The colon is physiologically exposed to osmotic stress, and the activation of mechanisms of cell volume regulation is fundamental in colonocyte physiology. This epithelium is characterized by a high cellular turnover ensured by a high division rate of stem cells and a corresponding high apoptotic rate of mature cells. Therefore, apoptosis is a crucial event in the tissue homeostasis.

The aim of this work was to study the role of cell volume regulation mechanisms in apoptosis in rat distal colon. The attention was focused on surface colonocytes.

Optical and confocal microscopy and image analysis were applied.

When colon explants were exposed to hypotonic stress, superficial colonocytes showed a Regulatory Volume Decrease (RVD) that was completely abolished by high K+ or iberiotoxin treatment. When they were exposed to the proapoptotic agent H₂O₂, colonocytes showed a multi-step isotonic decrease of the cell volume. The early normotonic volume decrease (observed within four hours) can be ascribed to Apoptotic Volume Decrease (AVD) and was shown to precede annexin V positivity. It was also inhibited by high K+ or iberiotoxin treatment.

In conclusion in rat superficial colonocytes K+ efflux through BK channels was demonstrated to be the main mechanism of RVD. In these cells AVD was detected preceding a key hallmark of apoptosis such as externalization of membrane phosphatidylserine. Interestingly, it showed to be dependent on the same ion transport mechanisms underlying RVD.