Transport of 5-hydroxy-3-indoleacetic acid by spinal cord during subarachnoid perfusion

The transport of [14C]5-hydroxy-3-indoleacetic acid (5-HIAA) by the spinal cord was investigated with spinal subarachnoid perfusions of 20 rhesus monkeys. For spinal white matter, [14C]5-HIAA was found to distribute in a tissue space of 50-55% and to exchange readily between tissue and blood across parenchymal capillaries. The data were analyzed with a model of the spinal cord as a cylinder and a mean capillary transfer half-time of 20 min determined. The spinal cord cleared [14C]5-HIAA from 19 mul/min of CSF. Administration of probenecid yielded a somewhat lower mean tracer clearance rate (14 mul/min) but did not alter the capillary exchange half-time or the distribution volume of [14C]5-HIAA in the spinal cord. The data suggest that capillaries within spinal white matter transport 5-HIAA by a mechanism which is insensitive to inhibition by probenecid and that lumbar CSF concentrations reflect only a small portion of total 5-HIAA production by the central nervous system.