1. Accumulation of calcium by extensor digitorum longus muscles from dystrophin-deficient mdx and control C57BL/10 mice has been studied in vitro by measurements of total muscle calcium and by following the retention of 45Ca resulting from the incubation of muscles with the isotope for up to 2 h.
2. The rate of influx of calcium, calculated from the retention of 45Ca, was linear over 2 h in muscles at rest with no significant difference between mdx and control muscles.
3. Repetitive tetanic stimuli caused a substantial increase in 45Ca flux into both mdx and control muscles. This elevated rate of influx was maintained by control muscle, but not by mdx muscle after stimulation resulting in a significantly smaller total calcium flux into mdx muscle compared with control muscle by 1 h after stimulation. Similar changes were also seen in the total muscle calcium content of mdx and control muscles. Comparison of these results with those for loss of cytosolic creatine kinase previously reported (McArdle, A., Edwards, R.H.T. & Jackson, M.J. Clin. Sci. 1991; 80, 367-71)  indicate that control and dystrophin-deficient muscles release equivalent amounts of intracellular creatine kinase in response to the same accumulation of intracellular calcium.
4. These results therefore do not support the hypotheses that dystrophin deficiency in muscle leads to increased calcium influx during contractile activity, or that dystrophin-deficient muscle shows any inherent increased permeability to cytosolic proteins.