Transient receptor potential vanilloid subtype 1 (
TRPV1) receptor is a
pain-sensing,
ligand-gated, non-selective
cation channel expressed in peripheral sensory neurons. Prolonged activation of TRPV1 by
capsaicin leads to cell swelling and formation of membrane
blebs in rat dorsal root ganglion (DRG) neurons. Similar results were obtained in NIH3T3 fibroblast cells stably expressing TRPV1. Here, we assessed the contribution of Ca(2+) and Na(+)
ions to TRPV1-mediated changes. Cell swelling was caused by a substantial influx of extracellular Na(+) via TRPV1 channels, causing concomitant transport of water. In the absence of extracellular Na(+), the membrane blebbing was completely inhibited, but Ca(2+) influx did not change under these conditions. Na(+) influx was modulated by the intracellular Ca(2+) concentration ([Ca(2+)]i). Elevation of [Ca(2+)]i by
ionomycin sensitized/activated TRPV1 channels causing cell swelling in TRPV1-positive cells. In the absence of extracellular Ca(2+),
capsaicin caused only little increase in [Ca(2+)]i indicating that the increase in [Ca(2+)]i observed after
capsaicin application is derived essentially from extracellular Ca(2+) and not from internal Ca(2+) stores. In the absence of extracellular Ca(2+) also the process of cell swelling was considerably slower.
Calretinin is a Ca(2+)
buffer protein, which is expressed in a subset of TRPV1-positive neurons.
Calretinin decreased the amplitude, but slowed down the decay of Ca(2+) signals evoked by
ionomycin. Cells co-expressing TRPV1 and
calretinin were less sensitive to TRPV1-mediated,
capsaicin-induced volume increases. In TRPV1-expressing NIH3T3 cells,
calretinin decreased the
capsaicin-induced Ca(2+) and Na(+) influx. Swelling and formation of membrane
blebs resulted in impaired plasma membrane integrity finally leading to cell death. Our results hint towards a mechanistic explanation for the apoptosis-independent
capsaicin-evoked neuronal loss and additionally reveal a protective effect of
calretinin; we propose that the Ca(2+)-buffering capacity of
calretinin reduces the susceptibility of
calretinin-expressing DRG neurons against cell swelling/death caused by overstimulation of TRPV1 channels. This article is part of a Special Issue entitled:12th European Symposium on
Calcium.