CGRP is one of the most potent vasodilators yet identified, with a particularly long duration, lasting several hours in human (Brain et al., 1985) and rodent (Brain and Williams, 1988) skin. However, the half-life of intravascular CGRP is only 7 min (Kraenzlin et al., 1985), suggesting that it is rapidly metabolised in the blood. It is unclear whether this difference is due to greater CGRP stability in the skin, or an alternative mechanism. In rat skin, CGRP is believed to be metabolised by mast cell tryptase (Brain and Williams, 1988). The ability of CGRP to potentiate plasma extravasation as a consequence of its vasodilator activity was studied.

Female CD1 mice (25g) were used. Anaesthesia was induced by urethane (2.5mg/g, i.v.). Oedema formation was measured by the accumulation of ¹²⁵I-BSA (45kBq i.v.). CGRP or vehicle were injected i.d. 30 min prior to substance P injection, or co-injected with histamine or compound 48/80. BIBN4096BS (a gift from Boehringer Ingelheim, Germany) or soybean trypsin inhibitor were co-injected with test agents as required. Plasma extravasation was allowed for 30 min after injection of oedematogenic agents, then blood samples and skin sites collected for measurement of radioactivity.

CGRP injected i.d. at 0 min (3pmol) significantly potentiated the plasma extravasation induced by substance P (300pmol) either co-injected with CGRP or administered 30 min afterwards (p<0.01), demonstrating that CGRP remains fully active for over 30 min in mouse skin. The potentiation was abolished by the CGRP antagonist BIBN4096BS (300pmol i.d) either co-injected with CGRP at 0 min or with substance P at 30 min (p<0.01), indicating the continued presence of active CGRP for 30 min, rather than prolonged activation of an intracellular signal.

CGRP (1 or 3pmol) also potentiated the plasma extravasation induced by co-injected histamine (3nmol; p<0.001). At the same doses CGRP was unable to potentiate the plasma extravasation induced by the mast cell activator compound 48/80 (500ng). These data suggest that the co-release of tryptase with the mast cell amines attenuates the potentiation by CGRP. This was confirmed by the finding that co-injection with the mast cell tryptase inhibitor soybean trypsin inhibitor (3µg) allowed potentiation of the compound 48/80-induced plasma extravasation by CGRP (3pmol; p<0.05).

In conclusion, these data suggest that CGRP can remain active in the skin for at least 30 min. The vasodilatation can be blocked either by a CGRP antagonist or by mast cell tryptase, supporting the concept that CGRP remains in an active form at extravascular sites This result may be related to the observation of prolonged neurogenic vasodilatation in the mouse ear (Grant et al., 2002).

Brain et al. (1985) Nature, 313, 54-56.
Brain and Williams (1988) Nature, 335, 73-75.
Grant et al. (2002) Br. J. Pharmacol., 135, 356-362.
Kraenzlin et al. (1985) Regul. Pept., 10, 189-197.

A.D. Grant is supported by the BBSRC.