Effect of clonidine on the turnover rate of noradrenaline in peripheral tissues of the rat

The turnover of noradrenaline (NA) was studied from the decline in specific activity of ³H-NA in heart, spleen, kidneys and sub-maxillary glands of rats maintained at room temperature (22°C) or exposed to 4°C. Cold-exposure enhanced NA turnover in heart, spleen and kidneys but not in submaxillary glands.Clonidine (2 × 50 μg/kg, i.p., 12 and 6 hr before sacrifice) did not change significantly the endogenous levels of NA in the various organs of animals placed at 22°C. In cold-exposed rats, a significant 15% decrease in the cardiac NA level was observed after clonidine treatment.In the submaxillary glands, clonidine significantly reduced NA turnover, the biological half-lives of ³H-NA increasing from 4.8 to 7.9 hr in the case of rats exposed to 22°C and from 4.8 to 7.6 hr in rats exposed to 4°C. NA turnover in the heart was not significantly altered by clonidine in animals at 22°C whereas it was markedly reduced in animals exposed to cold, the ³H-NA half-lives rising from 4.2 to 6.8 hr. In spleen and kidneys, reductions in NA turnover were also observed after clonidine treatment but the variations were never significant.The mechanisms by which clonidine reduces the peripheral NA turnover are discussed.