Elevated extracellular [K+] inhibits death-receptor- and chemical-mediatedapoptosis prior to caspase activation and cytochrome c release

Efflux of intracellular K+ and cell shrinkage are features of apoptosis in many experimental systems. and a regulatory role has been proposed For cyto plasmic [K+] in initiating apoptosis, We have investigated this in both dea th-receptor-mediated and chemical-induced apoptosis. Using jurkat T cells p re-loaded with the K+ ion surrogate Rb-86(+) We have demonstrated an efflux of intracellular K+ during apoptosis that was concomitant with, but did no t precede, other apoptotic changes, including phosphatidylserine externaliz ation, mitochondrial depolarization and cell shrinkage. To further clarify the role of K+ ions in apoptosis, cytoprotection by elevated extracellular [K+] was studied. Induction of apoptosis by diverse death-receptor and chem ical stimuli in two cell lines was inhibited prior to phosphatidylserine ex ternalization, mitochondrial depolarization, cytochrome c release and caspa se activation. Using a cell-free system, we have demonstrated a novel mecha nism by which increasing [K+] inhibited caspase activation. In control dATP -activated lysates, Apaf-1 oligomerized to a biologically active caspase pr ocessing approximate to 700 kDa complex and an inactive approximate to 1.4 MDa complex. Increasing [K+] inhibited caspase activation by preventing for mation of the approximate to 700 kDa complex, but not of the inactive compl ex. Thus intracellular and extracellular [K+] markedly affect caspase activ ation and the initiation of apoptosis induced by both death-receptor ligati on and chemical stress.