Factors which affect cerebral uptake and retention of 13NH3.

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Abstract
The single pass extraction of NH3 (E) by cerebral capillaries was studied in vivo in rhesus monkeys with 13N. The value of E for 13N-NH3 was less than 100%, inversely related to cerebral blood flow and limited by the permeability of the blood brain barrier for NH3. A value of the permeability surface area product was 0.0040 cm3/s per g. The single pass extraction fraction, E, for 13N-NH3 was independent of arterial blood pH (in the range of 7.2-7.6) and of arterial blood NH3 concentration (in the range of 80-1400 .mu.g/100 ml). An insulin induced hypoglycemic reduction in the cerebral metabolic rate for glucose and O2 of 54% produced a reduction in E of about 24%. When a condition of elevated arterial blood NH3 was added to hypoglycemia, the value of E and cerebral metabolic rate for O2 remained low while the cerebral metabolic rate for glucose increased by a factor of 2.2 indicating the presence of a detoxifaction shunt for NH3. Positron tomographic images of the equilibrium cross section distribution of 13N-NH3 appeared to reflect regional differences in variations of capillary density in the cerebral tissue.