Intracellular Signaling In Endocrine Cells

该项目涉及内分泌和神经内分泌细胞中的细胞信号级联，以及质膜电事件和受体介导的细胞内信号和分泌之间的相互作用。目前的重点是细胞类型特异性基础垂体激素分泌的特征及其潜在的机制。实验在垂体促生长激素细胞、催乳素细胞和促性腺激素细胞中进行。虽然这些细胞表现出自发的和细胞外钙依赖的动作电位(AP)放电，但电压门控钙内流仅在哺乳细胞和生长激素细胞中触发分泌。这些细胞类型中的分泌囊泡对电压门控钙内流也有类似的敏感性。然而，AP驱动的钙内流模式在所研究的不同细胞类型中有所不同。促性腺激素细胞的自发活动以高幅度、锐利的AP为特征，其促进钙内流的能力有限，而哺乳细胞和生长激素细胞则触发平台破裂的AP，产生高幅度的钙信号。此外，促性腺激素细胞的放电模式从尖锐的尖峰转变为平台状的尖峰，触发了黄体生成素的分泌。这些细胞表达相似的离子通道组，但其中一些单独通道的表达水平有显著差异。具体地说，与促性腺激素细胞相比，哺乳细胞和生长激素细胞表现出低水平的钠通道和高水平的大电导钙激活钾(BK)通道的表达水平。相反，瞬时钾通道在促乳素细胞和促性腺激素细胞中的功能表达远远高于在促生长激素细胞中的表达。此外，在生长激素细胞和乳汁营养细胞中，BK通道的激活延长了膜的去极化，导致平台破裂活动的产生，促进了钙离子的内流。BK通道的这种矛盾作用是由其被膜钙快速激活所决定的，它截断了AP的幅度，从而限制了延迟整流钾通道在膜复极过程中的参与。相反，促性腺激素细胞表达相对较少的BK通道，并激发单个AP，但促进钙内流的能力较低，而在促性腺激素模型系统中，BK电流表达的增加导致平台破裂活动和高幅度钙瞬变的产生。这些结果表明，垂体细胞的细胞型特异性AP分泌偶联是由其质膜振荡器产生阈值钙信号的能力决定的。

This project addresses the cellular signaling cascade in endocrine and neuroendocrine cells, and the interactions between plasma membrane electrical events and receptor-mediated intracellular signaling and secretion. Current emphasis is on the characterization of cell type-specific basal pituitary hormone secretion and its underlying mechanism. Experiments were performed in pituitary somatotrophs, lactotrophs, and gonadotrophs. Although these cells exhibited spontaneous and extracellular calcium-dependent firing of action potentials (APs), voltage-gated calcium influx triggered secretion only in lactotrophs and somatotrophs. The secretory vesicles in these cell-types also had a similar sensitivity to voltage-gated calcium influx. However, the pattern of AP-driven calcium influx differed among cell types studied. Spontaneous activity in gonadotrophs was characterized by high amplitude, sharp APs that had a limited capacity to promote calcium influx, whereas lactotrophs and somatotrophs fired plateau-bursting APs, which generated high-amplitude calcium signals. Furthermore, a shift in the pattern of firing from sharp spikes to plateau-like spikes in gonadotrophs triggered LH secretion. These cells express similar groups of ionic channels, but there were marked differences in the expression levels of some of the individual channels. Specifically, lactotrophs and somatotrophs exhibited low expression levels of sodium channels and high expression levels of the large-conductance, calcium-activated potassium (BK) channel compared to those observed in gonadotrophs. In contrast, functional expression of the transient potassium channel was much higher in lactotrophs and gonadotrophs than in somatotrophs. Furthermore, in somatotrophs and lactotrophs, BK channel activation prolonged membrane depolarization, leading to the generation of plateau-bursting activity and facilitated calcium entry. Such a paradoxical role of BK channels was determined by their rapid activation by domain calcium, which truncated the AP amplitude and thereby limited the participation of delayed rectifying potassium channels during membrane repolarization. Conversely, gonadotrophs expressed relatively few BK channels and fired single APs with a low capacity to promote calcium entry, whereas elevation in BK current expression in a gonadotroph model system led to the generation of plateau-bursting activity and high amplitude calcium transients. These results indicate that the cell-type specific AP-secretion coupling in pituitary cells is determined by the capacity of their plasma membrane oscillator to generate threshold calcium signals.