Stimulation-regulated fusion of vesicles to the plasma membrane is an essential step for hormone secretion but may also serve for the recruitment of functional proteins to the plasma membrane. While studying the distribution of G protein-gated K ؉ (K G ) channels in the anterior pituitary lobe, we found K G channel subunits Kir3.1 and Kir3.4 localized on the membranes of intracellular dense core vesicles that contained thyrotropin. Stimulation of these thyrotroph cells with thyrotropinreleasing hormone provoked fusion of vesicles to the plasma membrane, increased expression of Kir3.1 and Kir3.4 subunits in the plasma membrane, and markedly enhanced K G currents stimulated by dopamine and somatostatin. These data indicate a novel mechanism for the rapid insertion of functional ion channels into the plasma membrane, which could form a new type of negative feedback control loop for hormone secretion in the endocrine system.The G protein-gated K ϩ (K G ) 1 channel, a member of the inwardly rectifying K ϩ (Kir) channel family, is directly activated by pertussis toxin-sensitive G proteins. This system was first discovered in the muscarinic deceleration of the heartbeat (1). More recently, it is considered to play an essential role in the hormone-mediated inhibitory regulation of neural excitability (1, 2). Electrophysiological studies have revealed that a variety of inhibitory receptors, including M 2 -muscarinic, A 1 -purinergic, D 2 -dopamine, ␣ 2 -adrenergic, serotonin, ␥-aminobutyric acid type B, opioid, and somatostatin receptors in the brain, are coupled to K G channels (3). In endocrine organs such as the anterior pituitary lobe and pancreatic islet, it was also reported that some neurotransmitters including dopamine and somatostatin hyperpolarize the membrane by activating K G channels, which results in the inhibition of hormone secretion (4 -7).The main subunit of K G channels has been cloned from the heart and designated GIRK1/Kir3.1 (8, 9). In the brain and heart, Kir3.1 forms functional K G channels by assembling with other Kir3.0 subunits such as GIRK2/Kir3.2, GIRK3/Kir3.3 (10, 11), and GIRK4/CIR/Kir3.4 (12). Because Kir3.1 mRNA was detected in the anterior pituitary lobe (13) and pancreatic islet (14), this subunit may also contribute to the formation of K G channels in endocrine cells. The anterior pituitary lobe contains several kinds of endocrine cells: lactotrophs, somatotrophs, corticotrophs, and thyrotrophs. Although electrophysiological experiments have shown that somatostatin or dopamine activate K G currents in lactotrophs (4) and also in several cell lines derived from the anterior pituitary lobe, such as GH3 (5) and AtT20 (6), the cellular and subcellular localizations of K G channels in in vivo pituitary endocrine cells have not been examined.In this study, using a polyclonal antibody specific to Kir3.1, we found that this subunit was expressed only in thyrotroph cells and, surprisingly, was localized predominantly on intracellular secretory vesicles. Kir3.4 was co-localized on the vesicles w...

Obesity, which disturbs lipid and glucose metabolism, is a recent medical concern. It threatens human health and also has adverse effects on reproductive functions by causing insulin resistance/hyperinsulinemia, especially in women with polycystic ovary syndrome (PCOS). For PCOS patients to prevent these adverse effects, it is important to take into account improving their lifestyles by exercise and proper diets. The relationship between insulin resistance/hyperinsulinemia and reproductive disorders should be understood as fully as possible in order to provide effective treatment. It is well known that insulin resistance and compensatory hyperinsulinemia can be triggered by obesity with visceral fat accumulation. Hyperinsulinemia affects granulosa cells in small follicles and theca cells. This condition induces early response to luteinizing hormones on granulosa cells of small follicles and causes premature differentiation of these cells, which eventually results in anovulation. For improvement of anovulation because of hyperinsulinemia, insulin-sensitizing agents (biguanide and thiazolidinedione derivatives) are useful. Hyperinsulinemia may adversely affect the endometrial functions and environment, and evoke implantation disturbance. Treatment with an insulin-sensitizing agent (metformin) improves the levels of glycodelin, insulin-like growth factor binding protein 1, and blood flow in spiral arteries during the peri-implantation period. It supports endometrial function, improves the endometrial environment, and facilitates embryo implantation. The rate of early pregnancy loss during the first trimester is 30-50% in women with PCOS, which is threefold higher than for normal women. Metformin treatment improves the levels of insulin, the homeostasis model assessment for insulin resistance, and plasminogen activator inhibitor activity, and decreases early pregnancy loss. It goes without saying that lifestyle change is fundamental for improving reproductive performance in addition to treatment with insulin-sensitizing agents.