EFFECTS OF HISTAMINE ON RESPONSES OF THE RABBIT EAR ARTERY TO ELECTRICAL-STIMULATION AND TO EXOGENOUS NORADRENALINE

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. Both intraluminal (I/L) and extraluminal (E/L) histamine potentiated responses to E.S. and to I/L NA to the same extent. 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. 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. The H2‐receptor antagonist, metiamide (4 × 10−6mol/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. These results are interpreted in terms of postsynaptic H1‐receptors which potentiate and presynaptic H2‐receptors which inhibit contractile responses in the ear artery. 1979 British Pharmacological Society