Differential H₁- and H₂-receptor-mediated histamine responses of canine epicardial conductance and distal resistance coronary vessels

The contributions of histamine (H₁ or H₂) receptor-mediated responses and, therefore, the effects of histamine blocking agents are unclear with regard to regulation of proximal epicardial and distal resistance coronary arteries. This study was designed to evaluate the effects of selective H₁- and H₂-receptor antagonists on epicardial and resistance vessels in the closed chest dog model. Histamine, diphenhydramine (H₁ blocker), and cimetidine (H₂ blocker) were infused into the left anterior descending coronary artery (LAD), and responses were studied by quantitative coronary angiography and flow measurements (¹³³Xe washout). Histamine infusion alone produced a significant dilation of the proximal LAD (13% above control) only at the highest dose (45 μg/min), while LAD flow was increased by 128%. In the presence of H₁ blocker, histamine produced significantly greater epicardial dilation (55% above control). The flow response curve was shifted to the right in the presence of H₁ blocker, but the flow attenuation was overcome by the highest histamine dose. In contrast, the H₂ blocker attenuated both epicardial dilation (6% below control) and flow response (31% above control) to the highest histamine dose. Results support a differential regulation of proximal epicardial and distal resistance vessels to histamine with epicardial arteries demonstrating H₁-mediated vasoconstriction and H₂-mediated vasodilation and distal resistance vessels showing H₁- and H₂-mediated vasodilation. In addition, these findings suggest that H₁ blockade may antagonize histamine-related vasoconstriction and vasospasm, while H₂ blockers may permit unopposed H₁-mediated vasoconstriction of epicardial arteries and also limit resistance vessel vasodilatory responsiveness in the presence of elevated tissue histamine, as may occur in atherosclerotic coronary artery disease.