Heat-induced release of CGRP from isolated rat skin and effects of bradykinin and the protein kinase C activator PMA

Abstract In the skin, noxious heating induces an axon reflex response which is commonly accepted to be due to the release of vasodilatory neuropeptides from polymodal nociceptors. In the present study, the quantitative assessment of calcitonin gene-related peptide (CGRP) release from rat skin serves as an integrative measure of primary afferent activation by noxious heat and the presumed sensitising action of bradykinin and an activator of protein kinase C (PKC). The isolated rat hairy skin of either hind paw was mounted on acrylic rods and exposed for 5 min periods to synthetic interstitial fluid of either 32°C for control or of higher temperatures up to 59°C during stimulation. In addition, experiments were performed in calcium free solution (containing 10 mM EGTA) or the skin was preloaded with the membrane permeant calcium chelator BAPTA-AM (1 mM). To look for modulatory effects on the heat responses, bradykinin or polymyristate-acetate (PMA) were added during heat stimulation in further experiments. Heating the skin induced a temperature-dependent release of CGRP from a threshold of 43°C which was absent in calcium free solution. Only at the highest temperatures (55 and 59°C) was a partially calcium-independent release observed. Inhibition of the release was also obtained with the intracellular calcium buffer BAPTA-AM. Bradykinin 10 but not 1 μM as well as PMA 1 and 10 μM significantly facilitated the heat-induced CGRP release at 47°C whereby BK caused a marginal and PMA a significant CGRP release by itself. Our results indicate that moderate noxious heat induces calcium-dependent CGRP release and this can be facilitated by bradykinin and by the activation of PKC. This suggests the same sensitising mechanism that affects nociceptor heat responses.