Viability of the vascularly perfused, recirculating rat intestine and intestine-liver preparations

Beyond glucose: alternative sources of energy in glioblastoma

Glioblastoma multiforme (GBM) is the most common malignant brain tumor in adults. With a designation of WHO Grade IV, it is also the most lethal primary brain tumor with a median survival of just 15 months. This is often despite aggressive treatment that includes surgical resection, radiation therapy, and chemotherapy. Based on the poor outcomes and prevalence of the tumor, the demand for innovative therapies continues to represent a pressing issue for clinicians and researchers. In terms of therapies targeting metabolism, the prevalence of the Warburg effect has led to a focus on targeting glucose metabolism to halt tumor progression. While glucose is the dominant source of growth substrate in GBM, a number of unique metabolic pathways are exploited in GBM to meet the increased demand for replication and progression. In this review we aim to explore how metabolites from fatty acid oxidation, the urea cycle, the glutamate-glutamine cycle, and one-carbon metabolism are shunted toward energy producing pathways to meet the high energy demand in GBM. We will also explore how the process of autophagy provides a reservoir of nutrients to support viable tumor cells. By so doing, we aim to establish a foundation of implicated metabolic mechanisms supporting growth and tumorigenesis of GBM within the literature. With the sparse number of therapeutic interventions specifically targeting metabolic pathways in GBM, we hope that this review expands further insight into the development of novel treatment modalities.

A model of the isolated perfused rat small intestine

Intestinal edema remains a serious clinical problem, and novel approaches to study its pathophysiology are needed. It was our aim to develop a long-term stable isolated perfused rat small bowel preparation permitting analysis of vascular, luminal, interstitial, and lymphatic compartments and to demonstrate the utility of this model by studying the effects of the proinflammatory mediator platelet-activating factor (PAF). A temperature-controlled chamber with an integrated balance was designed to perfuse isolated intestines through the mesenteric artery and the gut lumen. Steroids or oxygen carriers were not needed. Functional and morphological integrity of the tissue was preserved for several hours as confirmed by oxygen consumption, venous lactate-to-pyruvate ratio, arterial and venous pH, lactose digestion and galactose uptake, intravascular and luminal pressures, maintained fluid homeostasis, gut motility, and quantitative light microscopic analysis. Administration of PAF caused typical effects such as vasoconstriction, gut atony, and loss of galactose uptake. PAF also elicited a transient loss of 20% of the perfusate liquid from the mesenteric vascular bed, two-thirds of which were transferred to the lumen. All these responses were entirely reversible. This new model provides detailed insights into the physiology of the small intestine and will allow to study fundamental processes such as fluid homeostasis, barrier functions, transport mechanisms, and immune responses in this organ. Using this model, here we show a dramatic and yet reversible response of the rat small bowel to PAF, suggesting luminal water clearance as a novel safety factor in the intestine that may be of clinical relevance.

Intestinal first-pass metabolism of eperisone in the rat

PURPOSE

The purpose of this study was to clarify quantitatively the contribution of the intestine to the first-pass metabolism of eperisone in rats.

METHODS

The systemic availabilities of eperisone were estimated by administering the drug into the duodenum, portal vein, and femoral vein in rats in vivo. The first-pass metabolism of eperisone was confirmed in the perfused rat small intestine in situ. Metabolism of eperisone to an omega-1-hydroxylated metabolite (HMO), the first step of eperisone metabolism, was studied using rat intestinal microsomes in vitro.

RESULTS

The bioavailabilities in the intestine were 0.176 and 0.0879 at administration rates of 100 and 25 mg/h/kg, respectively, whereas those in the liver were 0.532 and 0.486, respectively. In the intestinal perfusion experiment, the appearance clearance to the portal vein from the intestinal lumen was much lower than the elimination clearance from the intestinal lumen, resulting in high metabolic clearance of eperisone in the small intestine. Eperisone was biotransformed to HMO by rat intestinal microsomes, and this was inhibited by alpha-naphthoflavone and an anti-rat CYP1A antibody.

CONCLUSIONS

Those data strongly suggest that eperisone may be metabolized to HMO by CYP1A in rat intestinal microsomes during the first-pass through the epithelium of the small intestine.

