Autoregulation of superior mesenteric artery is blocked by adenosine antagonism

Pressure-flow autoregulation of the intact superior mesenteric artery (SMA) was demonstrated in the fasted, pentobarbital-anesthetized cat by use of a micrometer-controlled screw clamp to produce progressive decreases in vascular pressure. Administration (ia) of bolus doses of 8-phenyltheophylline (8-PT) were followed by infusion of adenosine to verify adenosine antagonism. 8-PT doses were progressively doubled until adenosine responses were blocked. If higher doses of 8-PT were used, SMA flow declined to very low levels and autoregulatory curves could not be obtained. Comparison of vasodilator responses to isoproterenol and adenosine before and after adenosine receptor blockade verified that, whereas adenosine responses were blocked, isoproterenol effects were not altered. The autoregulation was quantitated using three methods (the autoregulatory index, the percent decrease in vascular resistance, and the slope index) as blood pressure was reduced from a standardized control pressure of 110 mmHg (1 mmHg = 133.3 Pa). Maximal vasodilation appeared at a blood pressure of 56 +/- 5 mmHg (range 34-70). 8-PT resulted in dose-related antagonism of the dilator response to exogenous adenosine and autoregulation. All indices of autoregulation were significantly reduced by 8-PT. The data are compatible with the hypothesis that pressure-flow autoregulation in the SMA is not myogenic (responding to altered transmural pressure) but is dependent upon local concentrations of adenosine.