Gastrointestinal tract membrane physiology

DC Dawson. Principles of membrane transport. In SG Schultz, M Field, RA Frizzell, eds. Handbook of Physiology—The Gastrointestinal Tract. American Physiological Society, 1991, vol. IV, pp 1—44. 

Hartshome, D.J. (1987). Biochemistry of the contractile process in smooth muscle. In Physiology of the Gastrointestinal Tract. 2nd Edition (Johnson. L.R. ed.), pp. 423-482, Raven Press, New York. Hille, B. (1992). Ionic Channels of Excitable Membranes. 2nd edn., Sinauer Associates, Sunderland, MA. 

Steroid hormones are produced by the adrenal cortex, testes, ovaries, and placenta. Synthesized from cholesterol, these hormones are lipid soluble therefore, they cross cell membranes readily and bind to receptors found intracellularly. However, because their lipid solubility renders them insoluble in blood, these hormones are transported in the blood bound to proteins. Furthermore, steroid hormones are not typically preformed and stored for future use within the endocrine gland. Because they are lipid soluble, they could diffuse out of the cells and physiological regulation of their release would not be possible. Finally, steroid hormones are absorbed easily by the gastrointestinal tract and therefore may be administered orally. 

A systemic effect is an effect that is normally observed distantly from the site of first contact, i.e., after the substance has passed through a physiological barrier (mucous membrane of the gastrointestinal tract or of the respiratory tract, or the skin) and becomes systemically available. It should be noted, however, that toxic effects on surface epithelia may reflect indirect effects as a consequence of systemic toxicity or secondary to systemic distribution of the substance or its active metabolite(s). 

Most substances that can be absorbed enter cells by passive diffusion, but the extent to which a substance is or can be absorbed from the skin, gastrointestinal tract, or lung is not necessarily uniform among these sites and, in fact, may vary substantially. Typically for a substance well absorbed from one site, less is absorbed (if absorbed at all) from one or both of the other possible sites of exposure. For example, a substance may be completely absorbed from the gastrointestinal tract, moderately absorbed from the lung, and poorly absorbed from the skin. On the other hand, there are some substances are well absorbed from each of the sites of exposure, and others are poorly absorbed from each site. Whether a given substance can be absorbed depends on its physicochemical properties, their effect on the substance s ability to cross cellular membranes, and the site s anatomical and physiological characteristics. 

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