Development of a dynamic system simulating pig gastric digestion

ABSTRACT : The objective of this study was to develop a model for simulating gastric digestion in the pig. The model was constructed to include the chemical and physical changes associated with gastric digestion such as enzyme release, digestion product removal and gastric emptying. Digesta was collected from the stomach cannula of pigs to establish system parameters and to document the ability of the model to simulate gastric digestion. The results showed that the average pH of gastric digesta increased significantly from 2.47 to 4.97 after feed consumption and then decreased 140 min postprandial. The model described the decrease in pH within the pigs' stomach as p[H.sub.t] = 5.182[e.sup.-0.0014t], where t represents the postprandial time in minutes. The cumulative distribution function of liquid digesta was [V.sub.t] = 64.509[e.sup.0.0109t]. The average pepsin activity in the liquid digesta was 317Anson units/mL. There was significant gastric emptying 220 min after feed consumption. The cybernetic dynamic system of gastric digestion was set according to the above data in order to compare with in vivo changes. The time course of crude protein digestion predicted by the model was highly correlated with observed in vivo digestion (r = 0.97; p = 0.0001), Model prediction for protein digestion was higher than that observed for a traditional static in vitro method (r = 0.89; p = 0.0001). (Key Words : Gastric Digestion Modelling, Pigs, Protein Digestion)

INTRODUCTION

Methodology for the evaluation of nutrient digestion is important in animal nutrition research because it allows not only estimation of the nutritive value of particular feedstuffs (Yang et al., 2007) but also the bioavailability of drugs (Hebrard et al., 2006) and feed supplements (Chiang et al., 2005; Fang et al., 2007). In vitro methods are economical and efficient because they do not require animals, they employ less manpower and decrease the variation associated with replicate measurements using traditional laboratory procedures. The in vitro digestibility of major dietary components in pig diets (Furuya et al., 1979; Babinszky et al., 1990; Boisen and Fernandez, 1995) were similar to those values reported in previous in vivo feeding trails.

Several in vitro methods of estimating feed digestibility have been developed, and can be divided into single-(AOAC, 1980; Mertz et al., 1984), two- (Babinszky et al., 1990; Cone and van der Poel, 1993), or three-step (Vervaeke et al., 1979; Boisen and Fernandez, 1991) models simulating the gastric digestion, the gastric/small intestinal digestion and the gastric/small intestinal/large intestinal digestion, respectively. In each, the first step simulates gastric digestion. In non-ruminants, such as pigs, digestion of crude protein begins in the stomach or gastric pouch (Keys and DeBarthe, 1974; Furuya et al., 1979). The current static models, which expose substrates to a digesta fluid or enzyme solution at a fixed pH and temperature for a fixed period of time, simulate digestion in the animal gut in a manner similar to a batch reactor. These conditions do not simulate the physiological environment of digestion in the pig gastrointestinal tract (GI). In the static in vitro gastric models, pepsin hydrolysis proceeds at pH 1.0 (Babinszky et al., 1990) to 2.0 (Boisen and Fernandez, 1995), or in a 0.1 N HCl solution (Cone and van der Poel, 1993). In vivo, gastric pH decreases from 4.8 to 2.1 and 1.7, one and two hours, respectively, after ingestion of milk (Marteau et al., 1990: human). The amount of digesta within the GI tract and digesta transit time significantly affects nutrient digestive capacity. None of these conditions can be properly simulated in a batch, static model of in vitro digestibility.

Previous in vivo research described the gastric empty rate (Hunt and Stubbs, 1975: human; Weisbrodt et al., 1969: dog) using the equation: V(t) = [V.sub.0] (1-[e.sup.-Kt]), where V(t) represents the gastric digesta volume at time t postprandial, [V.sub.0] the initial volume and K a constant of digesta emptying rate. To date, the in vivo gastric residence time has only been realistically simulated in the pre-ruminant calf gastric model (Yvon et al., 1992) where liquid flow and the pH are controlled continually in real time.

The objective of the present study was to develop a dynamic in vitro model of digestion within the pig which would simulate, as closely as possible, the actual physiological processes which occur within the lumen of the pig stomach during nutrient digestion. This model was compared to digestibility parameters measured in vivo as well as a traditional static in vitro incubation technique.

MATERIALS AND METHODS

Experimental design and digestibility estimates

Animal preparation : The animal feeding protocol, surgery procedure and care were approved by the Animal Care and Use Committee of National Chung Hsing University. Materials required and procedure for cannulation were similar to those described by Low et al. (1985). Experiments were carried out on four Landrace castrated male pig. All surgery was performed with full aseptic precautions under halothane anaesthesia. The pigs were fitted, at a weight of 40-45 kg, with a cannula (polypropylene; barrel outer diameter 25 mm) in the fundic region of the stomach for a further study of the control of gastric emptying. The pigs were given water and antibiotic powder, for the first 24 hours after surgery. The normal diet was then gradually re-introduced and full intake was achieved 4-5 days after surgery. Collections of gastric digesta began 14 days after surgery.