Effects of antihypertensive medication on the cardiovascular response to ketamine in rats

The effects of hydralazine, clonidine, propranolol and methyldopa medications on the cardiovascular response to ketamine anaesthesia were studied in spontaneously hypertensive (SH) rats. Except in propranolol- and methyldopa-treated animals, blood pressure was increased by ketamine in all groups. The heart rate was increased only in the normotensive control rats, whereas it was reduced in both the unmedicated and all of the medicated SH rats. Plasma renin activity under ketamine anaesthesia was markedly higher in normotensive control rats than in any of the SH groups. The pressor responses to dopamine and metaraminol were not altered by propranolol medication, but hydralazine suppressed these responses, while they were markedly intensified by clonidine and methyldopa treatment. Similarly, small additional doses of ketamine caused a pressor response only in clonidine- and methyldopa-treated animals, whereas in the other groups the response was depressive. The tolerance to haemorrhagic shock during ketamine anaesthesia was notably improved by hydralazine and methyldopa medication, but impaired by propranolol treatment. The results suggest that, with special reference to shock tolerance, hydralazine and methyldopa, and to a lesser extent also clonidine, have a favourable effect on the circulation under ketamine anaesthesia, whereas the action of beta-blockade may be harmful.