Differences in the control of breathing between Himalayan and sea-level residents

Om hvordan langvarig høydeopphold utvisker sensitiviteten til CO2.

http://jp.physoc.org/content/588/9/1591.full

Highlanders had lower mean ± S.E.M.ventilatory sensitivities to CO2 than lowlanders at both isoxic tensions (hyperoxic: 2.3 ± 0.3 vs. 4.2 ± 0.3 l min−1 mmHg−1, P = 0.021; hypoxic: 2.8 ± 0.3 vs. 7.1 ± 0.6 l min−1mmHg−1, P < 0.001), and the usual increase in ventilatory sensitivity to CO2 induced by hypoxia in lowlanders was absent in highlanders (P = 0.361).

Furthermore, the ventilatory recruitment threshold (VRT) CO2 tensions in highlanders were lower than in lowlanders (hyperoxic: 33.8 ± 0.9 vs. 48.9 ± 0.7 mmHg, P < 0.001; hypoxic: 31.2 ± 1.1 vs. 44.7 ± 0.7 mmHg, P < 0.001).

We conclude that control of breathing in Himalayan highlanders is distinctly different from that of sea-level lowlanders. Specifically, Himalayan highlanders have decreased central and absent peripheral sensitivities to CO2. Their response to hypoxia was heterogeneous, with the majority decreasing their VRT indicating either a CO2-independent increase in activity of peripheral chemoreceptor or hypoxia-induced increase in [H+] at the central chemoreceptor.

Control of breathing in humans can be broadly divided into chemoreflex and non-chemoreflex drives to breathe (Fig. 1) (Lloyd & Cunningham, 1963). Non-chemoreflex breathing stimuli include a wakefulness drive (Longobardo et al. 2002), voluntary (cortical) drive (Shea, 1996) and hormonal factors (Jensen et al. 2008), as well as neural and humoral mediating factors that are especially important in the control of breathing during exercise (Bell, 2006; Dempsey, 2006; Haouzi, 2006). The chemoreflex drive to breathe can be further divided into central and peripheral chemoreceptor drives. Both central and peripheral chemoreceptors respond to changes in the hydrogen ion concentration ([H+]) in their immediate environments (Torrance, 1996; Nattie & Li, 2009).

In contrast to the central chemoreceptors, peripheral chemoreceptors are also sensitive to changes in arterial Graphic (Graphic) via a hypoxia-mediated increase in their sensitivity to [H+] (Cunningham, 1987; Torrance, 1996; Kumar & Bin-Jaliah, 2007), and hyperoxia (Graphic ≥ 150 mmHg) effectively silences this response (Lloyd & Cunningham, 1963; Mohan & Duffin, 1997).

Central and peripheral chemoreceptor neural drives are integrated in the medulla to provide the total chemoreflex neural drive (Fink, 1961; Shea, 1996; Mohan & Duffin, 1997; Orem et al. 2002) that, in combination with non-chemoreflex drives, provides ventilatory drive to respiratory muscles.


Figure 3. Hypercapnic ventilatory response The graph displays two isoxic responses: hyperoxic (Graphic = 150 mmHg) representing central chemoreflex response, and hypoxic (Graphic = 50 mmHg) representing the addition of central and peripheral chemoreflexes responses. The slope of each isoxic response represents sensitivity of the chemoreflex to CO2. The inflection point at which ventilation starts to increase in response to increasing Graphic is the ventilatory recruitment threshold (VRT), where the chemoreflex neural drive to breathe exceeds a drive threshold and starts to produce an increase in pulmonary ventilation. Ventilation below VRT represents non-chemoreflex drives to breathe and is known as the basal ventilation. The differences in ventilation between isoxic rebreathing lines at any given isocapnic Graphic can be used to calculate the hypoxic ventilatory response (indicated by vertical arrows). Note that the choice of isocapnic Graphic affects the magnitude of the measured HVR even within the same subject (Duffin, 2007), with higher HVRs measured at higher isocapnic Graphicvalues in the illustrated example. Note also that the magnitude of HVR provides little information about the characteristics of the control of breathing model, as HVR magnitude is dependent on the combination of central and peripheral chemoreflex responses.

There was no difference in the non-chemoreflex drives to breathe between highlanders and lowlanders, as indicated by similar basal (below VRT) ventilations in the two populations (Table 2).

