INTRACELLULAR SIGNALING IN ENDOCRINE CELLS

G蛋白偶联和酪氨酸激酶的作用机制 受体控制钙信号传导， 在几种细胞中研究了这种信号传导含义 类型观察到三种钙信号模式：基线 振荡，具有尖峰脉冲的频率但不具有尖峰脉冲的振幅 由激动剂浓度控制;缓慢振荡， 尖峰脉冲的恒定频率和可变幅度;以及 振荡的钙信号的模式不是由 受体亚型，但通过后受体事件。基线钙 通过注射三磷酸肌醇引发振荡 （InsP 3），以及暴露于化合物如离子霉素， 毒胡萝卜素和硫柳汞。 然而，当细胞质 振荡器被这些代理激活，它只在 5/min的基础速率及其尖峰频率不随 增加药物浓度，如在激动剂中常见的那样， InsP 3刺激的细胞。相反，两种类型的振荡 受到管腔内钙耗竭的影响， [Ca ~（2+）]i的变化，但不受ryanodine的抑制。的 电压敏感性钙离子进入途径也影响InsP 3- 兴奋性促性腺激素细胞中依赖的钙振荡。在 激动剂和β 3刺激的细胞，持续的钙振荡 在3-15分钟后被超极化熄灭， 细胞外介质中钙的可用性。 单个 去极化脉冲瞬时恢复的幅度 二氢吡啶和细胞外钙离子浓度持续升高 敏感的方式。 对去极化的反应显示出明显的 依赖于膜电位，这与 稳态内向钙电流。 此外，重复 应用短暂的去极化脉冲调制的频率， 激动剂和InsP 3控制的加标。这些外部驱动 和细胞外钙依赖性振荡敏感， 钙泵阻断剂毒胡萝卜素但对ryanodine无效一 基于这些实验观察的数学模型给出了 对广泛的激动剂剂量的反应，包括 阈下反应，阈上基线振荡 频率由[InsP 3]确定的反应，双相 振荡和双相非振荡反应。该模型还 预测存在非受体介导的钙离子 振荡 钙信号足以触发 主要反应基因（PROS）在几种类型的 内分泌细胞在垂体促性腺激素细胞中，蛋白激酶C- PRO的依赖性诱导被发现是调节的， （Ca ~（2+）]i. 因此，钙信号传导的模式可能代表了一种有效的 控制内分泌和其他组织中基因表达的机制 细胞类型。

The mechanisms by which G protein-coupled and tyrosine kinase receptors control calcium signaling, and the physiological implications of such signaling, were investigated in several cell types. Three patterns of calcium signaling were observed: base-line oscillatory, with the frequency but not the amplitude of spiking controlled by agonist concentration; slow-oscillatory, with a constant frequency and variable amplitude of spiking; and non- oscillatory. The pattern of calcium signaling was not determined by the receptor subtype but by post-receptor events. Base-line calcium oscillations were initiated by injection of inositol trisphosphate (InsP3), as well as by exposure to compounds such as ionomycin, thapsigargin, and thimerosal. However, when the cytoplasmic oscillator was activated by these agents, it operated only at the basal rate of 5/min and its spiking frequency did not change with increasing drug concentrations, as commonly occurs in agonist-and InsP3-stimulated cells. In contrast, both types of oscillations were affected by the depletion of intra-luminal calcium and by changes in [Ca2+]i, but were not inhibited by ryanodine. The voltage-sensitive calcium entry pathway also affected InsP3- dependent calcium oscillations in excitable gonadotrophs. In agonist- and Insp3-stimulated cells, sustained calcium oscillations were extinguished by hyperpolarization after 3-15 min despite the availability of calcium in the extracellular medium. Single depolarizing pulses transiently restored the amplitude of the sustained spiking in a dihydropyridine- and extracellular calcium- sensitive manner. The responses to depolarization showed a marked dependence on membrane potential that was correlated with the steady-state inward calcium current. In addition, the repetitive application of brief depolarizing pulses modulated the frequency of agonist and InsP3-controlled spiking. These extrinsically driven and extracellular calcium-dependent oscillations were sensitive to the calcium-pump blocker, thapsigargin, but not to ryanodine. A mathematical model based on these experimental observations gave responses to a wide range of agonist doses, including the subthreshold responses, superthreshold base-line oscillatory response with frequency determined by [InsP3], biphasic oscillatory, and biphasic non-oscillatory response. The model also predicted the existence of non-receptor-mediated calcium oscillations. Calcium signaling is sufficient to trigger the expression of primary response genes (PROS) in several types of endocrine cells. In pituitary gonadotrophs, the protein kinase C- dependent induction of PROs was found to be modulated both positively and negatively by physiological changes in (Ca2+]i. Thus, the pattern of calcium signaling may represent an efficient mechanism for the control of gene expression in endocrine and other cell types.