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Hypoxic inhibition of O-sensitive K channels plays a key role in mediating numerous cellular responses which counteract the deleterious effects of hypoxia. In type I cells of the carotid body (CB), a neurosecretory organ that responds to hypoxia by releasing neurotransmitters from specialized O-sensing type I cells onto sensory nerve endings, hypoxic inhibition of K channels underlies the membrane depolarisation (Lopez-Barneo et al., 1988) that stimulates Ca entry and neurotransmitter release (Urena et al., 1994). In other neurosecretory cells, such as those located in the neuroepithelial cell bodies of the lung (Youngson et al., 1993) and the adrenal medulla (Thompson and Nurse, 1998), hypoxic inhibition of K channels provides a critical link between O levels and the appropriate cellular responses.

Oxygen sensitivity of voltage-dependent K channels (Kv channels) in chemosensory glomus cells is responsible for hypoxic chemotransduction processes in the carotid body. Human studies in twins and in individuals over time suggest that hypoxic sensitivity of the carotid body is genetically controlled (Collins et al., 1978; Kawakami et al., 1982; Nishimura et al., 1991; Thomas et al., 1993). The concept is further confirmed in the studies using inbred strains of mice (Tankersley et al., 1994; Campen et al., 2004) and rats (Weil et al., 1998) which are genetically almost identical within a strain. In these studies, respiratory or cardiovascular responses to hypoxia vary among several strains, but are similar within a strain. Thus, some proteins which are differentially expressed in individuals due to genetic differences likely cause variable carotid body responses. We have hypothesized that differential expression of oxygensensitive Kv channels contributes to the differences in hypoxic sensitivity of DBA/2J and A/J strains of mice.

Carotid body (CB) chemoreceptors sense arterial PO and PCO/pH becoming activated in hypoxic hypoxia and in all types of acidosis. The sensing structures in the CB are chemoreceptor cells (CBCC), which are connected synaptically with the sensory nerve endings of the carotid sinus nerve (CSN). In situations of hypoxia and acidosis, CBCC are activated and their rate of release of neurotransmitters (NT) increase, promoting an increase in the activity of the CSN and subsequent ventilatory and cardiovascular reflexes (5).

In control ventilation, chemical stimuli are paramount in setting the level of ventilation. These are primarily changes in concentration of hydrogen ions, as well as changes in PO2 and PCO2. A number of receptors, both in the periphery and in the central nervous system, respond to these changes. Of the three socalled “chemical” stimuli, the response to CO2 is most prominent and for any 1 mm change in PCO2, ventilation changes by about 2 to 2.5 L/min. The ventilatory response to hypoxia becomes quite prominent once arterial PO2 has reached values of about 60 mm Hg. It has been well documented that the primary effect of hypoxia is stimulation of the carotid chemoreceptors with transmission of signal to the NTS. With acute hypoxia, there is a biphasic ventilatory response with an initial hyperventilation followed by a fall in ventilation, the so-called “roll-off”, to values above those in the pre-hypoxic level. This biphasic response is present in man as well as in a large number of other mammals tested and central neurotransmitters are essential in this response (4,5,7-10). This presentation will concentrate on the effects of acute hypoxia on the ventilatory response in anesthetized dog and rat, and the relationship between the ventilatory response and the release of neurotransmitters in the central nervous system, but in particular, in the medial chemosensitive area on the ventral surface of the medulla and summarizes the work from our laboratory from the past decade. The amino acids of interest are those that excite ventilation, which are primarily glutamate and aspartate; and those that depress ventilation, which include GABA, taurine, and glycine (3). Earlier work from this laboratory showed that inhibition of glutamate by intravenous administration of the specific NMDA receptor antagonist MK801 in anesthetized dog leads to a significant reduction in the hyperventilatory response to hypoxia (1). The subsequent studies in anesthetized rat showed that ventricular cisternal perfusion of MK801 abolished hyperventilatory response of acute hypoxia and infusion of GABA antagonist bicuculline caused augmentation of the hyperventilatory response to acute hypoxia and the “roll-off” was no longer observed (10).

Chronic intermittent hypoxia (CIH), characterized by short episodes of hypoxia followed by normoxia, is a common feature of obstructive sleep apnea (OSA). It has been proposed that CIH enhances the hypoxic ventilatory response (HVR) leading to hypertension, upregulation of catecholaminergic and reninangiotensin systems (Fletcher, 2000; Prabhakar and Peng, 2004). Most of the information of the effects of CIH on peripheral chemoreflex control of cardiovascular and respiratory systems has been obtained from studies performed on OSA patients. However, conclusions from these studies are conflictive because of comorbidities associated with OSA (Narkiewicz et al., 1999). Experiments performed in rats showed that CIH enhances HVR (Ling et al., 2001) and produces long-term facilitation of respiratory motor activity (McGuire et al., 2003; Peng & Prabhakar, 2003). The facilitator effect of CIH on HVR has been attributed to a potentiation of the carotid body (CB) chemosensory responses to acute hypoxia. However, it is a matter of debate if the ventilatory potentiation induced by CIH is due to a CB enhanced activity or secondary to central facilitation of chemosensory input. Peng et al., (2001) found that basal CB discharges and chemosensory responses to acute hypoxia were enhanced in rats exposed to a pattern of 5% O for 15s followed by normoxia for 5 min, repeated 8 hours/day for 10 days. However, this observation has not been confirmed in other animal models of CIH. Using a protocol of short hypoxic episodes, we studied the effects of CIH on cat cardiorespiratory reflexes and CB chemosensory responses induced by hypoxia.

