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Am J Physiol Regul Integr Comp Physiol (March 6, 2003). doi:10.1152/ajpregu.00112.2003
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Submitted on March 4, 2003
Accepted on March 5, 2003

Tonic pulmonary stretch receptor feedback modulates both eupnea and gasping in an in situ rat preparation

Michael B Harris* and Walter M St.-John1

1 Department of Physiology, Dartmouth Medical School, Dartmouth Hitchcock Medical Center, Lebanon, New Hampshire, USA

* To whom correspondence should be addressed. E-mail: michael.b.harris{at}dartmouth.edu.

The perfused in situ juvenile rat preparation produces phrenic discharge patterns comparable to eupnea and gasping in vivo. These ventilatory patterns of eupnea and gasping differ in multiple aspects including, most prominently the rate of rise of inspiratory activity. Since gasping, but not eupnea, appeared similar following vagotomy in spontaneous breathing preparations, it has been assumed that gasping was unresponsive to afferent stimuli from pulmonary stretch receptors. In the present study, efferent activity of the phrenic nerve was recorded during eupnea and gasping in the in situ juvenile rat preparation. Gasping was induced in hypoxic-hypercapnia or ischaemia. An increase in the pressure of tonic lung inflation from 1 to 10 cm H20 caused a prolongation of the duration between phrenic bursts in both eupnea or gasping. Bilateral vagotomy eliminated these changes. We conclude that the neural substrate mediating the Hering-Breuer reflex is retained in the in situ preparation, and that the brainstem circuitry generating the respiratory patterns respond to tonic activation of pulmonary stretch receptors in a similar manner in eupnea and gasping. These findings support the homology of eupnea-like phrenic discharge patterns in the reduced in situ preparation and eupnea in vivo, and disprove the common supposition that gasping is insensitive to vagal afferent feedback from pulmonary stretch receptor mechanisms.




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