title: Influence of nifedipine on xylazine-induced acute pressor response in halothane-anesthetized dogs.
abstract: Effects and interaction of nifedipine (Ca channel blocker) and xylazine (mixed alpha agonist) during halothane anesthesia were examined in 6 dogs. After achievement of steady-state halothane (1.35%) anesthesia, blood pressure (BP) and heart rate (HR) were recorded in these dogs during 3-minute saline or nifedipine (20 micrograms/kg) infusion periods. Seven minutes after the end of saline or nifedipine infusion, xylazine (1.1 mg/kg of body weight) was infused over a 2-minute period. After saline pretreatment, xylazine administration increased diastolic BP (33.67 +/- 3.91 mm of Hg) and decreased HR. Nifedipine infusion induced a transient reduction in BP, accompanied by a more persistent increase in HR. Compared with saline pretreatment, nifedipine pretreatment significantly decreased the acute increase in diastolic BP (33.67 +/- 3.91 vs 14.00 +/- 2.94 mm of Hg) which occurred during xylazine injection. After saline and nifedipine infusions, xylazine administration decreased HR 30 +/- 15.02 and 36.5 +/- 10.36 beats/min, respectively. A pronounced sinus arrhythmia and/or 2nd-degree atrioventricular block developed in all dogs during xylazine injection after saline infusion. Arrhythmias were not observed in the dogs after nifedipine infusion. Nifedipine's Ca blocking action depressed xylazine-induced acute vasoconstriction and concomitant increase in diastolic BP. Because alpha 2-, but not alpha 1-adrenoceptor-mediated vasoconstriction is Ca-dependent, these results indicate that a portion of the acute pressor response induced by IV xylazine in halothane-anesthetized dogs may be alpha 2-mediated. Seemingly, nifedipine-induced hypotension and damping of xylazine-induced increases in BP attenuated xylazine's actions on cardiac rate and rhythm.
mesh_terms: D000768:Anesthesia, General / Q000662:veterinary*; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D002245:Carbon Dioxide / Q000097:blood; D004285:Dogs / Q000502:physiology*; D004347:Drug Interactions; D005260:Female; D006221:Halothane; D006339:Heart Rate / Q000187:drug effects; D006439:Hemodynamics / Q000187:drug effects*; D008297:Male; D009543:Nifedipine / Q000494:pharmacology*; D010100:Oxygen / Q000097:blood; D010313:Partial Pressure; D013843:Thiazines / Q000494:pharmacology*; D014991:Xylazine / Q000494:pharmacology*
chemical_list: D013843:Thiazines; D002245:Carbon Dioxide; D014991:Xylazine; D009543:Nifedipine; D010100:Oxygen; D006221:Halothane