Normotonic cell shrinkage because of disordered volume regulation is an early prerequisite to apoptosis

A major hallmark of apoptosis is normotonic shrinkage of cells. Here, we studied the relation between apoptotic cell shrinkage and apoptotic cell death. Induction of the apoptotic volume decrease (AVD) under normotonic conditions was found to be coupled to facilitation of the regulatory volume decrease (RVD), which is known to be attained by parallel operation of Cl(−) and K(+) channels, under hypotonic conditions. Both the AVD induction and the RVD facilitation were found to precede cytochrome c release, caspase-3 activation, DNA laddering, and ultrastructural alterations in three cell types after apoptotic insults with two distinct apoptosis inducers. Also, the AVD was not prevented by a broad-spectrum caspase inhibitor. When the AVD induction and the RVD facilitation were prevented by blocking volume-regulatory Cl(−) or K(+) channels, these cells did not show succeeding apoptotic biochemical and morphological events and were rescued from death. Thus, it is concluded that the AVD, which is caused by disordered cell volume regulation, is an early prerequisite to apoptotic events leading to cell death.