Chemoreceptors maturation

Figure 2 Summary of rat carotid chemoreceptor maturation. All plots depict responses to a strong hypoxia stimulus. Hypoxia-induced CSN activity, catecholamine secretion, and intracellular calcium responses all increase over approximately the same time course. (Data adapted from Refs. 10,12,45,68.)...

An obvious possibility to explain carotid chemoreceptor maturation is that the sensitivity of the O2 sensor itself increases with age. This question cannot be addressed at this time because the identity of the type 1 cell O2 sensor remains unknown, even for the mature carotid body. As discussed above, available evidence suggests that type I cell membrane potential is regulated, in large part, by several K" " currents, some of which are inhibited by hypoxia (89,92,97,98) (Fig. 1). Whether the K" " channels themselves are O2 sensitive or linked to a separate O2 sensor is unknown. Maturational changes in O2 sensitivity of hypoxia-sensitive K" " eurrents could occur by shifts in K" " channel subiuiits, developmental shifts in expression of K" " channel subtypes with different O2 sensitivity, and a variety of other meeh-anisms. However, these possibilities have not been explored. 

IV. Fetal Arterial Oxygen Tension Implications for Carotid Chemoreceptor Maturation... 

Carroll JL, Bamford OS, Fitzgerald RS. Postnatal maturation of carotid chemoreceptor responses to O2 and CO2 in the cat. J Appl Physiol 1993 75 2383-2391. 

Kholwadwala D, Donnelly DF. Maturation of carotid chemoreceptor sensitivity to h3q)oxia in vitro studies in the newborn rat. J Physiol 1992 453 461-473. 

Postnatal Maturation of the Carotid Chemoreceptor O2 Sensitivity at the Cellular Level... 

In every species studied to date, the carotid chemoreceptor response to hypoxia increases with postnatal age. In in vitro carotid body preparations, the peak nerve discharge in response to a potent hypoxia stimulus increases about fourfold in neonatal rats during the first month, with most of that change occurring by 2 weeks (10,12,45) (Fig. 2). Similar patterns of postnatal maturation of carotid sinus nerve... 

Hypoxia-induced neurosecretion firom glomus cells is mediated by a rise in intracellular calcium, which resembles the CSN activity response to hypoxia in its hyperbolic shape (37,44,68), PO2 range, and relationship to neurosecretion (69,70). Because neurosecretion and (presumably) afferent nerve activity depend on the type I cell [Ca ]i response to hypoxia, age-related changes in this crucial transduction step could account for maturation of O2 sensitivity. The original hypothesis of the first studies in this area was simply that a major site of chemoreceptor resetting and development of oxygen sensitivity, within the carotid body, lies in the type I cell. 

Maturation of [Ca +] Responses to CO2 and O2-CO2 Interaction Several studies (in vivo) have shown that the carotid chemoreceptor neural response to CO2 is also weak in the newborn and increases with age (11,14). Therefore, it was proposed that maturation of carotid chemoreceptor hypoxic sensitivity may be related to development of O2-CO2 interaction (74). In an early study of rats 1-14... 

Fetal sheep carotid sinus nerve (13) and fetal rat type I cell [Ca ]i (68) responses to hypoxia are weak compared to mature postnatal responses. The consensus view is that carotid chemoreceptor O2 sensitivity is suppressed or adapted to the normally low Pa02 of the mammalian fetal environment ( 23-27 mmHg). Nature s design makes sense, from a teleological perspective to disable carotid chemoreceptor... 

Wasicko MJ, Stemi LM, Bamford OS, Montrose MH, Carroll JL. Resetting and posmatal maturation of oxygen chemosensitivity in rat carotid chemoreceptor cells. J Physiol 1999 514(Pt 2) 493-503. 

Schramm CM, Grunstein MM. Respiratory influence of peripheral chemoreceptor stimulation in maturing rabbits. J Appl Physiol 1987 63 1671-1680. 

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