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The single pass extraction of ammonia (E) by cerebral capillaries was studied in vivo in rhesus monkeys with 13N. The value of E for 13N-ammonia was found to be less than 100%, inversely related to cerebral blood flow and to be limited by the permeability of the blood brain barrier for ammonia. A value of the permeability surface area product was determined to be 0.0040 cm3/sec/gm. The single pass extraction fraction, E, for 13N-ammonia was found to be independent of arterial blood pH (in the range of 7.2 to 7.6) and of arterial blood ammonia concentration (in the range of 80-1400 μms/100 ml). An insulin induced hypoglycemic reduction in the cerebral metabolic rate for glucose and oxygen of 54% produced a reduction in E of about 24%. When a condition of elevated arterial blood ammonia was added to hypoglycemia, the value of E and cerebral metabolic rate for oxygen remained low while the cerebral metabolic rate for glucose increased by a factor of 2.2 indicating the presence of a detoxification shunt for ammonia. Positron tomographic images of the equilibrium cross section distribution of 13N-ammonia appeared to reflect regional differences in variations of capillary density that are found in the cerebral tissue.

Factors which affect cerebral uptake and retention of 13NH3.