Contribution of histamine to tyramine induced depressor response in the mean arterial blood pressure of tortoise

Purpose: Tyramine has a biphasic action consisting of depressor and presser components in the mean arterial blood pressure of the tortoise. The study was performed to investigate whether histaminergic mechanism contributes to the depressor component. Methods: Both sexes of tortoises (Testudo greaca) weighing 1200-1400 g (n = 77) were used. Each animal was anesthetized with subcutaneous injection of 5-7 ml urethan solution (%25). The mean arterial blood pressure was recorded on a smoked drum by using a Condon manometer connected to the left carotid artery. All drugs were given intravenously. Results: The depressor component of the response to tyramine was inhibited by metiamide and compound 48/80 but not by pyrilamine. The presser component was unaffected by histamine receptor antagonists and depletion of histamine stores. Conclusion: Findings suggest that endogenous histamine release may contribute to the depressor component of tyramine induced response in the tortoise