Effect of graded vagal blockade and pulmonary volume on tonic inspiratory activity in rabbits

Abstract

Reflex respiratory responses to increased and decreased tracheal pressure have been studied at bilateral local vagus temperatures between 30 degrees C and 0 degrees C in anaesthetized spontaneously breathing rabbits. The temperature of each cervical vagus nerve was governed by separate thermodes, which were cooled by Peltier elements and heated by transistors. In this study, the lowest activity level of the diaphragm during expiration was considered to reflect tonic inspiratory activity. As an indicator for this level, the longest interspike interval in the electromyogram of the diaphragm during a breath was used. Tonic inspiratory activity at tracheal pressures below -1.2 kPa was higher with vagi intact than after vagotomy. This observation indicates that disinhibition, due to the decreased activity of pulmonary stretch receptors, occurring during the Hering-Breuer deflation reflex, is an insufficient explanation for the facilitation of tonic inspiratory activity. When local vagus temperatures are 8 degrees C, and tracheal pressure has been increased or decreased, tonic inspiratory activity is higher than it would be with vagi at 0 degrees C or cut. Two reflex mechanisms that might explain the observed high tonic inspiratory activity are discussed.

Histamine-induced end-tidal inspiratory activity and lung receptors in cats

“…In rabbits, ETIA has been evoked in response to a decreased Ptr [7] and after administration of histamine, both in the diaphragm and in ICMs [10]. In man, ETIA has been observed after administration of histamine [6].…”

“…These studies have shown a significant correlation between the increase in thoracic gas volume and the increase in ETIA. Recent studies have shown that, with intact vagus nerves, lowering of tracheal pressure (P tr ) could evoke ETIA in rabbits [7] and cats [8,9]. Stimulation of thin vagal afferents by administration of histamine increased the magnitude of ETIA in rabbits [10].…”

Effects of local nerve cooling on conduction in vagal fibres shed light upon respiratory reflexes in the rabbit

“…The present study originated from an earlier one [ 10] in which lung inflation as well as deflation reflexly increased the tonic activity of the diaphragm after bilateral local vagus cooling to 8 ~ We hypothesized that the vagal A fibres were completely blocked at 8 ~ and that the C fibres conducted the reflex stimuli. The present results are consistent with this hypothesis since in eight nerves the A fibres were completely blocked at 8 ~ in the other two this occurred at 7.0 and 6.8 ~ (Table 1).…”

“…Vagotomy abolished these effects. Previous studies in the rabbit [6,10] showed that bilateral local vagus cooling to 8 ~ blocked the Hering-Breuer reflex and increased the tonic activity of the diaphragm, that is the level of activity to which the diaphragm returns during expiration. The tonic activity of the diaphragm could be increased further by both lung inflation [2,10] and deflation [10].…”

“…Previous studies in the rabbit [6,10] showed that bilateral local vagus cooling to 8 ~ blocked the Hering-Breuer reflex and increased the tonic activity of the diaphragm, that is the level of activity to which the diaphragm returns during expiration. The tonic activity of the diaphragm could be increased further by both lung inflation [2,10] and deflation [10]. The influence of local vagus cooling on conduction in myelinated (A) and non-myelinated (C) vagal fibres has been studied in the cat 11,7] and the dog [4,5].…”