Analytical methods for evaluating episodic secretory activity within neuroendocrine axes

We review here our recent analytical methods designed to: (a) detect abrupt increases and decreases in serially measured concentrations of hormones, substrates, or metabolites in body fluids (discrete peak detection); (b) carry out combined calculations of in vivo hormone secretion and metabolic clearance without the infusion of labeled compounds (deconvolution analysis); and (c) assess nonequilibrium concentrations of total, bound, and free hormones or effectors following the injection or secretion of finite amounts of one or more ligands into a closed compartment containing high-affinity transport or binding proteins. First, in relation to discrete peak detection, we illustrate the application of Cluster analysis to immunoradiometrically assayed growth hormone (GH) profiles collected in blood sampled every 30 s over night in healthy young men. Secondly, as examples of deconvolution analysis, both a waveform-independent and a waveform-specific methodology are presented, and their applications discussed (e.g., for episodic GH secretion). Thirdly, to illustrate nonequilibrium interactions between a single blood transporter protein and a hormone/ligand, we highlight the impact of the high-affinity GH binding protein in plasma on the nonsteady-state time course of GH secretion and removal. Collectively, these novel neuroendocrine tools permit a more demanding and quantitative analysis of the temporal regulation of hormone, substrate, or metabolite secretion, distribution, interconversion, irreversible metabolic removal, and association with one or more high- or low-affinity plasma binding proteins. Based upon these dynamic principles, the complex operation of neuroendocrine feedback axes in vivo can be defined and assessed quantitatively and relevant hypotheses of regulation formulated and tested specifically.