The alpha(2)-adrenoceptor agonists xylazine and guanfacine exert differentcentral nervous system, but comparable peripheral effects in calves

Acute pharmacodynamic effects of the alpha(2)-adrenoceptor agonists, xylazi ne and guanfacine, were investigated in nine healthy calves in an open cros sover trial, Xylazine (100 mu g/kg body weight intravenously (i.v.)) and gu anfacine (20 mu g/kg body weight i.v,) were equi-effective in lowering hear t rate by 25-30% at 5 min. Under these conditions, xylazine induced strong sedation and increased plasma growth hormone levels, indicating central ner vous system mediated actions, whereas guanfacine was not sedative and did n ot induce release of growth hormone, Oxygen consumption was decreased by bo th drugs, but respiratory exchange ratio decreased only in response to xyla zine. However, in response to both drugs, plasma levels of noradrenaline, a drenaline, insulin and non esterified fatty acids decreased similarly and g lucose increased comparably, These results demonstrate marked differences i n the central nervous system-mediated effects of the two alpha(2)-adrenocep tor agonists, whereas peripheral actions are similar.