Physiological and neurochemical adaptations following abrupt termination of chronic hypercapnia in goats

Abstract

Chronic hypercapnia (CH) is a hallmark of respiratory diseases such as chronic obstructive pulmonary disease. In such patients, mechanical ventilation is often used to restore normal blood-gas homeostasis. However, little is known regarding physiological changes and neuroplasticity within physiological control networks after termination of CH. Utilizing our goat model of increased inspired CO₂-induced CH, we determined whether termination of CH elicits time-dependent physiological and neurochemical changes within brain stem sites of physiological control. Thirty days of CH increased [Formula: see text] (+15 mmHg) and steady-state ventilation (SS V̇i; 283% of control). Within 24 h after terminating CH, SS V̇i, blood gases, arterial [H⁺], and most physiological measurements returned to control. However, the acute ventilatory chemoreflex (ΔV̇i/Δ[H⁺]) was greater than control, and measured SS V̇i exceeded ventilation predicted by arterial [H⁺] and ΔV̇i/Δ[H⁺]. Potentially contributing to these differences were increased excitatory neuromodulators serotonin and norepinephrine in the nucleus tractus solitarius, which contrasts with minimal changes observed at 24 h and 30 days of hypercapnia. Similarly, there were minimal changes found in markers of neuroinflammation and glutamate receptor-dependent neuroplasticity upon termination of CH, which were previously increased following 24 h of hypercapnia. Thus, following termination of CH: 1) ventilatory, renal, and other physiological functions rapidly return to control; 2) neuroplasticity within the ventilatory control network may contribute to the difference between measured vs. predicted ventilation and the elevation in the acute ventilatory [H⁺] chemoreflex; and 3) neuroplasticity is fundamentally distinct from acclimatization to CH.NEW & NOTEWORTHY In healthy adult goats, steady-state ventilation and most physiological measures return to control within 24 h after termination of chronic hypercapnia (CH). However, the acute [H⁺] chemoreflex is increased, and measured ventilation exceeds predicted ventilation. At 24 h of recovery, excitatory neuromodulators are above control, but other measured markers of neuroplasticity are unchanged from control. Our data suggest that CH elicits persistent physiological and neurochemical changes for up to 24 h after termination of CH.

Keywords: deacclimatization; hypercapnia; neuromodulation; ventilatory control.

Figure 1.

Deacclimatization processes from chronic hypercapnia rapidly occurred for steady-state minute ventilation (V̇i), breathing frequency (f), and tidal volume (Vt). A: 4 h after abrupt termination of chronic hypercapnia, V̇i decreased from ∼283% of control to ∼10% of control and changed minimally thereafter. B and C: during the transition from 30 days of hypercapnia to 4 h of room air recovery, return to control in V̇i was mediated by a decline in both f and Vt. Dotted line provides a reference to control values obtained prior to the hypercapnic period. Ventilatory measurements were obtained from 9 female adult goats. *Significant difference compared with room air control. #Significant difference compared with day 30 of hypercapnia. Significance (P < 0.05) was derived from one-way repeated-measures ANOVA.

Figure 2.

Recovery of arterial blood gases and acid-base status was completed within 24 h of deacclimatization from chronic hypercapnia. Due to a nonfunctioning catheter in 2 goats, blood gases were measured in only 7 of the 9 female adult goats. A: at 4 h of room air recovery, arterial [H⁺] decreased by 3 nmol/L from chronic hypercapnia, returning values to control values, and did not significantly change thereafter. B: at 4 h of recovery, arterial [$\text{HC}{\text{O}}_3^{-}$] was still significantly elevated compared with room air controls but had returned to room air control levels by 24 h of recovery. C: $\text{P}{\text{a}}_{\text{CO}}{}_{{}_2}$ was elevated relative to room air controls at 4 h of recovery but returned to control levels by 24 h of recovery. D: arterial Po₂ immediately returned to control levels 4 h following the return of inspired CO₂ (InCO₂) to room air and changed minimally thereafter. Dotted line provides a reference to control values obtained prior to the hypercapnic and normocapnic recovery period. *Significant difference compared with room air control. #Significant difference compared with day 30 of hypercapnia. Significance (P < 0.05) was derived from one-way repeated-measures ANOVA.

Figure 3.

