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.