Contribution of intestine and kidney to glucose fluxes in different nutritional states in rat

Abstract

The liver is considered the main contributor of endogenous glucose production (EGP) in the postabsorptive (PA) state in mammals. However, it has been shown that the kidney, in PA and fasting states, and the intestine, in insulinopenia states, could make significant contributions to EGP. Using glucose tracer dilution combined to a vessel ligaturing approach, we studied the respective role of these organs in glucose turnover under various nutritional conditions in the rat (Rattus norvegicus). Both organs constitute key sites of glucose disposal in all situations in the non-moving rat. The kidney makes a small (12%) contribution to EGP in the PA state (9.6 ± 1.3 μmol/kg min, means ± SEM, n = 5), which is dramatically increased (p < 0.01) in 24 h-fasting (18.8 ± 1.0 μmol/kg min) or streptozotocin diabetes (48 ± 3 μmol/kg min). The small intestine contributes to EGP via two ways: a direct glucose contribution that may only take place in fasting and diabetes; an indirect contribution via the supply of alanine and lactate to liver gluconeogenesis that may account for up to 5 μmol/kg min in both PA and fasted states in the rat. These data emphasize the coordinate interactions among the three gluconeogenic organs in glucose homeostasis when nutritional conditions are changing.

Introduction

The liver is generally considered the main contributor of endogenous glucose production (EGP) in postabsorptive (PA) and fasting states in mammals (Mittelman and Bergman, 1998). However, it has long been recognized that the kidney may have a key role in EGP in both PA and fasting states in rat, dog and human (Minassian and Mithieux, 1994, Cersosimo et al., 1994, Stumvoll et al., 1995, Ekberg et al., 1999). More recently, the small intestine (SI) has been shown to be a third gluconeogenic organ, able to contribute to EGP in the fasting and diabetic states in rat (Croset et al., 2001), reviewed in (Mithieux et al., 2004a). Moreover, a recent study suggested that intestine glucose production might occur during the anhepatic phase of liver transplantation in humans (Battezzati et al., 2004).

Seminal studies were based on the determination of the net release of glucose (i.e. arteriovenous balance measurements). This approach has the disadvantage that glucose uptake may hide glucose production. Recent approaches combined tracer dilution and arteriovenous balance to separate the uptake and release of glucose by the organ considered. The estimation of kidney glucose production has remained somewhat controversial. According to the study, the participation of the kidney to EGP in PA humans would vary from about 30% (Stumvoll et al., 1995) to 5% at most (Ekberg et al., 1999). A major limitation in these studies relates to the determination of significant tracer dilution and/or fractional extraction of glucose by the kidney, because of the high blood flow through the organ and the relative imprecision of the tracer approach. It has been suggested that this could prevent a correct estimation of glucose uptake and consequently of glucose release (Ekberg et al., 1999, Moller et al., 2001). A comparable impediment has been encountered for similar reasons in the determinations of glucose fluxes in the rat SI (Croset et al., 2001, for review, see Mithieux, 2001). Significant glucose tracer dilution could be evidenced in the 48 h-fasted and diabetic states, demonstrating that the SI is a true glucose producer organ in insulinopenia states in rat (Croset et al., 2001). In contrast, tracer dilution could not be shown in PA rats, preventing us to estimate the contribution of SI to EGP in the PA state accurately (Croset et al., 2001).

In addition to the problems linked to blood flow, the proper contribution of each gluconeogenic organ to glucose appearance has never been determined in unique species. Intestine fluxes were determined in rat (Croset et al., 2001, Mithieux et al., 2004b), kidney fluxes in dog or human (Cersosimo et al., 1994, Stumvoll et al., 1995, Ekberg et al., 1999, Moller et al., 2001), under nutritional conditions that are not easily comparable. There are thus multiple interests in this work. 1) By isolating the organ studied by ligaturing the blood vessels and estimating shortly after the repercussions on systemic glucose fluxes, we freed us from the problems bound to high blood flow through these organs, and thus at least in part from the problem of the relative imprecision of specific activity determinations of glucose tracers. 2) The respective contributions of the kidney and intestine to systemic glucose appearance in various nutritional states were estimated using unique models of rats with ligatured intestines or kidneys. 3) We were able to compare the contribution of each isolated organ using two completely different approaches.