Abstract
Respiratory long-term facilitation (LTF) is a long-lasting (> hr) augmentation of respiratory motor output that occurs even after cessation of hypoxic stimuli. Although ventilatory LTF has been reported in mice, phrenic LTF has not been reported in this species. We examined whether phrenic LTF can be observed in mice, a promising animal species for the future study with genetic engineering. Phrenic nerve activity was recorded with a pair of silver hook electrodes in urethane (1.3 g/kg, ip)-anesthetized, vagotomized, paralyzed (0.2 mg/kg panchronium bromide for every hour, ip), and artificially ventilated C57BL/6 male mice. Volume and frequency of the artificial ventilation was set so that the animal's PaCO2 would be just above the apneic threshold. Isocapnic condition was maintained throughout the experiment by adding appropriate CO2 to the inhaled gas mixture so that changes in expired PCO2 would not exceed 0.2% from the baseline. We examined possible effect of episodic hypoxia (EH, inhalation of the gas mixture of FIO2 = 0.10-0.15 and FICO2 = 0.005-0.01 for 45 sec) and intermittent hypoxia (IH, 5 times of EH separated by 5 min of 100% O2) on phrenic nerve activity for 1 hour. Amplitude of the integrated phrenic nerve activity gradually increased after cessation of IH. An increase in the burst frequency of the phrenic nerve activity was not so apparent. EH, on the other hand, did not affect amplitude or frequency of the phrenic nerve activity. We conclude that phrenic LTF occurs in mice as in a similar manner to that in cats and rats. [J Physiol Sci. 2008;58 Suppl:S118]
