The effects of histamine on responses of the rabbit ear artery to electrical stimulation and to exogenous noradrenaline

1 The effects of a subconstrictor dose of histamine (9 × 10^-7 mol/l) on the responses of the isolated perfused ear artery of the rabbit to electrical stimulation (E.S.) and to exogenous noradrenaline (NA) were investigated.

2 Both intraluminal (I/L) and extraluminal (E/L) histamine potentiated responses to E.S. and to I/L NA to the same extent.

3 Mepyramine alone (2.5 × 10^-6 mol/l) had no effect on the response of the ear artery to either stimulus, but in the presence of this concentration of mepyramine, the potentiation by histamine of the response to I/L NA was significantly decreased and that to E.S. was replaced by inhibition.

4 The H₁-receptor agonist, 2(2-pyridyl) ethylamine, applied I/L potentiated responses to I/L NA at both concentrations used (5.1 and 51 × 10^-7 mol/l), but only potentiated the effects of E.S. at the higher concentration.

5 The H₂-receptor antagonist, metiamide (4 × 10^-6 mol/l), alone did not alter the extent of potentiation of responses to either E.S. or I/L NA by histamine. This suggests relatively weak H₂-receptor activity in the rabbit ear artery. In the presence, but not the absence of metiamide, the potentiation by histamine of the I/L NA response was reversible, an observation suggesting an interaction between metiamide and the non-reversible component of the potentiating effect of histamine.

6 These results are interpreted in terms of postsynaptic H₁-receptors which potentiate and presynaptic H₂-receptors which inhibit contractile responses in the ear artery.