Respiratory responses to pH of neonatal hamster and mouse in vitro compared with that of opossum
© BioMed Central Ltd 2001
Received: 2 August 2001
Published: 17 August 2001
In the isolated CNS from neonatal opossum Monodelphis domestica, low pH superfusion increases both respiratory rate and amplitude. One unusual control mechanism in the isolated Monodelphis CNS is that the frequency of respiration depends primarily on changes in the duration of the expiratory rather than the inspiratory phase . To reveal whether this is a characteristic common to other species, we analyzed in isolated hamster and mouse CNS the respiratory responses to pH in terms of variables for amplitude and for timing.
Newborn Golden hamsters and CF1 mice (0-5 days old) were anesthetized with ether and cooled on ice. Their CNS were removed and decerebrated by pontobulbar transections. A vaseline seal at the level of the spinal cord segment C1 allowed selective superfusion of the brainstem. Basal conditions were obtained during brainstem superfusion with a continuos flow of 0.8 ml · min-1 artificial cerebrospinal fluid (aCSF) gassed with 95% O2 and 5% CO2, at 20-24°C (pH7.35-7.40). Fictive respiration was recorded from C3-C5 ventral roots with suction electrodes. Acidification (pH7.3 or lower) of the brainstem superfusion was attained by reducing the aCSF's bicarbonate final concentration.
Acidification of the brainstem superfusion (from pH7.4-7.3) increased the frequency and decreased the amplitude of respiration in both hamster and mouse (P < 0.01, Wilcoxon test). A common feature of these results is that changes in rate are brought about solely by changes in the duration of the expiratory phase. The inspiratory phase remains constant in duration. Thus, cycle duration can be represented by a linear function exclusively depending on the expiratory duration. The slope of the straight line is very close to 1.
These results indicate that the timing of the respiratory cycle in the neonatal hamster and mouse, as well as in the opossum, are controlled by an expiratory instead of an inspiratory "off-switch". Increase in respiratory amplitude in response to chemical stimulation is a distinctive feature of the isolated CNS preparation obtained from newborn opossum.
This work was funded by grants FONDECYT #1010242 and DICYT #029743EL and approved by Bioethics Committee of the Universidad de Santiago de Chile.