The highlanders had decreased VRTs compared to lowlanders during both hypoxic and hyperoxic rebreathing tests. The leftward shift of the VRTs in highlanders suggests that a lower Graphic was required to exceed the VRT in highlanders compared to lowlanders. Since both central and peripheral chemoreceptors are actually [H+] sensors, interpretation of this result should consider the acid–base status in both populations. According to the Henderson–Hasselbach equation (Nunn, 1993), the relationship between [H+] and Graphiccan be described as follows:Formulawhere [HCO3] is the bicarbonate ion concentration. In a hypothetical sea-level resident at sea-level, [H+] is approximately 40 nM l−1, Graphic is 40 mmHg and [HCO3] is 24 mM l−1. At altitude, hypoxia-induced hyperventilation results in a reduction of Graphic that leads to a reduction in [H+] and respiratory alkalosis according to the above equation. Highlanders compensate for respiratory alkalosis by presumably reducing their [HCO3] through increased renal excretion, thereby restoring the Graphic/[HCO3] ratio to sea-level values and normalizing [H+].

For example, if hypoxia-induced hyperventilation reduced CO2 from 40 to 30 mmHg, and HCO3− fell from 24 to 18 mM l−1, then the overall ratio of CO2/HCO3− would be maintained at 5/3 as in sea-level lowlanders, but normal [H+] of 40 nM l−1 would be achieved at a lower Graphic of 30 mmHg rather than 40 mmHg, as at sea-level. Considering that the highlanders in our study have an adapted acid–base status (Santolaya 1989), the observed difference in VRTs can be explained by the altered [H+]–Graphic relationship in highlanders compared to lowlanders, with the assumption that the chemoreceptor thresholds [H+] are similar (Duffin, 2005).

The sensitivity of the central chemoreceptor to CO2, as indicated by the ventilatory sensitivity during hyperoxic rebreathing, was lower in highlanders compared to lowlanders (2.5 ± 0.4 vs. 4.2 ± 0.3 l min−1 mmHg−1, P = 0.011). Hyperoxia effectively silences the peripheral chemoreceptor (Lloyd & Cunningham, 1963; Mohan & Duffin, 1997), and therefore the ventilatory sensitivity measured during hyperoxic rebreathing can be taken as a measure of central chemoreceptor sensitivity (Duffin, 2007).

Since the ventilatory response to hypercapnia decreases with age (Nishimura et al. 1991; Jones et al. 1993; Poulin et al.1993; McGurk et al. 1995) and increases with weight (Marcus et al. 1994), the observed lower central CO2 chemosensivity in our highlander subjects may be the result of their older age and smaller body size.

Ventilatory response to hypoxia in highlanders was markedly different from that in lowlanders. Unlike lowlanders, who responded to hypoxia by increasing the sensitivity of their ventilatory response to CO2 (Mohan & Duffin, 1997), the highlanders seemed to decrease their VRT in response to hypoxia with no change in the sensitivity to CO2.

The ventilatory response to hypoxia in sea-level residents is mediated by peripheral chemoreceptors via an increase in the peripheral chemoreceptor sensitivity to [H+] (Cunningham, 1987; Torrance, 1996; Kumar & Bin-Jaliah, 2007). A lack of increase in ventilatory sensitivity to CO2 with induction of hypoxia in highlanders suggests that their peripheral chemoreceptors are relatively insensitive to CO2.

Other possible mechanisms of CO2-independent peripheral responses to hypoxia may include a hypoxia-induced increase in the carotid body tonic drive to breathe, changes in systemic hormonal mediators, an altered cerebral spinal fluid-buffering capacity at the central chemoreceptor or an alteration in cerebral vascular reactivity leading to a higher [H+] at central chemoreceptor.

Specifically, we showed that Himalayan highlanders have decreased central and absent peripheral chemoreceptor sensitivity to CO2, and that they are sensitive to hypoxia, albeit via a different mechanism than that observed in lowlanders at sea-level. A blunted central and an absent peripheral ventilatory sensitivity to CO2 in Himalayan highlanders may stabilize their ventilatory controller by reducing the overall gain in the feedback part of the controller circuit, thereby reducing altitude-related breathing instability (Ainslie & Duffin, 2009)

The non-chemoreflex drives to breathe were similar between Himalayan highlanders and sea-level lowlanders.

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