ULTRASTRUCTURAL EVIDENCE IN RABBIT EAR ARTERIES THAT BARIUM ENTERS SMOOTH MUSCLE CELLS THROUGH CALCIUM CHANNELS

¹ Department of Physiology, Medical Biology Centre, 97 Lisburn Road, Belfast BT9 7BL

The calcium channel blocker methoxyverapamil, at 10^-5 mol/l, abolished contractile responses of isolated perfused rabbit ear arteries to barium. Entry of barium into smooth muscle cells was detected by the presence of mitochondrial granules specific for barium. Methoxyverapamil (10^-5 mol/l) prevented the appearance of barium granules. In a further series of experiments, eight out of twelve arteries incubated with barium, 2·5 mmol/l, showed granules; when the incubate contained 2·5 mmol/l of both barium and calcium, granules were found in only one of twelve specimens; thus calcium appears to compete with barium for entry. These results are consistent with the hypothesis that contractile responses to barium depend on entry of barium into the cells through calcium channels.