Role of blood flow in carbon monoxide- and hypoxic hypoxia-induced alterations in hexobarbital metabolism in rats.

The flow dependency of hepatic hexobarbital metabolism was examined in the isolated perfused rat liver. At low flow rates (0.5-1.0 ml/min/g of liver) hexobarbital clearance was found to depend on perfusion fluid flow, whereas at higher flow rates drug clearnace approached flow independence. Calculation of the in vivo hepatic blood flow rate suggested that hexobarbital metabolism in vivo should be highly dependent upon flow. Blood flow in the conscious rat was measured by use of radiolabeled microspheres during acute exposure to levels of hypoxic hypoxia (lowered pO2) or carbon monoxide which resulted in equal alterations in arterial oxyhemoglobin content (approximately 65% oxyhemoglobin). Hypoxic hypoxia (8% O2) caused a massive redistribution of flow away from the splanchnic area, resulting in a 45% decrease in hepatic blood flow. Carbon monoxide (500 ppm) was without significant effect on hepatic blood flow. These data would appear to explain the relatively greater inhibitory potency of hypoxic hypoxia on drug metabolism in vivo, since drug delivery to the liver is depressed by hypoxic hypoxia but unaffected by carbon monoxide exposure.