Window current through the T-type Ca²⁺ channel triggers the mechanism for cellular apoptosis via mitochondrial pathways

Abstract

We hypothesized that Ca²⁺ entry through the window T-type Ca²⁺ current causes apoptosis. To test this hypothesis, we transfected human embryonic kidney (HEK) 293 cells to express recombinant Ca_v3.2 T-type Ca²⁺ channels (hereafter called HEK-Ca_v3.2 cells). After incubation in media containing a high concentration (7.2 mM) of Ca²⁺, intracellular Ca²⁺ levels increased in HEK-Ca_v3.2 cells without electrical stimulation but not in untransfected HEK293 cells. In quiescent HEK-Ca_v3.2 cells exposed to high Ca²⁺ media, apoptosis, as indicated by the appearance of hypodiploid cells, loss of mitochondrial transmembrane potential, and activation of caspases-3 and -9 was observed, while caspase-8 was not activated. These apoptosis-associated changes were blunted by pretreatment with the R(−)-isomer of efonidipine, a selective blocker of T-type Ca²⁺ channels. High Ca²⁺ did not induce apoptosis in untransfected HEK293 cells. Our findings show that Ca²⁺ entry through the steady-state window current of T-type Ca²⁺ channels causes apoptosis via mitochondrial pathways, and suggests that T-type Ca²⁺ channels may be novel therapeutic targets for several diseases associated with abnormal apoptosis.