Basic anatomical and physiological differences between species should be considered when choosing combinations for use in models of hepatic xenotransplantation: an investigation of the guinea pig-to-rat combination

Animal organs for human transplantation: how close are we?

Solid organ transplantation has been, by most measures, a phenomenal success. Nonetheless, the field is plagued by extreme shortages of available organs from a very limited number of donors. One potential solution to this organ availability crisis is the use of animals as organ donors for humans (xenotransplantation). Though the concept remains theoretical, significant advances are being made in the field of genetics and in our understanding of the immunological barriers to xenotransplantation. With these advances also comes increased knowledge about the potential risks of xenotransplants, especially disease transmission. The eventual clinical application of animal-to-human transplants will require a careful, balanced appraisal of these issues.

Heterotopic cardiac xenotransplantation in rodents: Report of a refined technique in a hamster-to-rat model

Hamster-to-rat heterotopic cardiac xenotransplantation is widely used as an experimental model to study xenograft rejection, accommodation, and tolerance, as well as in studies aimed at developing immunosuppressive strategies in xenotransplantation. Despite its widespread application, no detailed description of a surgical technique for this model has been provided in the literature. Indeed, all publications so far on the use of this species combination refer to the rat allotransplantation technique. Hence the present paper provides a detailed, up-to-date description of the surgical method adopted at our center for the hamster-to-rat heterotopic cardiac xenotransplantation model. Considerable effort went into developing a reliable, reproducible experimental model in rodents, and the description given here is enriched with "tips" that we learned in the process. The discussion of the technique also addresses several significant related issues, e.g., the anesthesia and organ preservation solution used (aspects that, in our experience, are crucial to a good surgical outcome).

Isolated perfused liver model: the rat and guinea pig compared

OBJECTIVE

Although the rat is the most commonly used species for the study of hepatic metabolism, the physiology of the guinea pig is closer to human physiology. We compared the model of isolated perfused guinea pig liver with the classic model of isolated perfused rat liver, especially with respect to amino acid metabolism.

METHODS

After validation of an anesthetic mixture of ketamine, diazepam, and xylazine for the guinea pig, isolated perfused livers were harvested for both species. Three groups of animals were compared for the study of liver metabolic fluxes: 6-wk-old male Sprague-Dawley rats (R; 230 +/- 10 g, n = 5), young male Hartley guinea pigs (YG; 223 +/- 8 g, n = 6) matched to rats by liver weight, and adult male Hartley guinea pigs (AG; 389 +/- 5 g, n = 6) matched to rats by age. Results (mean +/- standard error of the mean) were compared by analysis of variance and Newman-Keuls tests.

RESULTS

Both models displayed a satisfactory hepatic viability, but differences were noted, with higher portal flows (R: 3.1 +/- 0.3 versus YG: 4.5 +/- 0.3 and AG: 4.2 +/- 0.3 mL. min(-1). g(-1); P < 0.05, YG and AG versus R) and bile flows (R: 0.34 +/- 0.01 versus YG: 2.38 +/- 0.22 versus AG: 3.17 +/- 0.28 microL. min(-1). g(-1); P < 0.05, YG and AG versus R, and YG versus AG) and higher amino acid fluxes (P < 0.05) leading to greater nitrogen uptake (P < 0.05) in guinea pigs. We performed a second set of experiments to evaluate the influence of anesthesia and portal flow on this last parameter. In these experiments, rats were anesthetized with ketamine, diazepam, and xylazine and guinea pig livers were perfused at rat blood flow. Apart from a 50% anesthesia-related mortality for rats, bile flow and metabolic parameters were only slightly modified. However, some amino acid fluxes were statistically different (aspartate, serine, and histidine; P < 0.05), as confirmed by a higher transfer constant.

CONCLUSION

Our results indicate that the isolated perfused guinea pig liver is a suitable model for the study of hepatic metabolism.

Different hepatic vascular response to noradrenaline and histamine between guinea-pig and rat

AIM

Hepatic xenotransplantation from guinea-pig to rat has not been established. This failure is partly ascribed to differences in hepatic vascular characteristics between two species. However, the differences in hepatic vascular resistance distribution and responses to vasoconstrictors are not known. The present study was designed to determine basal levels of segmental vascular resistances and the responses to histamine and noradrenaline in isolated guinea-pig and rat livers.

METHODS

The livers were haemoperfused (Hct 8.3%) via the portal vein at a constant flow. The sinusoidal pressure was measured by the double occlusion pressure, and was used to determine the pre- (Rpre) and post-sinusoidal (Rpost) resistances.

