Figure 7.
ATPo-mediated Ca2+ influx in thymocytes is dependent upon [Mg2+]o. The number of thymocytes exhibiting an ATPo- dependent rise in [Ca2+]i increases as [Mg2+]o is halved from 2 to 1 mM, suggesting that ATP4−, rather than MgATP2−, is the active form of the agonist. The top graph presents two representative single-cell traces that illustrate that the threshold of activation for the [Ca2+]i rise varies between thymocytes. Interestingly, the reduction in [Mg2+]o elicits a further rise in [Ca2+]i in many cells. The lower graph displays average [Ca2+]i in ATPo-sensitive and -insensitive thymocytes. Three times as many thymocytes exhibit a rise in [Ca2+]i as [Mg2+]o is reduced. Reducing Mg2+ did not affect [Ca2+]i levels within unresponsive thymocytes (experiment 71295-3; Cell 15 and 21).