The mechanism underlying potentiation of the pressor action of noradrenaline by some drugs which depress sympathetic tone

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Abstract
The potentiation of the action of noradrenaline on the blood pressure of spinal cats by intravenously injected hexamethonium or bretylium and intrathecally injected procaine, all of which depress sympathetic tone, was accompanied by an increase in cardiac output but no significant change in total peripheral resistance. Acute cardiac sympathectomy itself caused increases in cardiac output and in the pressor action of noradrenaline which were similar to those produced by the drugs. After cardiac sympathectomy or after treatment of spinal cats with pronethalol, the blood pressure responses to noradrenaline were not further modified by bretylium, hexamethonium, or intrathecal procaine. It is concluded that potentiation of the pressor action of noradrenaline by these drugs in the spinal cat results from inhibition of cardiac sympathetic tone.