RESULTS

There was no significant difference in basal total hepatic vascular resistance (Rt) between two species, whereas Rpre in rat (69% of Rt) was significantly greater than that in guinea-pig (61% of Rt). The responses to noradrenaline were similar; Rpre increased in a greater magnitude than Rpost, and liver weight was reduced. However, the noradrenaline-induced increase in Rt was greater in rats than in guinea-pigs. In contrast, histamine increased predominantly Rpost over Rpre, and increased liver weight in guinea-pig, while it affected no haemodynamic variables in rat.

CONCLUSION

There exist species differences in the hepatic vasculature between rat and guinea-pig. Basal pre-sinusoidal resistance in rat is greater than that in guinea-pig. Although noradrenaline predominantly contracts pre-sinusoidal vessels in both species, histamine causes predominant post-sinusoidal vasoconstriction in guinea-pig liver, while it has no vasoactive effects on rat liver.

Preconditioning with the prostacyclin analog epoprostenol and cobra venom factor prevents reperfusion injury and hyperacute rejection in discordant liver xenotransplantation

Liver xenografts transplanted from guinea pig to rat suffer from inadequate organ reperfusion and initial dysfunction, despite sufficient complement depletion using cobra venom factor (CVF). Reperfusion injury is prevented when complement depleted donors are treated with the prostacyclin analog epoprostenol. Histological analysis suggests that epoprostenol preconditioning prevents post-reperfusion spasms of the intrahepatic branches of the portal vein and strongly reduces appearance of hepatocyte apoptosis shortly after transplantation. Cobra-venom-treated rats show breakdown of glucose metabolism and die in acute hypoglycaemia, whereas the additional application of epoprostenol restores gluconeogenesis. Consequently, recipient survival after epoprostenol and CVF treatment is significantly improved compared with animals receiving CVF only (5.1 +/- 2.6 h vs. 17.9 +/- 5.1 h). These data demonstrate that initial dysfunction of discordant liver grafts in the guinea-pig-to-rat species combination, can be overcome by the application of epoprostenol combined with CVF. Using this pharmacologic regimen, the discordant guinea-pig-to-rat model appears useful to study further questions concerning functional and immunological compatibility of a discordant liver xenograft.

Animal models in xenotransplantation

The severe shortage of donor organs has provided a strong impetus to push the investigation into the use of animal organs for humans. Xenotransplantation will not only benefit patients, but also represents a unique and potentially profitable business opportunity. However, there are many barriers to successful clinical xenotransplantation, including immunological barriers, physiological incompatibility, zoonosis and ethical concerns. This overview will focus on currently available animal models used in attempts to break through the immunological barriers to xenotransplantation. There are many advantages to using small animal, namely rodent, models in xenotransplantation research. For example, the use of the mouse model allows the use of knockout mice and careful dissection of rejection mechanisms at the molecular level. The following models can be used to study hyperacute rejection (HAR): guinea-pig-to-rat, mouse-to-rabbit, guinea-pig-to-mouse, rat-to-presensitised mouse and rat-to-alpha-Gal knockout mouse. The hamster-to-rat, mouse-to-rat and rat-to-mouse models are commonly used to study acute vascular rejection. Large animal models are complex and expensive, but they are more relevant to clinical xenotransplantation. Based on experiments using transgenic pig-to-primate models, HAR can be overcome. However, acute vascular rejection remains a major barrier at the present time. A pig cartilage-to-monkey model has been developed to study chronic rejection. Other novel models such as pig venous segment-to-monkey model and rat-to-primate model may represent viable options to study immunological barriers following xenotransplantation. Like many other medical breakthroughs, animal research will continue to make enormous contributions towards the eventual success of xenotransplantation.

Impaired recoloration of a discordant liver xenograft in the guinea pig-to-rat combination: physiological insults or immunological responses

BACKGROUND

The aim of this study was to clarify, using guinea pig (GP) syngeneic and xenogeneic liver graft models, the mechanisms of impaired recoloration caused by immunological responses or physiological insults; and to examine the effect of an anti-complement agent, FC43 emulsion, on xenograft rejection.

METHODS AND RESULTS

GP syngeneic and xenogeneic liver grafts flushed with 4 degrees C lactated Ringer's solution were poorly perfused around the porta hepatis, whereas those flushed with 15 degrees C solution were immediately recolored, and the portal venography confirmed homogeneous perfusion throughout the GP livers. Impaired recoloration of GP xenografts was also ameliorated by reduction of ischemic time. Pretreatments with FC43 emulsion significantly extended xenograft survivals. Rejected GP liver xenografts showed C3 deposits on sinusoids and central veins and IgM deposits faintly stained only on some small vessels, but there were no detectable deposits of IgG.

CONCLUSIONS

The impaired recoloration in this model may be caused by physiological insults during the initial stage of revascularization, and the immunological responses that probably followed.