Effects of histamine bolus injections and continuous infusions on the H1- and H2-receptors in the hindlimb vessels of the rabbit.

1. Hindlimb vascular resistance (HVR) was continuously measured after pharmacological block of the autonomic effectors in unanesthetized rabbits with previously implanted Doppler ultrasonic flowmeters. 2. Histamine bolus injections caused a dose-related short lived fall in HVR followed by a more sustained rise. The fall was due to H2-receptor stimulation (blocked by burimamide or metiamide) and the rise to H1-receptor stimulation (blocked by mepyramine). At the doses of histamine tested the magnitude of the H1-mediated vasoconstriction had a larger peak effect than the H2-mediated vasodilatation. 3. Histamine infusions up to 200 microgram kg-1 min-1 did not alter HVR significantly but both increases and decreases in HVR were observed after giving H2- or H1-antagonists, respectively. 4. From the double reciprocal plots of 1/peak HVR change and 1/dose of histamine the magnitude of the predicted H1- and H2-mediated peak HVR effects at large doses were the same. This suggested that the number of H1- and H2-receptors were similar in the hindlimb vascular bed, in agreement with the infusion data.