Influence of nifedipine on xylazine-induced acute pressor response in halothane-anesthetized dogs.
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.[1]References
- Influence of nifedipine on xylazine-induced acute pressor response in halothane-anesthetized dogs. Tranquilli, W.J., Thurmon, J.C., Paul, A.J., Benson, G.J. Am. J. Vet. Res. (1985) [Pubmed]
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