SPATIAL GRADIENTS OF CYTOSOLIC CALCIUM-CONCENTRATION IN NEURONS DURING PARADOXICAL ACTIVATION BY CALCIUM

4-Br-A23187 caused a calcium influx into chick sensory neurones and ra ised cytosolic calcium from a rest level of 97 +/- 7 nM to a peak of 2 96 +/- 30 nM. Despite the continued presence of ionophore, however, cy tosolic calcium concentrations then fell. After 30 min in ionophore, c ytosolic calcium concentration had returned to 105 +/- 5 nM, not signi ficantly different from the value before ionophore addition. The perme ability of the plasmalemma to divalent cations, as estimated by the ma nganese quench technique, was no lower at 30 min than at the peak of t he cytosolic calcium transient. Thus the fall of calcium from its peak was not due to a slowing of calcium influx, but was due to an upregul ation of mechanisms that remove calcium from the cytosol - an upregula tion that persists even though cytosolic calcium has apparently return ed to pre-stimulus levels. We used a novel fixed slit confocal microsc ope to examine the calcium concentration profile close to the plasmale mma. We found that after 25-30 min ionophore treatment, calcium concen tration was elevated only in the cytoplasm within 1 mu m of the plasma lemma. A maintained, elevated calcium under the plasmalemma can help e xplain the phenomenon of paradoxical activation seen in this and other cell types.