Summary: The effects of histamine on coronary vasomotor tone and on myocardial blood flow distribution were studied in the anaesthetised rabbit in the absence of histamine H1-preceptor blockade and calcium channel blockade. Two groups of rabbits were used, those fed a normal diet and those fed a high cholesterol diet (2%) for 8 to 12 weeks. Continuous recordings of standard limb lead electrocardiograms (ECG) were obtained, and all animals were pretreated with cimetidine, an H2-receptor blocker, to minimise the intervening systemic effects of histamine. In the absence of H1-receptor blockade, histamine produced a marked (40 to 50%) reduction in coronary blood flow without significantly affecting other cardiovascular variables. This effect was seen uniformly across the free wall of the left ventricle, ie endo-epi flow ratios did not significantly change. Concomitant with the coronary vasoconstriction were significant depressions (≥0.1 mV) of the ECG ST-segment and elevation of cardiac tissue lactate and lactate:pyruvate ratio. These histamine-mediated responses were independently abolished by chlorpheniramine (1.5 mg·kg-1 iv) and verapamil (0.5 mg·kg-1 iv). Atherosclerosis reduced the average dose of histamine needed to induce these ischaemic changes from 55 ± 6 to 34 ± 6 μg·kg-1·min-1 (p<0.05). These findings suggest that in the H2-receptor blocked rabbit coronary vascular bed histamine causes tissue ischaemia by an H1-receptor mechanism. The decrement in myocardial blood flow appears to involve activation of plasma membrane calcium.
All Science Journal Classification (ASJC) codes
- Cardiology and Cardiovascular Medicine
- Physiology (medical)
- Coronary vasospasm
- Myocardial ischaemia