三聚氰胺混合三聚氰酸(1:1)對雄鼠短時間及28天重複餵食造成腎毒性之研究

Abstract

Melamine is composed of multi-amine, was added into food to false increase protein contents. Furthermore, cyanuric acid is an aderivarative of melamine. The objective of this study was conducted to evaluate the possible acute toxic mechanisms of melamine and cyanuric acid (MCA) crystals formation related to renal failure. The time course of acute oral toxicity of male rats was singly gavaged with 400 mg/kg MCA (1:1) and observed at different time-course of 30 minutes, 1, 3, 12, 24 and 48 hours, respectively. Results revealed that degeneration/necrosis were found in the proximal tubules starting from 12 hour post-administration and increased severity with timing. After that, a small amount of yellow-greenish crystals were observed in the dilated distal renal tubules at 48 hour. Hematology and serum chemistry parameters showed that acute inflammatory cells, BUN, ceatinine and serum potassium ion values significantly increased. Immunohistochemistry, the numbers of proliferating cell nuclear antigen (PCNA) increased and accompanied with renal tubular injury. Malondialdehyde (MDA), IL-1β and IL-6 levels elevated after MCA (1:1) treatment indicating reduction of antioxidant ability and acute inflammation in kidney. Ultrastructurally, nucleus pyknosis, lost microvilli, mitochondria vesicles and cell swelling were mainly found in the proximal tubular cells. Two kinds of crystals were observed; small needle-like crystals in the cytoplasm were presented in dying cells, and large crystals in the lumen of tubule could induce physical damage or directly penetrated cell membrane. This study suggests that administration of MCA poses different toxicity to the target cells of both the proximal and distal tubules in rats. The proximal tubular cells may be injured directly by MCA crystals. In order to understand the no observed adverse effect level (NOAEL) of MCA combination, three different doses of 100, 200 and 300 ppm MCA in diet were fed to male rats for either 14 or 28 days. Results revealed moderate to severe tubulointerstitial nephritis with yellow-greenish MCA crystals in the median and high dose groups. The NOAEL of MCA subacute toxicity is 2.99 (~3) mg/kg bw/day in male rats and the ADI value of MCA is recommended to equal or less than 0.03 mg/kg bw/day for human. The ADI (0.03 mg/kg bw/day) based on the nephrotoxicity of MCA in rats from the study are lower than published ADI from WHO (0.2 mg/kg bw/day), and suggested that ADI with melamine alone may underestimate the risk from co-exposure to melamine and cyanuric acid.