Factors which affect cerebral uptake and retention of 13NH3.

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.