Absorption of insulin from perfused rabbit small intestine in vitro

Regulation of Drug Absorption from Small Intestine by Enteric Nervous System I: a Poorly Absorbable Drug Via Passive Diffusion

To investigate the regulation of drug absorption from the small intestine by the enteric nervous system (ENS), the vascular-luminal perfusion study and the in-vitro transport study were performed by employing phenol red as a poorly absorbable model compound. The effect of ENS on the intestinal absorption of phenol red was examined by adding epinephrine, an adrenergic agonist, or bethanechol, a cholinergic agonist into the vascular perfusate in the vascular-luminal perfused rat small-intestine preparation. The viability of the perfused intestine was checked by the recovery of the vascular perfusate, net water flux and absorbability of antipyrine, a well absorbable drug, and it was confirmed that the function of the perfused small-intestine preparation was maintained for at least 1 hr. The effect of epinephrine or bethanechol on the function of the small intestine was recognized as the increase in net water absorption, or the promotion of the water secretion, respectively. These phenomena are ones that are typically observed when adrenergic or cholinergic neuron is stimulated. Then, we investigated the small-intestinal absorption of phenol red in the vascular-luminal perfused preparation. Absorption clearance (CL(abs)) of phenol red was gradually increasing during the perfusion for 1 hr, but the 20-min vascular perfusion with the perfusate containing epinephrine made CL(abs) of phenol red constant and significantly lower than those for control study. Furthermore, after the perfusate was changed with the one without any agonist, again, CL(abs) of phenol red started to increase. These results clearly indicate that the stimulation of adrenergic neuron by epinephrine leads to the decrease in the small-intestinal absorption of phenol red. On the other hand, the vascular perfusion of bethanechol resulted in the increase in CL(abs) of phenol red comparing to the control study. Removing bethanechol from the vascular perfusate decreased CL(abs) of phenol red, again. The in-vitro transport study using the isolated jejunum sheet also showed that epinephrine in the serosal solution significantly decreased the transport of phenol red, which can be ascribed to the paracellular pathway tightened by the action of epinephrine because of the increase in transmucosal electrical resistance (TER). On the other hand, although the effect of bethanechol on both the transport of phenol red and TER was not statistically significant, the transport of phenol red tended to increase and the values of TER are smaller than those of control study.

New developments in insulin delivery

A vigorous research effort has been undertaken worldwide to replace injectable insulin by a more comfortable and painless delivery method. Several routes have been explored for their suitability with respect to insulin degradation in the human body. Considerable progress has been made in achieving the common goal for a convenient and equally effective insulin delivery. This article reviews the different routes available for insulin administration and the many successful developments that have been made in recent years for improving that particular route for a much better insulin delivery.

Carrier-enhanced human growth hormone absorption across isolated rabbit intestinal tissue

Small molecular weight alpha acid derivatives are able to enhance the intestinal absorption of human growth hormone through isolated rabbit intestinal tissue. The enhancement is not through the usual tissue modification associated with traditional penetration enhancers nor is it through an active transport process. Rather these small molecules associate with human growth hormone in solution to make it more transportable through intestinal tissue. It is shown that the enhancer has specificity for a particular protein and the enhancer and human growth hormone must be in solution together to be effective, i.e. pretreating the tissue with enhancer and then adding the protein does not increase tissue permeability. Moreover, the enhancer does not increase the permeability of mannitol or progesterone, thus providing additional evidence of specificity and establishing that these agents are not classical penetration enhancers.

Pharmacodynamics of insulin following intravenous and enteral administrations of porcine-zinc insulin to rats

Previous work from this laboratory showed site-dependent variations in the apparent permeability of insulin as measured using the everted rat gut sac technique, with the greatest permeability in the distal jejunum and the lowest in the duodenum (5). To quantify better the rate and extent of insulin absorption from the small intestine, closed-loop in situ experiments were performed in nondiabetic rats. Results correlated with the everted gut sac technique in that the absolute bioavailability determined in situ was higher for insulin solution administered to the more distal region of the intestine (0.133%) than that absorbed from an earlier portion of the intestine (0.059%). While the difference in regional bioavailabilities was not significant (P = 0.08), the blood glucose response showed highly significant differences (P = 0.0015), with severe and prolonged hypoglycemia resulting from insulin delivered to the distal jejunum/proximal ileum. Insulin administered iv followed a two-compartment pharmacokinetic model. Whole-body elimination rate constants were similar for both iv and enteral insulin. Although therapeutic quantities of insulin were absorbed from the distal small intestine, absorption enhancers would be necessary to decrease the dose of insulin required.

Intracellular digestion of saturated and unsaturated phospholipid liposomes by mucosal cells. Possible mechanism of transport of liposomally entrapped macromolecules across the isolated vascularly perfused rabbit ileum

The mechanism of intestinal absorption of liposomally entrapped [14C]inulin and 125I-labelled poly(vinylpyrrolidone) was studied using the isolated rabbit intestinal loop with intact perfused vasculature, a system more closely resembling an in vivo system than the everted sac technique. [14C]Inulin or 125I-poly(vinylpyrrolidone) was entrapped in liposomes prepared from unsaturated egg phosphatidylcholine and soya phosphatidylcholine, and saturated distearoylphosphatidylcholine (18:0), dipalmitoylphosphatidylcholine (16:0) and dimyrostoylphosphatidylcholine (14:0). Free and liposomally entrapped macromolecules were introduced in the ileum and the transport of liposomes and entrapped macromolecules into the venous effluent was monitored by measuring the presence of the aqueous marker 125I-poly(vinylpyrrolidone) or [14C]inulin, and lipid marker [3H]cholesterol. The results show that intact liposomes are not transported across intestine into the venous effluent, but they are taken up by mucosal cells and digested intracellularly, releasing the entrapped markers 125I-poly(vinylpyrrolidone) and [14C]inulin. These markers are then transported into the venous effluent as free molecules. The absorption of liposomally entrapped [14C]inulin into the venous effluent is biphasic, first slow for 30 min (i.e., a lag period of 30 min), followed by a rapid linear increase. The duration of the lag period and the rate of absorption of the entrapped [14C]inulin are dependent on the degree of saturation and the transition temperature of the phospholipids used to prepare liposomes. The possible explanation of the lag period based on the evidence presented here is that it is the time required for the liposomes to be taken up by mucosal cells and digested intracellularly. Intracellular digestion of liposomes prepared from saturated phospholipids is more rapid than from those prepared from unsaturated phospholipids, and the greater the fatty acid chain length of the saturated phospholipids the more rapid the intracellular degradation of liposomes.