Autoradiographische Untersuchungen zur Toxikokinetik von Myosmin in Ratten
Autoradiographic studies on myosmine toxicokinetics in rats Myosmine, a minor tobacco alkaloid was recently detected in a wide variety of nutritional plants including cereals, nuts, cocoa and dairy products. Under acidic conditions myosmine is easily nitrosated leading to pyridyloxobutylation either directly or through the formation of N'-nitrosonornicotine (NNN). In NNN-treated rats DNA adducts releasing 4-hydroxy-(3-pyridyl)-1-butanone (HPB) are made responsible for tumours in oesophagus, oral and nasal cavities. Whereas the pharmacokinetics and metabolism of NNN have been studied in detail no such data have been available for myosmine. In the present study time-dependent distribution, binding and excretion of radiolabelled myosmine was investigated in pigmented and albino rats by quantitative whole-body autoradiography. One rat each was sacrificed by CO2 asphyxiation six and 15 minutes as well as 1, 4 and 24 hours after intravenous injection Zusammenfassung / Summary 91 of myosmine (5-3H: 1.4 µg/kg; 8.49 MBq/kg or 2’-14C: 0.27 mg/kg; 3.74 MBq/kg). Sagittal sections of 40 µm thickness obtained after deep freezing of the rats, were exposed to image plates either before or after serial extraction with solvents and water. Additionally, sections of eyes and oesophagus of untreated rats were incubated in vitro with 2’-14C myosmine. The rats killed four and 24 hours after myosmine injection were separately housed in metabolism cages for collection of urine and faeces. High concentrations of radioactivity were present six minutes after myosmine application in Harder's gland, lacrimal, salivary and accessory genital glands, nasal cavity, liver, kidney cortex, wall of the stomach fundus as well as the melanin rich parts of the eye and skin. An accumulation of radioactivity was also seen in the contents of the stomach and bladder. A similar distribution pattern was evident after 15 minutes and one hour with stronger accu-mulation in oesophagus, medulla of kidneys and accessory genital glands. An increasing labelling was observed throughout the contents of the gastrointestinal tract. Four hours after myosmine application the labelling intensity of most organs and tissues decreased conside-rably with exception of the accessory genital glands showing an extraordinary high concen-tration of radioactivity. Additionally, the oesophagus showed more marked labelling at this time. After 24 hours nearly all of the applied labelled myosmine has eliminated from the body. However, a significant labelling was still present in melanin rich tissues and in the preputial gland. The high accumulation of radioactivity in the accessory genital glands within the first four hours after myosmine injection is a new finding which has not been reported for similar compounds such as nicotine, cotinine and tobacco-specific nitrosamines. Direct determina-tion of radioactivity in the excised glands confirmed the result obtained by quantitative autoradiography. After extraction of the slices most of the radioactivity was removed from the tissues. An increased proportion of radioactivity remained in melanin rich tissues, nasal cavity, oesopha-gus, lung and liver. A distinct labelling of oesophageal mucosa as well as pigmented parts of the eyes could also be observed after in vitro incubation of slices from untreated rats. In most of the tissues, the drop of radioactivity followed a first order elimination kinetic with half lives between 0.4 hours for salivary glands and 1.2 hours for skin melanin. Within 24 hours 72 and 15 percent of the dose have been excreted in urine and faeces, respectively. The results demonstrate the suitability of saliva and hair for biomonitoring human myosmine uptake. The increased proportion of non-extractable radioactivity in the oesophagus supports our hypothesis of the possible implication of myosmine as an initiator of human oesophageal adenocarcinoma.
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