The vasoactive properties of pentobarbital (PB) were studied in intracerebral arterioles and venules (diameter, 30-90 microns). These vessels mediate changes in cerebrovascular resistance and capacitance, respectively. Mean control vessel diameters of arterioles and venules at pH 7.3 were 53.9 +/- 2.8 microns and 78.4 +/- 4.3 microns, respectively. Both arterioles and venules dilated when the pH of the extraluminal solution was lowered to 6.8 and constricted when the pH was raised to 7.6. PB, 10(-6) to 10(-2) mol/L, dilated intracerebral arterioles in a dose-dependent manner at pH 7.3, reaching a maximal dilation of 129.7 +/- 3.1% of control diameter at a dose of 10(-3) mol/L. In contrast, PB at 10(-6) to 10(-2) mol/L failed to produce significant changes in the diameter of intracerebral venules. In addition, PB at 10(-3) mol/L significantly inhibited arteriolar constriction induced by KCl (120 mmol/L), but not venular constriction. The present study suggests that intracerebral venules are relatively less responsive to PB than cerebral arterioles and peripheral veins. In addition to its effect on cerebral metabolism. PB may act to redistribute venous blood volume from cerebral veins to more responsive peripheral veins, thereby decreasing intracranial blood volume and intracranial pressure.
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