Cerebral Extraction of N-13 Ammonia: Its Dependence on Cerebral Blood Flow and Capillary Permeability, Surface Area Product

摘要

exp 13 N-labeled ammonia was used to investigate: (1) the cerebral extraction and clearance of ammonia; (2) the mechanicsm by which capillaries accommodate changes in cerebral blood flow (CBF); and (3) its use for the measure of CBF. This was investigated by measuring the single pass extraction of exp 13 NH sub 3 in rhesus monkeys during P/sub a/CO sub 2 induced changes in CBF, and with dog studies using in vitro tissue counting techniques to examine exp 13 NH sub 3 extraction in gray and white matter, mixed tissue, and cerebellum during variations in CBF produced by combinations of embolization, local brain compression, and changes in P/sub a/CO sub 2 . The single pass extraction fraction of exp 13 NH sub 3 varied from about 70 to 20% over a CBF range of 12 to 140cc/min/100gms. Capillary permeability-surface area product (PS) estimates from this data and the dog experiments show PS increasing with CBF. The magnitude and rate of increase in PS with CBF was highest in gray matter > mixed tissue > white matter. Tissue extraction of exp 13 NH sub 3 vs CBF relationship was best described by a unidirectional transport model in which CBF increases by both recruitment of capillaries and by increases of blood velocity in open capillaries. Glutamine synthetase, which incorporates exp 13 NH sub 3 into glutamine, appears to be anatomically located in astrocytes in general and specifically in the astrocytic pericapillary end-feet that are in direct contact with gray and white matter capillaries. The net exp 13 NH sub 3 extraction subsequent to an i.v. injection increases nonlinearly with CBF. Doubling or halving basal CBF produced from 40 to 50% changes in the exp 13 N tissue concentrations with further increases in CBF associated with progressively smaller changes in exp 13 N concentrations. exp 13 NH sub 3 appears to be a good tracer for the detection of cerebral ischemia with positron tomography but exhibits a poor response at high values of CBF. (ERA citation 05:007695)