[Blocking by naloxone of the antihypertensive effect of clonidine in hypertensive and normotensive animals].

Antagonism between cardiovascular effects of clonidin (10-30 micrograms/kg) and naloxon (0.15-1.0 mg/kg) was studied on anesthetized spontaneously hypertensive rats (SHR) as compared to Wistar Kyoto (WKY) rats and on awake cats. Naloxon was shown to completely block the clonidin hypotensive effect in SHR and cats and to partially block it in WKY rats. In some experiments, naloxon was administered after the blockade of peripheral M-cholinoreceptors by metacin (2 mg/kg). In these experiments naloxon evoked elevation of blood pressure, cardiac output and heart rate (in cats). It is concluded that the opioid link participates in the development of clonidin-evoked bradycardia and hypotension both in hypertensive and in normotensive animals.