Intracellular site of Sr2+ and Ba2+ accumulation in frog twitch muscle fibres as determined by electron probe X-ray microanalysis.

Strontium and barium can substitute for calcium at different levels of the excitation-contraction-relaxation cycle. The problem of sequestration of these ions in cellular microcompartments may be settled only by direct evidence obtained with analytical methods. Isolated frog twitch muscle fibres were perfused with increasing concentrations of potassium in Ca-free solution supplemented with Sr2+ (10 mmol/l) or Ba2+ (5 mmol/l). After equilibration in a Ca-free Ringer with Sr2+ or Ba2+ for 30 to 60 min the fibres were frozen in liquid propane (at 80 K) to immobilise ions. Ultrathin (150 nm) cryosections were cut at 170 K, freeze-dried, carbon-coated and analysed in an electron microscope equipped with an X-ray spectrometer. The ultrastructure of the superficial layer of the fibres was satisfactorily preserved. The terminal cisternae (t.c.) of the sarcoplasmic reticulum (SR) were dark and contained various amounts of Sr or Ba in addition to Ca. In Sr loaded fibres the longitudinal SR occasionally showed electron dense content with significant amounts of Ca; no Sr was present. The results suggest that t.c. is the common sequestering compartment for Ca, Sr and Ba. Essentially the same distribution pattern of Sr was found following precipitation of Sr with a solution containing digitonin and Koxalate.