Hawaii Institute of Marine Biology, University of Hawaii, Kaneohe, HI 96744, USA.
Gen Comp Endocrinol. 2011 Jan 15;170(2):401-7. doi: 10.1016/j.ygcen.2010.10.022. Epub 2010 Nov 2.
In accord with its role in freshwater osmoregulation, prolactin (PRL) release from the tilapia pituitary is stimulated by small, physiologically relevant reductions in plasma osmolality, a response that is mediated by an acute influx of intracellular Ca(2+) through stretch-activated Ca(2+)channels. In the present study, the role of the calcium and cyclic AMP (cAMP) messenger system in the transduction of a response to a hyposmotic stimulus was examined using dispersed PRL cells and PRL cell membrane preparations from freshwater-acclimated tilapia. When PRL cells were treated with the phosphodiesterase (PDE) inhibitor, 3-isobutyl-1-methylxanthine (IBMX) (100μM), significant increases in cAMP levels and PRL release were observed at 1h. Exposure to reduced medium osmolality (300 mOsmolal) in the presence of IBMX further augmented PRL release. Depletion of Ca(2+) from the incubation medium blocked PRL release even in the presence of IBMX. By contrast, exposure of PRL cells to cholera toxin (CTX), an activator of adenylyl cyclase (AC), stimulated PRL release and cAMP accumulation in both the presence and absence of extracellular Ca(2+). On the other hand, treatment with the Ca(2+) ionophore A23187, which elicits a large rise in intracellular free Ca(2+), reduced cAMP accumulation. Likewise, the AC activity of a PRL cell membrane preparation was reduced as extracellular Ca(2+) concentration increased from 0.1 to 1 μM. These results indicate that: (1) the stimulation of PRL release and cAMP formation by a fall in extracellular osmolality are Ca(2+)-dependent; (2) large increases in intracellular Ca(2+) attenuate cAMP formation; (3) direct agonists of cAMP messenger system, such as cholera toxin, however, stimulate PRL release independently of the extracellular Ca(2+). These findings add to the evidence that the osmosensitive response of the tilapia PRL cell is mediated through a Ca(2+)-dependent mechanism. Nevertheless, the present findings also suggest that tilapia PRL cells have the ability to rapidly augment release PRL both via a Ca(2+)-dependent manner and via a cAMP-dependent pathway in the absence of extracellular Ca(2+).
与催乳素（PRL）在淡水渗透压调节中的作用一致，鱼垂体中 PRL 的释放受到血浆渗透压小幅度、生理相关降低的刺激，这种反应是通过通过伸展激活的 Ca(2+)通道引起的细胞内 Ca(2+)的急性内流介导的。在本研究中，使用来自淡水适应的罗非鱼的分散 PRL 细胞和 PRL 细胞膜制剂，研究了钙和环腺苷酸 (cAMP)信使系统在对低渗刺激的反应中的转导作用。当 PRL 细胞用磷酸二酯酶 (PDE)抑制剂 3-异丁基-1-甲基黄嘌呤 (IBMX)（100μM）处理时，在 1 小时时观察到 cAMP 水平和 PRL 释放的显著增加。在 IBMX 的存在下暴露于降低的培养基渗透压（300 mOsmolal）进一步增强了 PRL 的释放。即使在存在 IBMX 的情况下，从孵育培养基中耗尽 Ca(2+)也会阻断 PRL 的释放。相比之下，霍乱毒素 (CTX)，一种激活腺苷酸环化酶 (AC)的激活剂，刺激 PRL 释放和 cAMP 积累，无论是在存在或不存在细胞外 Ca(2+)的情况下。另一方面，用钙离子载体 A23187 处理，该载体引起细胞内游离 Ca(2+)的大量增加，降低了 cAMP 积累。同样，当细胞外 Ca(2+)浓度从 0.1 增加到 1μM 时，PRL 细胞膜制剂的 AC 活性降低。这些结果表明：(1) 细胞外渗透压降低刺激 PRL 释放和 cAMP 形成是 Ca(2+)-依赖性的；(2) 细胞内 Ca(2+)的大量增加会减弱 cAMP 的形成；(3) 然而，cAMP 信使系统的直接激动剂，如霍乱毒素，独立于细胞外 Ca(2+)刺激 PRL 释放。这些发现增加了这样的证据，即罗非鱼 PRL 细胞的渗透压敏感反应是通过 Ca(2+)-依赖性机制介导的。然而，目前的发现还表明，罗非鱼 PRL 细胞具有通过 Ca(2+)-依赖性方式和在不存在细胞外 Ca(2+)的情况下通过 cAMP 依赖性途径快速增强 PRL 释放的能力。
