Activation of rat intestinal mucosal mast cells by fat absorption

Abstract

Previous studies have linked certain types of gut mucosal immune cells with fat intake. We determined whether fat absorption activates intestinal mucosal mast cells (MMC), a key component of the gut mucosal immune system. Conscious intestinal lymph fistula rats were used. The mesenteric lymph ducts were cannulated, and the intraduodenal (i.d.) tubes were installed for the infusion of Liposyn II 20% (an intralipid emulsion). Lymphatic concentrations of histamine, rat MMC protease II (RMCPII), a specific marker of rat intestinal MMC degranulation, and prostaglandin D(2) (PGD(2)) were measured by ELISA. Intestinal MMC degranulation was visualized by immunofluorescent microscopy of jejunum sections taken at 1 h after Liposyn II gavage. Intraduodenal bolus infusion of Liposyn II 20% (4.4 kcal/3 ml) induced approximately a onefold increase in lymphatic histamine and PGD(2), ∼20-fold increase in lymphatic RMCPII, but only onefold increase in peripheral serum RMCPII concentrations. Release of RMCPII into lymph increased dose dependently with the amount of lipid fed. In addition, i.d. infusion of long-chain triacylglycerol trilinolein (C18:2 n-6, the major composite in Liposyn II) significantly increased the lymphatic RMCPII concentration, whereas medium-chain triacylglycerol tricaprylin (C8:0) did not alter lymph RMCPII secretion. Immunohistochemistry image revealed the degranulation of MMC into lamina propria after lipid feeding. These novel findings indicate that intestinal MMC are activated and degranulate to release MMC mediators to the circulation during fat absorption. This action of fatty acid is dose and chain length dependent.

Fig. 1.

Lymph flow rate (A), protein output (B), lymphatic histamine (C), and rat mucosal mast cells (MMC) protease II (RMCPII) concentrations (D) after intraduodenal (i.d.) infusion of saline (n = 6) or Liposyn II 20% (4.4 kcal) (n = 8). Data are expressed as means ± SE, *P < 0.05, **P < 0.01 vs. saline.

Fig. 2.

Immunofluorescence staining for RMCPII in the rat jejunum. The representative images show that RMCPII immunoreactivity occurred in MMC in intestinal lamina propria of saline-gavaged rats (A). In Liposyn II rats (n = 3) (B), the release of RMCPII stained granules diffusely surrounding the cells (arrow with round head), and the decreased staining of the RMCPII granules in the cytoplasm (arrow with diamond head) of the mast cells demonstrate mast cell degranulation. Right: ×5 the selected images of left. C: negative control without primary anti-RMCPII antibody.

Fig. 3.

Peripheral serum RMCPII concentrations after lipid (n = 10) or saline (n = 7) infusion . Data are expressed as means ± SE. *P < 0.05, **P < 0.01 vs. saline.

Fig. 4.

Lymphatic prostaglandin D₂ (PGD₂) concentrations (A), triglyceride contents (B), and IL-6 concentrations (C). Values are means ± SE. *P < 0.05, **P < 0.01, Lipid (n = 6) vs. saline (n = 6). C: dashed line represents the value of 78 pg/ml.

Fig. 5.

Lymphatic releases of RMCPII (A) and PGD₂ (B) in response to repeated lipid feeding. Data are expressed as means ± SE. **P < 0.01, Lipid (n = 5) vs. saline (n = 4), ##P < 0.01 values of second dose vs. values of first dose of Liposyn II.

Fig. 6.

Effect of the amount of Liposyn II (A and B) and the type of fatty acids (C and D) on lymphatic RMCPII release. A: response to four infused lipid doses (0.55, 1.1, 2.2, and 4.4 kcal). B: correspondent areas under the curve. Values (fold amounts above saline) are means ± SE. *P < 0.05, **P < 0.01 vs. 1/4 dose (1.1 kcal, n = 6), ##P < .01, full dose (4.4 kcal, n = 8) vs. half dose (2.2 kcal, n = 8) Liposyn II. C and D: infusion of trilinolein or tricaprylin on lymphatic RMCPII concentrations (C) and triacylglycerol (TG) contents (D). Values are means ± SE; n = 6. *P < 0.05, **P < 0.01 vs. lipid emulsion (control).

Fig. 7.

Effect of pluronic L-81 (L-81) on lymphatic TG contents (A) and RMCPII concentrations (B). Means ± SE, n = 6. *P < 0.05, **P < 0.01 vs. saline; ##P < 0.01, L-81+Liposyn II vs. Liposyn II.