Central control of posture: reciprocal discharge by two pontine neuronal groups leading to suppression of decerebrate rigidity

Spinal subarachnoid perfusions of rhesus monkeys were performed to study the transport of 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) by the spinal cord. The perfusate contained ³H]MHPG and a reference extracellular space marker (¹⁴C]sucrose or ¹⁴C]ethylenediaminetetraacetic acid-calcium complex). The mean (±S.D.) clearance of MHPG by the monkey spinal cord was 33 (± 4) μl/min. Analysis of serial sections of spinal tissue after each perfusion facilitated determination of several basic parameters of MHPG transport. MHPG has a spinal tissue distribution space of 70–80%. MHPG enters capillaries withing spinal tissus throughout the entire spinal cord. The mean capillary exchange half-time was 8.2 min ± 1.9 min. MHPG crosses cell membranes easily and diffuses through cells rapidly. The apparent tissue diffusion coefficient approximated the diffusion coefficient in water (8.5 × 10^?6 sq.cm/sec). The data suggests that CSF levels of MHPG are an uncertain indicator of norepinephrine metabolism and that lumbar CSF levels of MHPG reflect in large part spinal cord rather than brain metabolism.