The group of Loeschcke established that the superficial ventrolateral medulla contains chemosensitive regions (see the reviews [1-3]). Later, the group of Nattie conducted experiments of acetazolamide microinjection that induced local tissue acidosis, and found that the midline region (raphe), nucleus tractus solitarii and locus coeruleus are also chemosensitive (see the review [4]). To map the medullary chemosensitive regions using a more physiological stimulation technique, we microinjected CO2-enriched saline into various regions of the in vivo and in vitro rat medulla, and found that the superficial midline, parapyramidal and ventrolateral regions are chemosensitive [5]. These findings extend other microinjection studies [6,7] that showed only that the superficial ventrolateral medulla is chemosensitive.

Considerations from control theory revealed that an elevated gain of the feedback loop may lead to instability of the respiratory system, e.g. during sleep [Longobardo et al.,1982; Honda et al., 1983; Khoo, 2000; Dempsey et al., 2004]. In respiratory medicine, the carbonic anhydrase (CA) inhibitor acetazolamide is known to reduce the incidence of apneas in mountain sickness [Swenson et al., 1991]or sleep disordered breathing [Tojima et al., 1988; Verbraecken et al., 1998]. Other clinical studies revealed that patients prone to sleep apnea showed an increased sodium/proton exchange activity in their lymphocytes [Tepel et al., 2000]. To predict possible protective effects of substances inhibiting either carbonic anhydrase activity or sodium/proton exchange, we evaluated steady state feedback loop characteristics of the respiratory control system from previous studies in anaesthetized rabbits [Kiwull-Schöne et al., 2001a,b]. Steady state loop gain (G) was assessed as ratio of the slope of the CO response (S) and that of the metabolic hyperbola (S ) [Honda et al., 1983; Khoo, 2000] at the intersection of both curves, by which also the arterial set point PCO (PspCO) is determined.

Alveolar ventilation rises proportionally with metabolic rate during exercise and thus arterial Pco remains constant or may even fall slightly. The mechanism underlying this isocapnic hyperpnea, by which ventilation is coupled so precisely to metabolism, however, remains unclear. We have shown recently (Bin-Jaliah et al., 2004), that an increased metabolic rate, induced by insulin infusion, could produce an isocapnic hyperpnoea in an anaesthetized rat and subsequently, we showed that this hyperpnoea was correlated with an increase in the CO sensitivity, or gain, of the carotid body such that ventilation could be increased without hypercapnia. Low glucose can stimulate catecholamine release from carotid body tissue (Pardal & Lopez Barneo, 2002) but we demonstrated that the effect we observed could not be due to an insulin-induced fall in blood glucose concentration (Bin-Jaliah et al., 2005). We speculated that some other blood borne factor may be involved, and we in this present study, we evaluated the role of circulating adrenaline in the augmentation of chemoreceptor gain. Adrenaline has long been mooted as a possible feed forward factor involved in exercise hyperpnoea (Linton et al., 1992) and is know to be released in both hypoglycaemic states (Vollmer et al., 1997) and during exercise (Christensen et al., 1983).

It has been reported that hypoxic ventilatory response can be enhanced by an increase in work rate of exercise (Weil et al., 1972; Grover et al., 2002). However, it is still vague about how the muscular exercise produces the enhanced ventilatory responsiveness to hypoxia. Previous studies suggested that afferent nerve activity from carotid body was stimulated by some humoral factors related to muscular exercise, i.e. circulating catecholamine, lactate (Wasserman et al., 1986) and K (Band et al., 1985).

Inhibitors of carbonic anhydrase (CA) have complex effects on respiration. Many cells and tissues that are involved in the control of breathing contain various isoforms of CA, ., red cells, carotid bodies, lung and brain capillary endothelial cells, muscle and neurons closely associated with central chemoreceptors (1-9). In human and cats, low intravenous doses of acetazolamide have both stimulatory and inhibitory effects on the control of breathing. (10, 11). One of the inhibitory effects applies to the peripheral chemoreceptors because acetazolamide has been shown to reduce the hypoxic response and also the O-CO interaction that is known to reside the carotid bodies (10,12,13).