca-free solutions. The amount of 45Ca remaining in the tissue at the end of the washout phase is taken as Ca2+ influx during the labeling period. Procedures including longer washout periods cannot yield information on Ca2+ influx, since the Ca store relevant for contractile activation rapidly adapts to changes in the extracellular Ca2+ concentrations.