Paradoxical Effect of Acetylcholine and Noradrenaline on Heart rate of Anaesthetized Rabbits, Introduction Normal function of biological tissues depends on perfusion of blood. While poor perfusion is associated with tissue damage like cerebral hypoperfus

Methods

The effects of ACh, NA and Adr were tested in vivo by intra venous injection into anaesthetized rabbits while blood pressure was continuously recorded through an arterial cannula. Rabbits were anaesthetized with ketamine (60 mg/kg, i.m.) and xylazine (8 mg/kg, i.m.) and maintained using 20 mg/kg ketamine and 2 mg/kg xylazine dosed to effect. The rabbits’ trachea were cannulated to maintain a patent upper airway and allow for manual ventilation if needed. Body temperature (by rectal thermometer), and end tidal O₂ and CO₂ monitored to determine anesthesia depth and the animals condition. All animal procedures were approved by the university animal ethics committee.

To measure BP the femoral artery was cannulated and connected to a pressure transducer (ADIstruments). Continuous BP was recorded on computer using Power Lab (ADInstruments) and Chart software (ADInstruments). Heart rate was determined by counting the pulse in the BP signal, pulse pressure (PP) was calculated from the average cyclic height of the pulse wave and used as an index of SV (Levy el al. 2005). Recorded BP was integrated over each heart beat to measure true MAP.

The rabbit’s ear vein was cannulated for the injection of ACh, NA and Adr. Drugs were diluted in normal saline (0.9% NaCl) and flushed in with an additional 1-2 ml of saline. A 30 second control period was recorded before the injection of each drug and 3 min of datawere recorded after each injections. At least 10 min was left after BP had returned to pre-injection levels before the next does was given. Control (pre-injection) values were the average of the beat by beat HR, PP & MAP over the 20 sec before injection of each drug. The post injection response was taken as the peak response to the injected drug in each animal. Differences before vs after injection were assessed by paired t-test with all data shown as mean ± SEM were n is the number of animals.