Diffusion insulin affecting

Hormones can also be classified by the way they get from the point of their release to their target tissue. Endocrine (from the Greek endon, within, and krinein, to release ) hormones are released into the blood and carried to target cells throughout the body (insulin is an example). Paracrine hormones are released into the extracellular space and diffuse to neighboring target cells (the eicosanoid hormones are of this type). Autocrine hormones are released by and affect the same cell, binding to receptors on the cell surface. 

Every hormone must have one or more receptors, most of which are proteins. These may be found embedded in the outer surface of the plasma membrane, in the cytoplasm, or in the cell nucleus. Binding of a hormone to its receptor often elicits both a rapid response and a slower one. For example, we have seen that glucagon, adrenaline, and vasopressin bind to cell surface receptors and promote the synthesis of cyclic AMP (Fig. 11-4). Tire cAMP induces rapid chemical modifications of many proteins. Some of these may diffuse into the nucleus and affect transcription of genes, a slower response. Insulin (Chapter 11, Section G) also exerts both rapid and slower responses. 

Disuccinimidyl suberate, a cross-linking agent, has been synthesized and used to probe the insulin-responsive D-glucose transport of the rat adipocyte. Incubation of cells with the reagent blocked methyl-a-D-glucopyranoside transport without affecting either basal hexose transport or simple diffusion as indicated by uptake in the presence of cytochalasin B. 

Transport to and from encapsulated cells of nutrients, respiratory gases, and similar small molecules is not affected by pore sizes commonly found in encapsulation motifs. However, transport of desired proteins, synthesized by the cells, out of the encapsulation matrix can be Hmited by the pore size of the material. Large secreted proteins (e.g.,MW 660 kDa) will be blocked by the MW cutoffs needed to shield the implanted cells from antibodies, whereas the diffusion of MW = 28 kDa and smaller proteins wiU not. Consequently, the molecular weight cutoff of the encapsulation material must be carefully chosen when transport of a secreted natural compound such as insulin is desired. 

What is the nature of the transfer mechanism, and how might it be affected by insulin At first glance, the problem would appear relatively straightforward. The cell membrane might be expected to be the cyto-structural barrier to glucose diffusion and the natural site of the transfer... 

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