Evans J J, Forrest-Owen W, McArdle C A
Department of Obstetrics and Gynaecology, Christchurch School of Medicine, New Zealand.
Endocrinology. 1997 May;138(5):2049-55. doi: 10.1210/endo.138.5.5138.
Gonadotropes synthesize and secrete LH and FSH under the control of GnRH, which acts via phosphoinositidase C (PIC)-linked G protein coupled receptors. Additionally, gonadotropin released from the pituitary is influenced by oxytocin, a peptide that has been shown to play a role in generation of the preovulatory LH surge. Although oxytocin receptors are present in the pituitary, studies have identified their presence on lactotropes but not on gonadotropes, raising the question of which cells act as the direct target of oxytocin in gonadotrope regulation. In this study, we examined effects of oxytocin on alphaT3-1 cells, a gonadotrope-derived cell line. Oxytocin, vasopressin, and vasotocin each stimulated accumulation of [3H]inositol phosphates in cells prelabeled with [3H]inositol, indicating activation of PIC. The rank order of potency (oxytocin > vasotocin > vasopressin) and sensitivity to inhibition by oxytocin and vasopressin receptor antagonists, revealed the effect to be mediated by oxytocin-selective receptors. Like other PIC activators, these nonapeptides caused biphasic (spike-plateau) increases in the cytosolic Ca2+. The spike response to oxytocin and GnRH were both retained in Ca2+-free medium, reflecting mobilization of intracellular Ca2+, and were comparably reduced by thapsigargin, implying mobilization of Ca2+ from a shared thapsigargin-sensitive intracellular pool. Brief stimulation with oxytocin, vasopressin, or vasotocin prevented subsequent Ca2+ responses to oxytocin, but not to GnRH, suggesting that the oxytocin receptor undergoes rapid homologous desensitization and reinforcing the interpretation that the nonapeptides act via the same receptor type. Oxytocin did not increase Ca2+ in cells stimulated with GnRH, whereas GnRH caused a spike Ca2+ increase even in the presence of oxytocin, implying that different mechanisms of desensitization (Ca2+ pool depletion and receptor uncoupling) are operating for two distinct PIC-coupled receptors in these cells. The demonstration that oxytocin acts directly via PIC-linked, oxytocin-selective receptors to increase cytosolic Ca2+ in a gonadotrope-derived cell line is consistent with the possibility that oxytocin has a comparable effect on nonimmortalized gonadotropes.
促性腺激素细胞在促性腺激素释放激素（GnRH）的控制下合成并分泌促黄体生成素（LH）和促卵泡生成素（FSH），GnRH通过与磷酸肌醇酶C（PIC）相关的G蛋白偶联受体发挥作用。此外，垂体释放的促性腺激素受催产素影响，催产素是一种已被证明在排卵前促黄体生成素激增过程中起作用的肽。尽管催产素受体存在于垂体中，但研究发现它们存在于催乳素细胞上，而不存在于促性腺激素细胞上，这就提出了在促性腺激素细胞调节中哪种细胞是催产素直接作用靶点的问题。在本研究中，我们检测了催产素对αT3 - 1细胞（一种源自促性腺激素细胞的细胞系）的影响。催产素、加压素和血管紧张素都刺激了预先用[³H]肌醇标记的细胞中[³H]肌醇磷酸的积累，表明PIC被激活。效价顺序（催产素>血管紧张素>加压素）以及对催产素和加压素受体拮抗剂抑制作用的敏感性，表明该效应是由催产素选择性受体介导的。与其他PIC激活剂一样，这些九肽导致胞质Ca²⁺出现双相（尖峰 - 平台）增加。对催产素和GnRH的尖峰反应在无Ca²⁺培养基中均得以保留，这反映了细胞内Ca²⁺的动员，并且两者均被毒胡萝卜素同等程度地降低，这意味着Ca²⁺是从一个共同的对毒胡萝卜素敏感的细胞内池动员而来。用催产素、加压素或血管紧张素进行短暂刺激可阻止随后对催产素的Ca²⁺反应，但不影响对GnRH的反应，这表明催产素受体经历快速同源脱敏，并进一步证明这些九肽通过相同类型的受体发挥作用。催产素不会增加用GnRH刺激的细胞中的Ca²⁺，而GnRH即使在存在催产素的情况下也会导致Ca²⁺尖峰增加，这意味着在这些细胞中，两种不同的与PIC偶联的受体存在不同的脱敏机制（Ca²⁺池耗竭和受体解偶联）。催产素通过与PIC相关的、催产素选择性受体直接作用以增加源自促性腺激素细胞的细胞系中的胞质Ca²⁺，这一发现与催产素对未永生化的促性腺激素细胞具有类似作用的可能性是一致的。
