Effects of dipyridamole and adenosine infusions on ovine pulmonary and systemic circulations

This study was designed to test the hypothesis that A2 adenosine receptors mediate the hemodynamic responses to intravenous infusions of dipyridamole. We tested the hypothesis using theophylline, which has been reported to block A2 adenosine receptors and thereby attenuate the vasodilation caused by adenosine. Twenty-four anesthetized lambs that were between 7 and 17 days of age were used. Basal vascular tone of each animal was increased with the thromboxane mimetic U-46619. A theophylline dose commonly used in humans (5.0 mg/kg infused over 30 min followed by 1.0 mg x kg(-1) x h(-1)) resulted in negligible changes in the vasodilation caused by either dipyridamole or adenosine. However, a 10-fold greater theophylline dose significantly attenuated the vasodilation caused by adenosine, yet the attenuation in vasodilation caused by dipyridamole remained negligible. In addition, dipyridamole caused a weakly preferential pulmonary vasodilation, whereas adenosine caused a strongly preferential systemic vasodilation. These findings suggest that dipyridamole dilates effectively both the pulmonary vasculature and the systemic vasculature via predominantly adenosine-independent mechanisms.