Tear film hyperosmolarity induces dry eye syndrome (DES) through transient receptor potential vanilloid type 1 (TRPV1) activation. L-carnitine is a viable therapeutic agent since it protects against this hypertonicity-induced response. Here, we investigated whether L-carnitine inhibits TRPV1 activation by blocking heat- or capsaicin-induced increases in Ca2+ influx or hyperosmotic stress-induced cell volume shrinkage in a human corneal epithelial cell line (HCE-T). Single-cell fluorescence imaging of calcein/AM-loaded cells or fura-2/AM-labeled cells was used to evaluate cell volume changes and intracellular calcium levels, respectively. Planar patch-clamp technique was used to measure whole-cell currents. TRPV1 activation via either capsaicin (20 µmol/L), hyperosmolarity (≈450 mosmol/L) or an increase in ambient bath temperature to 43 °C induced intracellular calcium transients and augmented whole-cell currents, whereas hypertonicity induced cell volume shrinkage. In contrast, either capsazepine (10 µmol/L) or L-carnitine (1-3 mmol/L) reduced all these responses. Taken together, L-carnitine and capsazepine suppress hypertonicity-induced TRPV1 activation by blocking cell volume shrinkage.
Keywords: L-carnitine; cell volume; human corneal epithelium; hypertonic cell shrinkage; intracellular Ca2+ signaling; planar patch-clamp technique; transient receptor potential channel vanilloid 1.