Tomé Angelo R, Castro Enrique, Santos Rosa M, Rosário Luís M
Center for Neurosciences and Cell Biology, University of Coimbra, Coimbra, Portugal.
BMC Neurosci. 2007 Jun 14;8:39. doi: 10.1186/1471-2202-8-39.
Adrenal chromaffin cells mediate acute responses to stress through the release of epinephrine. Chromaffin cell function is regulated by several receptors, present both in adrenergic (AD) and noradrenergic (NA) cells. Extracellular ATP exerts excitatory and inhibitory actions on chromaffin cells via ionotropic (P2X) and metabotropic (P2Y) receptors. We have taken advantage of the actions of the purinergic agonists ATP and UTP on cytosolic free Ca2+ concentration ([Ca2+]i) to determine whether P2X and P2Y receptors might be asymmetrically distributed among AD and NA chromaffin cells.
The [Ca2+]i and the [Na+]i were recorded from immunolabeled bovine chromaffin cells by single-cell fluorescence imaging. Among the ATP-sensitive cells 40% did not yield [Ca2+]i responses to ATP in the absence of extracellular Ca2+ (Ca2+o), indicating that they expressed P2X receptors and did not express Ca2+- mobilizing P2Y receptors; the remainder expressed Ca2+-mobilizing P2Y receptors. Relative to AD-cells approximately twice as many NA-cells expressed P2X receptors while not expressing Ca2+- mobilizing P2Y receptors, as indicated by the proportion of cells lacking [Ca2+]i responses and exhibiting [Na+]i responses to ATP in the absence and presence of Ca2+o, respectively. The density of P2X receptors in NA-cells appeared to be 30-50% larger, as suggested by comparing the average size of the [Na+]i and [Ca2+]i responses to ATP. Conversely, approximately twice as many AD-cells expressed Ca2+-mobilizing P2Y receptors, and they appeared to exhibit a higher (20%) receptor density. UTP raised the [Ca2+]i in a fraction of the cells and did not raise the [Na+]i in any of the cells tested, confirming its specificity as a P2Y agonist. The cell density of UTP-sensitive P2Y receptors did not appear to vary among AD- and NA-cells.
Although neither of the major purinoceptor types can be ascribed to a particular cell phenotype, P2X and Ca2+-mobilizing P2Y receptors are preferentially located to noradrenergic and adrenergic chromaffin cells, respectively. ATP might, in addition to an UTP-sensitive P2Y receptor, activate an UTP-insensitive P2Y receptor subtype. A model for a short-loop feedback interaction is presented whereby locally released ATP acts upon P2Y receptors in adrenergic cells, inhibiting Ca2+ influx and contributing to terminate evoked epinephrine secretion.
肾上腺嗜铬细胞通过释放肾上腺素介导对应激的急性反应。嗜铬细胞功能受几种受体调节，这些受体存在于肾上腺素能（AD）和去甲肾上腺素能（NA）细胞中。细胞外ATP通过离子型（P2X）和代谢型（P2Y）受体对嗜铬细胞发挥兴奋和抑制作用。我们利用嘌呤能激动剂ATP和UTP对胞质游离Ca2+浓度（[Ca2+]i）的作用，来确定P2X和P2Y受体是否可能在AD和NA嗜铬细胞中不对称分布。
通过单细胞荧光成像记录免疫标记的牛嗜铬细胞的[Ca2+]i和[Na+]i。在对ATP敏感的细胞中，约40%在无细胞外Ca2+（Ca2+o）时对ATP不产生[Ca2+]i反应，表明它们表达P2X受体且不表达可动员Ca2+的P2Y受体；其余细胞表达可动员Ca2+的P2Y受体。相对于AD细胞，约两倍数量的NA细胞表达P2X受体而不表达可动员Ca2+的P2Y受体，这分别由在无Ca2+o和有Ca2+o时缺乏[Ca2+]i反应且对ATP产生[Na+]i反应的细胞比例表明。通过比较对ATP的[Na+]i和[Ca2+]i反应的平均大小表明，NA细胞中P2X受体的密度似乎大30 - 50%。相反，约两倍数量的AD细胞表达可动员Ca2+的P2Y受体，且它们似乎表现出更高（约20%）的受体密度。UTP使一部分细胞中的[Ca2+]i升高，且在所测试的任何细胞中均未使[Na+]i升高，证实了其作为P2Y激动剂的特异性。UTP敏感的P2Y受体的细胞密度在AD和NA细胞之间似乎没有差异。
尽管两种主要的嘌呤受体类型都不能归因于特定的细胞表型，但P2X和可动员Ca2+的P2Y受体分别优先定位于去甲肾上腺素能和肾上腺素能嗜铬细胞。除了UTP敏感的P2Y受体外，ATP可能还激活一种UTP不敏感的P2Y受体亚型。提出了一个短环反馈相互作用模型，据此局部释放的ATP作用于肾上腺素能细胞中的P2Y受体，抑制Ca2+内流并有助于终止诱发的肾上腺素分泌。
