Heat Sensing by the Retinal Pigment Epithelium in the Eye Through Activation of Trpv Channels

Abstract

Purpose: : The retinal pigment epithelium (RPE) fulfils a variety of tasks which are essential for retinal function. This requires a multitude of different regulatory pathways and the ability to react to a large variety of stimuli. One of these stimuli is heat. Puposer of this study was to identify the heat-sensing pathway in the RPE.

Methods: : The expression profile of probable heat-sensing ion channels was investigated by RT PCR. Responses from RPE cells to heat-stimuli were investigated by measurements of intracellular free Ca²⁺ with fura-2 and by the patch-clamp technique.

Results: : RT-PCR analysis of cDNA from freshly isolated human RPE cells revealed the expression of TRPV1,2, 3 and 4 channels. Increasing the temperature from room temperature to either 45°C or 56°C led to an increase in [Ca²⁺]_i in human RPE cells in primary culture. These temperature-induced rises in intracellular Ca²⁺ were blocked by 100 µM La³⁺ or 1 µM ruthenium-red. Application of the TRPV1 opener capsaicin (1 µM) did not increase [Ca²⁺]_i whereas application of the TRPV opener 2-ABP [Ca²⁺]_i. Whole-cell currents in ARPE-19 cells showed at a holding potential -40 mV, the transient activation of an inward-current by rising the temperature to 45°C. In the presence of 20 µM ruthenium-red, rising the temperature to 45°C failed to activate this inward current.

Conclusions: : In summary, we conclude that increase in the temperature leads to rises in [Ca²⁺]_i by activation of TRPV2 or TRPV3 channels. Thus, the RPE is able to sense heat. Since the RPE absorbs the major of the light energy which is focused by a lens onto the retina heat sensing by the RPE is a novel important mechanism to adopt its function to changes in the light intensity.

Keywords: calcium • ion channels • retinal pigment epithelium