Transient increases in extracellular K ⁺ produce two pharmacological distinct cytosolic Ca ²⁺ transients

Transient increases in extracellular K ⁺ are observed under various conditions, including repetitive neuronal firing, anoxia, ischemia and hypoglycemic coma. We studied changes in cytoplasmic Ca ²⁺ ([Ca ²⁺] _cyt) evoked by pulses of KCl in human neuroblastoma SH-SY5Y cells and rat dorsal root ganglia (DRG) neurons at 37°C. A "pulse" of KCl evoked two transient increases in [Ca ²⁺] _cyt, one upon addition of KCl (K ⁺ _on) and the other upon removal of KCl (K ⁺ _off). The K ⁺ _on transient has been described in many cell types and is initiated by the activation of voltage-dependent Ca ²⁺ channels followed by Ca ²⁺-evoked Ca ²⁺ release from intracellular Ca ²⁺ stores. The level of KCl necessary to evoke the K ⁺ _off transient depends on the type of neuron, in SH-SY5Y cells it required 100 mM KCl, in most (but not all) of dorsal root ganglia neurons it could be detected with 100-200 mM KCl and in a very few dorsal root ganglia neurons it was detectable at 20-50 mM KCl. In SH-SY5Y cells, reduction of extracellular Ca ²⁺ inhibited the K ⁺ _on more strongly than the K ⁺ _off and slowed the decay of K ⁺ _off. Isoflurane (1 mM) reduced the K ⁺ _on- but not the K ⁺ _off-peak. However, isoflurane slowed the decay of K ⁺ _off. The nonspecific cationic channel blocker La ³⁺ (100 μM) had an effect similar to that of isoflurane. Treatment with thapsigargin (TG) at a concentration known to only deplete IP3-sensitive Ca ²⁺ stores did not affect K ⁺ _on or K ⁺ _off, suggesting that Ca ²⁺ release from the IP3-sensitive Ca ²⁺ stores does not contribute to K ⁺ _on and K ⁺ _off transients and that the thapsigargin-sensitive Ca ²⁺ ATPases do not contribute significantly to the rise or decay rates of these transients. These findings indicate that a pulse of extracellular K ⁺ produces two distinct transient increases in [Ca ²⁺] _cyt.