No abstract

Plasma cortisol is largely bound to corticosteroid-binding globulin (CBG), which regulates its bioavailability by restricting exit from capillaries. Levels of CBG may be altered by several factors including stress and this can influence the amount of cortisol reaching cells. This study investigated the effect of social instability on plasma concentrations of CBG, total and free (not protein bound) cortisol in horses. Horses new to our research herd ('newcomers') were confined in a small yard with four dominant resident horses for 3-4 h daily for 3-4 (n=5) or 9-14 (n=3) days. Jugular blood was collected in the mornings from newcomers before the period of stress began ('pre-stress'), and then before each day's stress. Residents were bled before stress on the first and thirteenth day.Residents always behaved aggressively towards newcomers. By the end of the stress period, all newcomers were subordinate to residents. In newcomers (n=8) after 3-4 days of social stress, CBG binding capacity had fallen (P=0·0025), while free cortisol concentrations had risen (P=0·0016) from pre-stress values. In contrast, total cortisol did not change. In residents, CBG had decreased slightly but significantly (P=0·0162) after 12 days of stress. Residents and newcomers did not differ in pre-stress CBG binding capacity, total or free cortisol concentrations. However, by the second week of stress, CBG binding capacity was lower (P=0·015) and free cortisol higher (P=0·030) in newcomers (n=3) than in residents. Total cortisol did not differ between the groups.In conclusion social stress clearly affected the adrenal axis of subordinate newcomer horses, lowering the binding capacity of CBG and raising free cortisol concentrations. However, no effect of stress could be detected when only total cortisol was measured. Therefore, to assess adrenal axis status accurately in horses, it is essential to monitor the binding capacity of CBG and free cortisol concentrations in addition to total cortisol levels.

Homeostasis in the intact organism is achieved implicitly by repeated incremental feedback (inhibitory) and feedforward (stimulatory) adjustments enforced via intermittent signal exchange. In separated systems, neurohormone signals act deterministically on target cells via quantifiable effector-response functions. On the other hand, in vivo interglandular signaling dynamics have not been estimable to date. Indeed, experimentally isolating components of an interactive network definitionally disrupts time-sensitive linkages. We implement and validate analytical reconstruction of endogenous effectorresponse properties via a composite model comprising (i) a deterministic basic feedback and feedforward ensemble structure; (ii) judicious statistical allowance for possible stochastic variability in individual biologically interpretable dose-response properties; and (iii) the sole data requirement of serially observed concentrations of a paired signal (input) and response (output). Application of this analytical strategy to a prototypical neuroendocrine axis in the conscious uninjected horse, sheep, and human (i) illustrates probabilistic estimation of endogenous effector dose-response properties; and (ii) unmasks statistically vivid (2-to 5-fold) random fluctuations in inferred target-gland responsivity within any given pulse train. In conclusion, balanced mathematical formalism allows one to (i) reconstruct deterministic properties of interglandular signaling in the intact mammal and (ii) quantify apparent signal-response variability over short time scales in vivo. The present proof-of-principle experiments introduce a previously undescribed means to estimate time-evolving signal-response relationships without isotope infusion or pathway disruption.I n contradistinction to the remarkable insights gained recently about signaling behavior in isolated systems, virtually nothing is known about quantitative properties of unperturbed interglandular control in vivo. This knowledge deficit is significant, because homeostasis in the whole organism implicitly proceeds via repeated incremental dose-responsive adjustments transduced by the exchange of inhibitory and facilitative signals (1-8). Thematic examples include reciprocal coupling between anorexigenic and satiety factors that govern body weight, sympathetic neuronal and adrenalglandular linkages that parse adaptations to stress, and glucose and insulin interactions that ration the distribution of metabolic fuels (9-11). The burgeoning repertoire of novel molecular signals establishes a need for integrative formalism to estimate such in vivo effector-response dynamics (12). The present analytical platform offers a first step toward this end. MethodsOverview. Analysis of isolated components of an interlinked system has provided important insights. However, this approach disrupts intrinsic control of spontaneously unfolding adaptive signal control. The current studies illustrate an analytical strategy to reconstruct unmanipulated in vivo dose-response attributes.Stochastic E...