G Protein Subunit G a 13 Binds to Integrin a IIb b 3 and Mediates Integrin “ Outside-In ” Signaling
G Protein Subunit G a 13 Binds to Integrin a IIb b 3 and Mediates Integrin “ Outside-In ” Signaling
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发表时间：
2010
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通讯作者：
Haixia Gong;Bo Shen;P. Flevaris;Christina R. Chow;S. Lam;T. Voyno-Yasenetskaya;T. Kozasa;Xiaoping Du
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作者：
Haixia Gong;Bo Shen;P. Flevaris;Christina R. Chow;S. Lam;T. Voyno-Yasenetskaya;T. Kozasa;Xiaoping Du
腹支气管进入中支气管 (1)。玻璃模型展示了这种几何形状如何产生单向气流 (1)。鳄鱼肺部单向流动的机制尚未确定，但我们的数据支持 Hazelhoff 的模型 (1)，其中支气管树的关键特征导致单向流动。在吸气期间，空气可以喷射经过 CVB 的倾斜前庭进入较大的背侧支气管开口，并降低 CVB 孔口处的侧向压力，以将空气从 CVB 吸入肺内支气管。呼气时，尾腹支气管中的空气可能会向背侧喷射（图1C中的蓝色箭头）进入背支气管口。这样，支气管本身的简单布置可能会产生单向气流。此外，鳄鱼的气体交换机制尚不清楚。已经假设了横流机制（11），但不能排除逆流机制。此外，单向气流对于鳄鱼肺部气体交换效率的重要性尚不清楚，也无法根据我们的数据（包括气流测量）来确定。之前关于单向气流进化的设想是，它出现在虚骨龙级恐龙中 (12)，集中出现在兽脚类恐龙和翼龙中 (13, 15)，或者由于肝脏活塞呼吸机制而根本不出现在恐龙中 (14)。我们的发现在几个方面与之前的观点形成鲜明对比。他们证明鳄鱼具有但鸟类缺乏的肝活塞呼吸机制并不排除单向流动的进化，而鳄鱼缺乏的气动性不能用于诊断化石类群中的单向气流，正如之前所建议的 (13, 15)。鳄鱼和鸟类均属于冠龙类。因此，与之前的观点相反，我们认为单向流在十字跗龙和恐龙主龙分化之前就已经进化，并且存在于基础主龙及其后代中，包括植龙类、埃托龙类、“劳伊鳄类”和鳄形类。十字跗龙和稍后的恐龙取代合弓动物，成为三叠纪陆生脊椎动物群的主要成员，而三叠纪哺乳动物则以小型鼠类形式存在 (16, 17)。二叠纪末大规模灭绝后发生的动物群更替中的偶然性和竞争的作用是有争议的。基础祖龙和祖龙型动物，例如 Euparkaria，在相对环境缺氧的时期 (19) 似乎增强了剧烈运动的能力 (18)。在鸟肺中，单向气流与气体交换的横流机制相结合，有助于在缺氧条件下提取氧气 (20)。如果这样的肺存在于主龙辐射的底部，那么这个进化枝可能比合弓动物更好地竞争需要剧烈运动能力的生态位。
the ventrobronchi into the mesobronchus (1). A glass model demonstrated how this geometry gives rise to unidirectional airflow (1). The mechanism of unidirectional flow in alligator lungs is yet to be determined, but our data support Hazelhoff 's model (1), in which key features of the bronchial tree give rise to unidirectional flow. During inspiration, air may jet past the obliquely oriented vestibule of the CVB to enter the larger dorsal bronchial openings and reduce lateral pressure at the CVB orifice to draw air from the CVB into the intrapulmonary bronchus. During exhalation, air in the caudoventral bronchi may jet dorsally (blue arrows in Fig. 1C) to enter the ostia of the dorsobronchi. In this way, a simple arrangement of the bronchi by themselves might give rise to unidirectional airflow. Also, the mechanismof gas exchange in crocodilians is not known; a crosscurrent mechanism has been hypothesized (11), but a countercurrent mechanism cannot be ruled out. Furthermore, the importance of unidirectional airflow for gas exchange efficiency in the alligator lung is not known and cannot be determined from our data, which consist of measurements of airflow. Previous scenarios for the evolution of unidirectional airflow are that it arose in dinosaurs of coelurosaurian grade (12), convergently in theropods and pterosaurs (13, 15), or not at all in dinosaurs because of a hepatic piston mechanism of breathing (14). Our findings contrast with these previous views in several ways. They demonstrate that the hepatic piston mechanism of breathing, which crocodilians have but birds lack, does not preclude the evolution of unidirectional flow and that pneumaticity, which crocodilians lack, cannot be used to diagnose unidirectional airflow in fossil taxa, as previously suggested (13, 15). Crocodilians and birds are crown-group Archosauria. Therefore, in contrast to previous views, we suggest that unidirectional flow evolved before the divergence of crurotarsan and dinosaurian archosaurs and was present in the basal archosaurs and their descendants, including phytosaurs, aetosaurs, “rauisuchians,” and crocodylomorphs. The crurotarsans and, somewhat later, the dinosaurs supplanted the synapsids as the dominant members of the Triassic terrestrial vertebrate assemblage, with Triassic mammals existing as diminutive mouselike forms (16, 17). The roles of contingency and competition in the faunal turnover that occurred in the aftermath of the End Permian mass extinction are controversial. The basal archosaurs and archosauromorphs, animals such as Euparkaria, appear to have expanded their capacity for vigorous exercise (18) during a period of relative environmental hypoxia (19). In bird lungs, unidirectional airflow coupled with a crosscurrent mechanism of gas exchange facilitates the extraction of oxygen under conditions of hypoxia (20). If such a lung was present at the base of the archosaur radiation, this clade may have been better able than the synapsids to compete for niches that required a capacity for vigorous exercise.