Hormone Transport Experiments

The hypothalamic releasing factors regulate release of the anterior pituitary trophic hormones. As summarized in Figure 52-1, the releasing factors are produced in various neuronal groups within the hypothalamus and are transported to the median eminence for release into the portal circulation to the anterior pituitary. Neurons in the hypothalamus also produce the hormones oxytocin and vasopressin, which are released by the posterior pituitary into the blood. Therefore, it is not surprising that behavior and experience, which influence the hypothalamus, sometimes alter the secretion of these hypothalamic releasing factors and hormones. 

For the last forty years, many reports have emerged on the hormone-like effects of chemical compounds such as pesticides and industrial chemicals upon wildlife and humans. The effects of these materials are believed to be either direct or indirect. Direct effects involve positive or negative interactions with the hormone receptors. Indirect effects may result when the synthesis of hormones or their receptors is altered, or the transport, metabolism, or elimination of hormones is modified in some way. The discovery of hormone-like properties of some compounds was made long after their release into the environment. It was shown soon after their introduction that aviation crop dusters handling DDT had low sperm counts and workers at a plant producing the insecticide kepone were reported to have low libido, sperm counts and to be impotent. Subsequently, experiments conducted in laboratory animals demonstrated unambiguously the oestrogenic activity of these pesticides. 9 refs. 

From preliminary in vitro experiments using rat renal slices, it appears that the hormone directly affects renal calcium transport. If insulin is the mediator of the hypercalciuria, it might be possible to reduce urinary calcium loss by lowering the intake of in-sulinogenic foods. This would be especially important in those individuals with a marked calciuric response to such foods. 

Polar Cell Systems for Membrane Transport Studies Direct current electrical measurement in epithelia steady-state and transient analysis, 171, 607 impedance analysis in tight epithelia, 171, 628 electrical impedance analysis of leaky epithelia theory, techniques, and leak artifact problems, 171, 642 patch-clamp experiments in epithelia activation by hormones or neurotransmitters, 171, 663 ionic permeation mechanisms in epithelia biionic potentials, dilution potentials, conductances, and streaming potentials, 171, 678 use of ionophores in epithelia characterizing membrane properties, 171, 715 cultures as epithelial models porous-bottom culture dishes for studying transport and differentiation, 171, 736 volume regulation in epithelia experimental approaches, 171, 744 scanning electrode localization of transport pathways in epithelial tissues, 171, 792. 

The third mechanism is peritubular extraction of peptides and proteins from postglomerular capillaries and intracellular metabolism. Experiments using iodi-nated growth hormone (125I-rGH) have demonstrated that, while reabsorption into endocytic vesicles at the proximal tubule is still the dominant route of disposition, a small percentage of the hormone may be extracted from the peritubular capillaries [79, 86]. Peritubular transport of proteins and peptides from the baso-lateral membrane has also been shown for insulin [87] and the mycotoxin ochra-toxin A [88]. 

Data presented in previous sections revealed that the concentration of FFAs in plasma may reach 2.0 mM during exercise. How is this possible when the highest attainable concentration in water is only about 0,1 mM This problem was resolved by nature by use of albumin as a vehicle for the transport of FFAs within the circulation. Albumin constitutes about 60% of the protein of blood plasma. It is a major carrier of FFAs, other metabolites, hormones, and drugs- Serum albumin has the capadty to carry several fatty adds. Figure 4.45 shows results from an experiment usingpurificdalbumin.Thenumberoffattyacid molecules bound per protein molecule is plotted versus the concentration of unbound fatty acids in solution. The study, conducted with lauric acid (12 carbons) and myristic add (14 carbons), demonstrates that one protein molecule is able to bind at least 8 or 9 molecules of fatty acid. Albumin has a molecular weight of 69 kDa and occurs in human plasma at a concentration of about 0.6 mM (40 mg/ml) (Halliwell, 1988). 