Twenty-four hours following termination of hypercapnia, the slope of the ventilatory (V̇i)/arterial [H⁺] relationship was increased compared with room air control. The first points for each line are steady-state V̇i and arterial [H⁺] during the chronic steady state. The additional three points are V̇i and [H⁺], obtained when inspired CO₂ (InCO₂) was increased above steady-state levels during an acute chemoreflex challenge. The slope of the line represents the sensitivity of the ventilatory system to changes in [H⁺] (acute chemoreflex; ΔV̇i/Δ[H⁺]). At 30 days of hypercapnia, the relationship was significantly shifted left compared with room air control without a change in sensitivity. At 24 h of recovery, there was a nonsignificant shift in the relationship between ventilation and arterial [H⁺] compared with room air control. However, at 24 h of recovery, the slope of the relationship or the sensitivity to acute changes in arterial [H⁺] was increased compared with room air controls. Inset: slopes (ΔV̇i/Δ[H⁺]) of goats studied at room air control and 24 h of recovery. Due to a nonfunctioning catheter in 2 goats, ΔV̇i/Δ[H⁺] was measured in only 7 of the 9 female adult goats. P value for the shift in the relationship was derived from an analysis of covariance (ANCOVA). Significance for ΔV̇i/Δ[H⁺] (P < 0.05) was derived from a paired t test.

Figure 4.

Actual ventilation significantly exceeded predicted minute ventilation during 30 days of chronic exposure to 6% inspired CO₂ (InCO₂) and 24 h after return to normocapnia. Predicted ventilation was calculated by use of the steady-state level of arterial [H⁺] as well as the acute chemoreflex obtained by increasing inspired CO₂ above the steady-state level. At 24 h following the transition from hypercapnia to normocapnia, measured ventilation significantly exceeded predicted ventilation, suggesting a disconnect between the steady-state ventilation and the acute chemoreflex and a maintained elevated ventilatory set point, as previously observed in chronic hypercapnia. Due to a nonfunctioning catheter in 2 goats, predicted ventilation was measured in only 7 of the 9 female adult goats. Dotted line provides a reference to control values obtained prior to the hypercapnic and normocapnic recovery period. *Significant difference of actual ventilation compared with predicted ventilation at that time point. P value shown was derived from a two-way ANOVA (predicted vs. actual ventilation and time as factors).

Figure 5.

Excitatory neuromodulators and their metabolites in the nucleus tractus solitarius (NTS) were greater after 24 h of recovery compared with 30 days of chronic hypercapnia. Bulk tissue HPLC analysis within the NTS showed a significant increase between 24-h recovery female adult goats (n = 6) compared with 30-day hypercapnic female adult goats (n = 6) in norepinephrine (A), serotonin (B), dopamine (D), and 3,4-dihydroxyphenyl acetic acid (DOPAC; E). There was a slight but nonsignificant increase in 5-hydroxyindoleacetic acid (HIAA; C) within the NTS (P = 0.063). P value represents the significance term from an unpaired t test. *P < 0.05.

Figure 6.

Serotonin (5-HT) metabolism is decreased within the medullary raphe (MR) at 24 h of recovery. Within the MR (a primary site of 5-HT synthesis) in the 24-h recovery goats, there was a significant decrease in the 5-hydroxyindoleacetic acid (HIAA)/5-HT ratio (index of 5-HT metabolism) compared with goats exposed to 30 days of hypercapnia. Within the ventrolateral medulla (VLM), there was a nonsignificant change in this ratio between the 2 groups of animals. Changes in the HIAA/5-HT ratio may contribute to elevated levels of 5-HT. P value represents the significance term from an unpaired t test. *P < 0.05.

Figure 7.

Selected enzymes of tryptophan metabolism and total number of neurons remained decreased 24 h after termination of chronic hypercapnia. Within the rostral (+2–4 mm from obex) regions of the medullary raphe (MR; A) and ventrolateral medulla (VLM; B), enzymes tryptophan hydroxylase (TPH), indolamine 2,3-dioxygenase (IDO), and total neuron number (NeuN) were significantly reduced in 24-h recovery goats compared with 30-day room air control goats. However, in both the rostral MR (C) and VLM (D), there was no significant difference between 30-day hypercapnia adult female goats (n = 6) and 24-h (n = 6) adult female recovery goats. Significance was derived from one-way ANOVA with *P < 0.05.