Cell volume regulation and apoptotic volume decrease in isolated eel enterocytes.

Aim: Programmed cell death is a physiological form of death that is crucial in growth, maintenance, and normal development of tissues. Despite the universal prominence of cell volume loss in cells undergoing apoptosis, the relationship between this event and ensuing cell death remains uncertain. The aim of the present work was to study the role of cell volume regulation in the Apoptotic Volume Decrease (AVD), which represents one of the first events in apoptosis. The study was carried out on enterocytes isolated from the intestine of the teleost European eel. In the intact tissue the ion transport mechanisms responsible for cell volume regulation have been previously characterized (Lionetto et al., Cell Physiol. Biochem., 11: 41-54, 200; Lionetto et al., J. Exp. Biol., 208: 749-760, 2005; Lionetto et al., Cell Physiol. Biochem., 21: 361-372, 2008). Methods: Video imaging was utilized for cell volume measurement, while confocal microscopy was applied on cells labelled with annexin V for monitoring the onset of apoptosis. Results: Isolated enterocytes performed Regulatory Volume Decrease (RVD) following hypotonic swelling by the activation of high conductance Ca2+ activated K+ channels (BK channels). On the other hand, they were unable to perform Regulatory Volume Increase (RVI) following hypertonic shrinkage. The cells underwent AVD (corresponding to a cell shrinkage in isotonic condition) after 4-5 hours after isolation. The phenomenon was reversed by iberiotoxin, specific inhibitor of BK channels. Iberiotoxin was also able to delay the onset of apoptosis. Conclusions: Induction of the apoptotic volume decrease (AVD) under normotonic conditions shares cellular mechanisms that are used by the cells in response to osmotic changes. In isolated eel enterocytes AVD is dependent on BK activated K+ efflux and it is facilitated by the inactivation of ion transport mechanisms involved in RVI.