Cardiovascular responses to phenylephrine and sodium nitroprusside during acute coronary occlusion in dogs

The effects of administration of pressor agent phenylephrine (PE) and depressor agent, sodium nitroprusside (SNP) (10-40 micrograms/kg) on arterial blood pressure (ABP) and heart rate (HR) were investigated during acute occlusion of left anterior descending coronary artery (LAD) in anaesthetized, artificially ventilated dogs with and without the influence of selective blockade of autonomic nervous system (ANS). ABP response to PE was significantly (P < 0.05) attenuated following 4 hrs of LAD occlusion in all the four groups of animals. SNP response at higher dose (40 micrograms/kg) was also significantly (P < 0.05) attenuated 4 hrs after LAD occlusion in ANS intact, beta-blocked and atropinized groups. The bradycardia response to PE after LAD occlusion was abolished in vagotomized group while in the other three groups, it was significantly attenuated following 4 hrs of LAD occlusion. The tachycardia response to SNP was significantly (P < 0.05) attenuated 4 hrs after LAD occlusion in ANS intact and atropinized animals. The response was abolished in beta-blocked animals and no significant change occurred (P > 0.05) in vagotomized group. This study suggests that the cardiovascular reflex effects of PE and SNP are significantly attenuated following acute LAD occlusion. Blocking any of the components of ANS changed this responsiveness.