Portal blood system, insulin

The fact that rectal administration in humans only partially avoids delivery of the drug to the portal system has been suggested as an advantage of rectal administration of insulin over parenteral administration.Delivery of insulin to the portal blood system is suggested to be more physiological than delivery to the peripheral cells from subcutaneous injections. 

Fig. 15. Hypothetical model of how initiators and modulators that affect insulin release may reach A-, B- and D-cells. The first target of arterial blood containing nutrients, hormones, peptides and drugs is the B-cell. From there, via an intraislet portal vein system, blood which now also contains released insulin flows to the mantle where A- and D-cells are localized and from there enters the circulation. Nerves derived from the autonomous nervous system which contain neurotransmitters (acetylcholine, noradrenaline) and neuropeptides (including vasoactive intestinal peptide (VIP), gastrin-releasing peptide (GRP), galanin) are connected to islet cells. Glucagon (A-cells) and somatostatin (D-cells) reach other endocrine cells in the islet in a paracrine manner. The B-cell may also be the target of previously released insulin via a short loop.

Portal delivery. In order to supply the Uver with all the insulin it needs, the pancreas delivers the insulin into the portal blood supply. Ideally, a replacement system would act similarly. 

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