2.2 | Animal preparation
Male rats weighing 400–500 g at the age of 12 weeks received general
anesthesia with isoflurane 2% at 2 L/min O2 plus
intraperitoneal injection with 25% urethane (5 ml/kg) before the
surgical procedures. Anesthetic efficacy was determined by the lack of
withdrawal from painful stimulus on the tail. All animals were placed in
a supine position with a subcutaneous injection of lidocaine
hydrochloride (2%) for local anesthesia. A mid-cervical tracheostomy
was performed using a 16-gauge cannula. A polyethylene tube was placed
into the femoral artery for the continuous recording of arterial blood
pressure and blood sampling. Another catheter was inserted into the
femoral vein for intravenous infusion. After these procedures, all
animals were paralyzed with an intravenous injection of cisatracurium
besylate (0.2 mg/kg), followed by a continuous intravenous infusion of
the same agent (0.05 mg/kg/min). The animals were then sedated with a
continuous intravenous infusion of propofol (0.7 mg/kg/min). The animals
were connected to a volume-controlled animal ventilator (Model 683,
Harvard Apparatus, Holliston, MA, USA) to establish conventional
ventilation. A tidal volume of 6 mL/kg, an inspiratory versus expiratory
ratio of 1:1, a positive end-expiratory pressure (PEEP) of 3 cm
H2O, and a fractional FiO2 concentration
of 1.0 were maintained throughout the experiments. The original
ventilation rate was set at 60 breaths/min, followed by an increment or
decrement of 2–3 breaths/min to maintain an arterial blood
PCO2 within 40 and 50 torr (5.33–6.67 kPa) and pH
> 7.25. The body temperature was maintained at 38 °C–39 °C
via a servo-controlled heating blanket with a probe monitoring the anal
temperature throughout the experiments.