Role of sympathetic nerve activity in endotoxin induced hypotension in cats.

Within 10 min of intravenous (iv) injection of E. coli endotoxin (1 mg X kg-1) in alpha-chloralose anaesthetised cats, mean blood pressure (MBP) fell significantly to 86% of the pre-endotoxin level. There followed a gradual decline in MBP, so that after 60 min MBP was depressed to 49% of the pre-endotoxin level. Preganglionic splanchnic nerve (PSN) activity decreased significantly before the rapid fall in MBP. There followed a gradual decrease in PSN activity coincident with the significant fall in MBP, so that 60 min after iv injection of E. coli endotoxin PSN activity fell to 40% of the pre-endotoxin level. Heart rate did not differ significantly from the pre-endotoxin level in the control group. However, the hypotension and the reduction of PSN activity following endotoxin were abolished by intracisternal (ic) pretreatment with phentolamine (0.5 mg X kg-1). The heart rate in this group increased significantly, by 15% at 10 min and was maintained until the end of the experiment. These results indicate that in E. coli endotoxin hypotension the blood pressure falls together with the reduction in sympathetic outflow; and suggests that stimulation of central alpha-adrenergic receptors leads to an inhibition of activity in brainstem sympathetic pathways.