Ultradian variations of plasma noradrenaline in humans

Abstract

Plasma noradrenaline (NA) levels were measured every 15 min for 4 h in 9 human volunteers kept at complete bed rest. Levels of plasma NA in each subject fluctuated with a median period of 107 min (range 75–188 min) for the group as a whole. Fluctuations with this periodicity were found whether subjects were tested in the morning or afternoon. The amplitude of these intrinsic fluctuations was at times greater than those brought about by postural change. NA levels showed a tonic downward trend from the beginning to the end of the survey, in both AM and PM groups, upon which the ultradian variations for NA were superimposed. Behavioral arousal tended to correlate negatively with plasma NA levels and pain had little effect on levels. These data suggest that plasma NA levels in humans fluctuate with both longer tonic and shorter phasic components which are subject to alteration by environmental factors. These fluctuations must be taken into account when using plasma levels of NA as indicators of sympathetic function.