Changes in dermal interstitial ATP levels during local heating of human skin.

Key points • Heating human skin results in a localized increase in blood flow (hyperaemia) to the skin. • Experiments in mouse skin and cultured human cells suggest that skin cells known as keratinocytes release the chemical ATP, a known cutaneous vasodilator, and that the release of this ATP increases when temperature-sensing vanilloid type III and IV transient receptor potential ion channels (TRPV3, TRPV4 respectively) are activated by heating. Studies also suggest that this ATP release is necessary for temperature sensation. • We hypothesized that, like mouse skin locally heating human skin to temperatures that activate heat-sensing TRPV3 and TRPV4 channels would be associated with an accumulation of ATP in the interstitial space that would be related to temperature sensation. • We also hypothesized that the accumulation of ATP would be associated with the magnitude of heat-induced hyperaemia to the area. • We report that, unlike mouse skin, such local heating does not result in an accumulation of ATP in the interstitial space of human skin, and therefore such an accumulation is not necessary for cutaneous temperature sensation and local reactive hyperaemia. • We also report that warming skin from 31°C results in dilatation at temperatures as low as 35°C, which is several degrees lower than previously reported. This suggests that TRPV3 and/or TRPV4 channels have a role in heat-induced hyperaemia or that such heating sensitizes TRPV1 channels to respond to temperatures below their typical threshold. Abstract Heating skin is believed to activate vanilloid type III and IV transient receptor potential ion channels (TRPV3, TRPV4, respectively), resulting in the release of ATP into the interstitial fluid. We examined the hypothesis that local skin heating would result in an accumulation of ATP in the interstitial fluid that would be related with a rise in skin blood flow (SkBF) and temperature sensation. Two microdialysis probes were inserted into the dermis on the dorsal aspect of the forearm in 15 young, healthy subjects. The probed skin was maintained at 31°C, 35°C, 39°C and 43°C for 8 min periods, during which SkBF was monitored as cutaneous vascular conductance (CVC). Dialysate was collected and analysed for ATP ([ATP]d) using a luciferase-based assay, and ratings of perceived warmth were taken at each temperature. At a skin temperature of 31°C, [ATP]d averaged 18.93 ± 4.06 nm and CVC averaged 12.57 ± 1.59% peak. Heating skin to 35°C resulted in an increase in CVC (17.63 ± 1.27% peak; P < 0.05), but no change in [ATP]d. Heating skin to 39°C and 43°C resulted in a decreased [ATP]d (5.88 ± 1.68 nm and 8.75 ± 3.44 nm, respectively; P < 0.05), which was accompanied by significant elevations in CVC (38.90 ± 1.37% peak and 60.32 ± 1.95% peak, respectively; P < 0.05). Ratings of perceived warmth increased in proportion to the increase in skin temperature (r2 = 0.75, P < 0.05). In conclusion, our data indicate that an accumulation of interstitial ATP does not occur during local heating, and therefore does not have a role in temperature sensation or the dilator response in human skin. Nevertheless, the low threshold of dilatation (35°C) indicates a possible role for the TRPV3, TRPV4 channels or the sensitization of other ion channels in mediating the dilator response.