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.