These experiments with enriched ecdysteroid receptor preparations demonstrate the potential of some brassinosteroids to interact with ecdysteroids at the target site of the hormone. However, this does not exclude other sites of interference with ecdysteroids. Theoretically, brassinosteroids may also affect the hormone system at the site of transportation in the haemolymph, at the sites of enzymatic conversion of ecdysteroids or at the excretory system. 

A possible consideration in the in vivo experiments where T3 and tryptophan were administered to rats was that T3 may inhibit tryptophan transport to the liver. A mutual competitive inhibition between the transport of tryptophan (mediated by the aromatic amino acid transport system T) and T3 has been reported.149150 Though system T transport activity has been studied mainly in erythrocytes, it is also expressed in hepatocytes.151 Interactions between thyroid hormone and tryptophan transport in rat liver have been reported to be modulated by thyroid status.152 For this reason, whether the administration of T3 and tryptophan under selected experimental conditions would affect free tryptophan levels in liver was investigated. The results revealed that rats tube-fed tryptophan and given T3 intraperitoneally at 0 time and killed after 1 h148 had the same increase (10%) in free tryptophan levels in liver as that of rats tube-fed tryptophan alone. 

Experiments with everted sacs of rat intestine have shown a fall in the transport of calcium-45 across the intestinal wall after thyroparathyroid-ectomy (D9) and after parathyroidectomy (R2), and it has been suggested that the hormone may promote calcium absorption. It is said that calcium is particularly well absorbed in hyperparathyroidism, but this may be a nonspecific response of the absorptive mechanism to hyper-calcuria (Jl). 

The use of tissue slices for experiments on histidine decarboxylation introduces the additional problem of the access of substrate, co-enzyme and inhibitors into the cells. In this connection, it should be noted that in practice the specificity of an enzyme within a cell may be increased by the specificity of the substrate-transporting system. Similar considerations apply to the in vivo inhibition of histidine decarboxylases there is, however, the additional possibility of modifying production of the apo-enzyme either by restricting the supply of amino acids or by altering the hormonal state of the animal. 

The available data from in vivo, as well as from in vitro, experiments clearly demonstrate that soy isoflavones act as inhibitors of TPO and thus may influence iodine incorporation in thyroid hormone biosynthesis. On the other hand, most of the cfinical trials, including our own, have shown only modest and transitory effects of soy isoflavones on circulating thyroid hormone levels. However, they may be important in the case of insufficient iodine supply. Besides thyroid hormone biosynthesis, soy isoflavones may influence other events involved in thyroid function, such as iodine transport, thyroid hormone deiodination, and signaling pathways. So far, the data available on these topics is scarce and more experiments addressing these issues are needed. 

Vitamin D functions in the process of calcium mobilization from previously formed bone making it available to the extracellular fluid upon demand by the calcium homeostatic system as described under the regulation of vitamin D metabolism. From the experiments described in the metabolism section it is clear that 1, 25-(0H)2D3, rather than 25-OH-D3 or vitamin D3 functions in the mobilization of calcium from bone under physiologic conditions. The mechanism whereby 1,25-(0H)2D3 initiates mobilization of calcium from bone is not at all understood. In contrast to the response of intestinal calcium transport to 1, 25-(0H)2D3 this process is blocked by the prior administration of actinomycin D suggesting that in fact a transcriptive event is involved in this activation As previously pointed out, bone possesses a specific receptor for 1,25bone cells has not yet been determined however. In vivo the 1,25-(OH)2D3 activation of bone calcium mobilization requires the presence of parathyroid hormone but the nature of... 

Chapter 9 shows how compartmental models may be used to describe physiological systems, for example, pharmacokinetics. The production, distribution, transport, and interaction of exogenous materials, such as drugs or tracers, and endogenous materials, such as hormones, are described. Examples of both linear and nonlinear compartmental models are presented, as well as parameter estimation, optimal experiment design, and model validation. 

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