Route-dependent metabolism of morphine in the vascularly perfused rat small intestine preparation

PURPOSE

1. To compare the disposition of tracer morphine ([3H]M) following systemic and intraduodenal administration in the recirculating, rat small intestine preparation in absence or presence of verapamil (V), an inhibitor of P-glycoprotein. 2. To develop a physiological model to explain the observations.

METHODS

A bolus dose of [3H]M was added to the reservoir or injected into the duodenum of the rat small intestine preparation. V (200 microM in reservoir) was either absent (control studies) or present. Intestinal microsomal, incubation studies were performed to evaluate the effect of V on morphine glucuronidation.

RESULTS

After systemic administration, [3H]M was not metabolized but was exsorbed into lumen. By contrast, both [3H]M and the 3beta-glucuronide metabolite, [3H]M3G, appeared in reservoir and lumen after intraduodenal administration. A physiologically-based model that encompassed absorption, metabolism and secretion was able to describe the route-dependent glucuronidation of M. The presence of V resulted in diminished levels of M3G in perfusate and lumen and mirrored the observation of decreased glucuronidation in microsomal incubations. Verapamil appeared to be an inhibitor of glucuronidation and not secretion of M.

CONCLUSIONS

M was secreted and absorbed by the rat small intestine. Route-dependent glucuronidation of M was explained by physiological modeling when M was poorly partitioned in intestinal tissue, with a low influx clearance from blood and a even poorer efflux clearance from tissue. The poor efflux rendered a much greater metabolism of M that was initially absorbed from the lumen. V increased the extent of M absorption through inhibition of M glucuronidation.

Evaluation of intestinal absorption into the portal system in enterohepatic circulation by measuring the difference in portal-venous blood concentrations of diclofenac

PURPOSE

We evaluated the first-pass effects in vivo by the intestine and liver during enterophepatic circulation (EHC) by simultaneously measuring the portal and venous plasma concentrations of the rat.

METHODS

The venous and upper portal blood vessels were cannulated through the jugular and the pyloric veins, respectively, to obtain simultaneously blood samples from both sites. After diclofenac was injected as a bolus through the jugular vein, the concentrations of diclofenac in the portal and jugular veins were measured at time intervals. The absorption rate from the intestinal tract into the portal system was determined using the portal-venous difference in plasma concentrations of diclofenac, considering 40% partitioning of diclofenac into erythrocytes.

RESULTS

After one hour, the plasma concentration in the portal vein was always higher than that in the jugular vein in awakening rats with intact EHC (portal-venous blood concentration difference). No portal-venous difference was observed in awakening rats with bile-duct cannulation. Therefore, it was concluded that this portal-venous concentration difference was not due to the hepatic clearance but to diclofenac reabsorption from the intestinal tract.

CONCLUSIONS

Approximately 40% of the dose of diclofenac was reabsorbed over 8 hours from the intestinal tract into the portal system. By comparing the reabsorbed amounts in the portal system and in the systemic circulation, the hepatic extraction ratio in vivo (FH) of diclofenac was estimated to be 63%.

Kinetic analysis of the drug permeation process across the intestinal epithelium

The rat intestinal lumen and the blood vessel were simultaneously perfused to study drug permeation across the intestinal epithelium. On the basis of drug disappearance from the intestinal lumen and its appearance into the vascular outflow, the mean time required for permeation across the intestinal membrane (MPT) and the permeation clearance (CLp) were calculated. MPT values of water, antipyrine, propranolol, imipramine and mannitol, varied from 0.45 min to 9.91 min depending on their physicochemical property. From both MPT and CLp, five drugs were classified as being (i) highly and rapidly absorbed (water, antipyrine), (ii) highly but slowly absorbed (propranolol, imipramine) and (iii) low and slowly absorbed (mannitol). Permeation profiles of these drugs were analyzed based on the diffusion model which defined the parameter for each permeation process, i.e. partitioning to and diffusion through the epithelium and clearance into the blood flow. Propranolol and imipramine partitioned into the membrane at a higher level than the other drugs. However, the clearance of both drugs from the epithelium was extremely slow, suggesting that this process is the rate-limiting step in their permeation. On the other hand, the rate-limiting step in the permeation of water and antipyrine was found to be the diffusion process in the epithelial layer.