Department of Physiology and Neurobiology, University of Connecticut, Storrs CT, USA.
J Physiol. 2018 Sep;596(17):4033-4042. doi: 10.1113/JP276104. Epub 2018 Jul 5.
Changes in CO result in corresponding changes in both H and HCO and despite evidence that HCO can function as an independent signalling molecule, there is little evidence suggesting HCO contributes to respiratory chemoreception. We show that HCO directly activates chemosensitive retrotrapezoid nucleus (RTN) neurons. Identifying all relevant signalling molecules is essential for understanding how chemoreceptors function, and because HCO and H are buffered by separate cellular mechanisms, having the ability to sense both modalities adds additional information regarding changes in CO that are not necessarily reflected by pH alone. HCO may be particularly important for regulating activity of RTN chemoreceptors during sustained intracellular acidifications when TASK-2 channels, which appear to be the sole intracellular pH sensor, are minimally active.
Central chemoreception is the mechanism by which the brain regulates breathing in response to changes in tissue CO /H . The retrotrapezoid nucleus (RTN) is an important site of respiratory chemoreception. Mechanisms underlying RTN chemoreception involve H -mediated activation of chemosensitive neurons and CO /H -evoked ATP-purinergic signalling by local astrocytes, which activates chemosensitive neurons directly and indirectly by maintaining vascular tone when CO /H levels are high. Although changes in CO result in corresponding changes in both H and HCO and despite evidence that HCO can function as an independent signalling molecule, there is little evidence suggesting HCO contributes to respiratory chemoreception. Therefore, the goal of this study was to determine whether HCO regulates activity of chemosensitive RTN neurons independent of pH. Cell-attached recordings were used to monitor activity of chemosensitive RTN neurons in brainstem slices (300 μm thick) isolated from rat pups (postnatal days 7-11) during exposure to low or high concentrations of HCO . In a subset of experiments, we also included 2',7'-bis(2carboxyethyl)-5-(and 6)-carboxyfluorescein (BCECF) in the internal solution to measure pHi under each experimental condition. We found that HCO activates chemosensitive RTN neurons by mechanisms independent of intracellular or extracellular pH, glutamate, GABA, glycine or purinergic signalling, soluble adenylyl cyclase activity, nitric oxide or KCNQ channels. These results establish HCO as a novel independent modulator of chemoreceptor activity, and because the levels of HCO along with H are buffered by independent cellular mechanisms, these results suggest HCO chemoreception adds additional information regarding changes in CO that are not necessarily reflected by pH.
CO 的变化导致 H 和 HCO 也相应变化，尽管有证据表明 HCO 可以作为独立的信号分子发挥作用，但几乎没有证据表明 HCO 有助于呼吸化学感受。我们表明 HCO 可直接激活化学敏感的 Retrotrapezoid 核（RTN）神经元。确定所有相关的信号分子对于了解化学感受器如何发挥作用至关重要，并且由于 HCO 和 H 被单独的细胞机制缓冲，因此能够同时感知两种模态会增加有关 CO 变化的额外信息，而 pH 本身不一定反映这些变化。当 TASK-2 通道（似乎是唯一的细胞内 pH 传感器）活性最小化时，HCO 可能特别重要，可调节 RTN 化学感受器在持续细胞内酸化期间的活性。
中枢化学感受是大脑根据组织 CO / H 的变化调节呼吸的机制。Retrotrapezoid 核（RTN）是呼吸化学感受的重要部位。RTN 化学感受的机制涉及 H 介导的化学敏感神经元激活和局部星形胶质细胞 CO / H 诱发的三磷酸腺苷嘌呤能信号转导，当 CO / H 水平较高时，该信号直接和间接通过维持血管张力来激活化学敏感神经元。尽管 CO 的变化导致 H 和 HCO 也相应变化，并且有证据表明 HCO 可以作为独立的信号分子发挥作用，但几乎没有证据表明 HCO 有助于呼吸化学感受。因此，本研究的目的是确定 HCO 是否独立于 pH 调节化学敏感 RTN 神经元的活性。在从出生后 7-11 天的大鼠幼崽中分离的脑片（300μm 厚）中，使用细胞附着记录监测低或高 HCO 浓度暴露期间化学敏感 RTN 神经元的活性。在一组实验中，我们还在内部溶液中加入 2'，7'-双（2-羧乙基）-5-（和 6）-羧基荧光素（BCECF），以在每种实验条件下测量 pHi。我们发现 HCO 通过独立于细胞内或细胞外 pH、谷氨酸、GABA、甘氨酸或嘌呤能信号转导、可溶性腺苷酸环化酶活性、一氧化氮或 KCNQ 通道的机制激活化学敏感 RTN 神经元。这些结果确立了 HCO 作为化学感受器活性的新型独立调节剂，并且由于 HCO 与 H 的水平被独立的细胞机制缓冲，这些结果表明 HCO 化学感受增加了有关 CO 变化的额外信息，而 pH 本身不一定反映这些变化。
