## 1 Introduction  
Utilizing autologous stem cells from human exfoliated deciduous teeth (SHED) as an insulin reservoir is an attractive option compared to islet allotransplantation procedures for patients with severe Type 2 diabetes or pancreatic fibrosis in pancreetogenic (Type 3c) forms of diabetes [1]. Properly reserved and seeded SHED \(\beta\)-cells may provide an ideal \(\beta\)-cell substitute for treating insulin-deficient diabetes if appropriate immune protective agents are guaranteed [2]. Zinc is acknowledged as an essential ion for various cellular functions, and some specific cell types accumulate a significant amount of zinc in the cytosol. Among these, for example, pancreatic \(\beta\)-cells may contain zinc at concentrations of 10 to 20 mM [3]. The role of zinc in pancreatic \(\alpha\)-and \(\beta\)-cells is not fully understood, although zinc is recognized as a principal ion in glucose homeostasis [4; 5; 6]. A recent report suggested that excessive secretion of insulin can deplete zinc in \(\beta\)-cells, and may distress glucose homeostasis, which may be associated with diabetes [7; 8]. Although the therapeutic potential of SHED-derived \(\beta\)-cells, i.e., SHED \(\beta\)-cells, has been suggested, mechanisms explaining the effect of zinc on calcium influx during insulin secretion by SHED \(\beta\)-cells are not clear. We previously reported a significant role of elevated zinc levels in the cytosol of primary \(\beta\)-cells harvested from rats, [9] as well as in pancreatic \(\beta\)-cell-like cells derived from SHED [10]. In the previous study, we showed that insulin secretion levels by rat pancreatic \(\beta\)-cells and SHED \(\beta\)-cells increased as cytosolic zinc levels increased. Since recent reports [11; 12; 13] suggested a remedial effect of \(\gamma\)-aminobutyric acid (GABA) on the regeneration of pancreatic cells, we postulated that there could be mutual influence between GABA and intracellular zinc in SHED \(\beta\)-cells. In this new report, we sought to investigate the relationship between GABA and zinc levels. GABA is an important and ubiquitous transmitter involved in \(\beta\)-cell signaling pathways that regulate autocrine/paracrine functions associated with insulin secretion [13; 14]. GABA\({}_{\text{A}}\) receptors function as chloride channels in the plasma membrane of \(\beta\)-cells. Since \(\beta\)-cells maintain high concentrations of intracellular chloride, GABA induces membrane depolarization resulting in calcium influx, influencing the proliferation of \(\beta\)-cells [15]. GABA has been thought to ameliorate diabetic symptoms [16; 17]. Despite accumulated knowledge, the interactions between GABA and zinc in insulin secretion are not fully understood [18]. Furthermore, it is important to understand whether exogenous GABA supplementation can potentially be utilized as an inducer of GABA-mediated insulin secretion in SHED \(\beta\)-cells for therapeutic use. Members of the SLC39A family of zinc transporters, including Zrt/irt-like protein (ZIP)8, facilitate movement of zinc ions from extracellular spaces or intracellular organelles to the cytosol of \(\beta\)-cells, as we and others have reported [19]. However, most research on zinc transporters and diabetes has been focused on zinc exporters, particularly ZnT8 [20]. In the current study, we have investigated the possibility that the ZIP3 transporter is a functional regulator of intracellular zinc levels in SHED \(\beta\)-cells, as shown previously for pancreatic \(\beta\)-cells [6]. We sought to investigate if ZIP3 could be another principal zinc uptake transporter in addition to ZIP8 in SHED \(\beta\)-cells. Further, we examined the effects of increased zinc levels in the cytosol on insulin secretion and on GABA-induced activation in SHED \(\beta\)-cells. We hypothesized that the ZIP3 transporter is a principal zinc uptake transporter, which collaborates with GABA in SHED \(\beta\)-cells for insulin secretion.