We describe a negative feedback autocrine regulatory circuit for glucose-stimulated insulin secretion in purified human islets in vitro. Using chronoamperometry and in vitro glucose-stimulated insulin secretion measurements, evidence is provided that dopamine (DA), which is loaded into insulin-containing secretory granules by vesicular monoamine transporter type 2 in human β-cells, is released in response to glucose stimulation. DA then acts as a negative regulator of insulin secretion via its action on D2R, which are also expressed on β-cells. We found that antagonism of receptors participating in islet DA signaling generally drive increased glucose-stimulated insulin secretion. These in vitro observations may represent correlates of the in vivo metabolic changes associated with the use of atypical antipsychotics, such as increased adiposity.

Abstractβ-Cell mass (BCM) influences the total amount of insulin secreted, varies by individual and by the degree of insulin resistance, and is affected by physiologic and pathologic conditions. The islets of Langerhans, however, appear to have a reserve capacity of insulin secretion and, overall, assessments of insulin and blood glucose levels remain poor measures of BCM, β-cell function and progression of diabetes. Thus, novel noninvasive determinations of BCM are needed to provide a quantitative endpoint for novel therapies of diabetes, islet regeneration and transplantation. Built on previous gene expression studies, we tested the hypothesis that the targeting of vesicular monoamine transporter 2 (VMAT2), which is expressed by β cells, with [ 11 C]dihydrotetrabenazine ([ 11 C]DTBZ), a radioligand specific for VMAT2, and the use of positron emission tomography (PET) can provide a measure of BCM. In this report, we demonstrate decreased radioligand uptake within the pancreas of Lewis rats with streptozotocininduced diabetes relative to their euglycemic historical controls. These studies suggest that quantitation of VMAT2 expression in β cells with the use of [ 11 C]DTBZ and PET represents a method for noninvasive longitudinal estimates of changes in BCM that may be useful in the study and treatment of diabetes.

T cells from an HLA-DR11/DR12 responder were stimulated in mixed lymphocyte culture with cells carrying the DR1 antigen. After priming, T cells proliferated in response to both DR1-positive-stimulating cells and a peptide derived from a polymorphic region of the HLA-DR beta 1*0101 chain presented by responder's antigen-presenting cells (APC). The dominant epitope recognized by the primed T cells corresponded to residue 21-42 and was presented by the responder's HLA-DR12 antigen. The DR1 peptide-reactive T cells express T cell receptor V beta 3. The results demonstrate that allopeptides derived from the processing and presentation of donor major histocompatibility complex molecules by host-derived APC trigger alloreactivity. The frequency of T cells engaged in the indirect pathway of allorecognition is about 100-fold lower than that of T cells participating in the direct recognition of native HLA-DR antigen. However, indirect allorecognition may play an important role in chronic allograft rejection, a phenomenon that is mediated by the activation of T helper cells and of alloantibody- producing B cells.

Although long-studied in the central nervous system, there is increasing evidence that dopamine (DA) plays important roles in the periphery including in metabolic regulation. Insulin-secreting pancreatic β-cells express the machinery for DA synthesis and catabolism, as well as all five DA receptors. In these cells, DA functions as a negative regulator of glucose-stimulated insulin secretion (GSIS), which is mediated by DA D 2 -like receptors including D 2 (D2R) and D 3 (D3R) receptors. However, the fundamental mechanisms of DA synthesis, storage, release, and signaling in pancreatic β-cells and their functional relevance in vivo remain poorly understood. Here, we assessed the roles of the DA precursor L-DOPA in β-cell DA synthesis and release in conjunction with the signaling mechanisms underlying DA’s inhibition of GSIS. Our results show that uptake of L-DOPA is essential for establishing intracellular DA stores in β-cells. Glucose stimulation significantly enhances L-DOPA uptake, leading to increased DA release and GSIS reduction in an autocrine/paracrine manner. Furthermore, D2R and D3R act in combination to mediate dopaminergic inhibition of GSIS. Transgenic knockout mice in which β-cell D2R or D3R expression is eliminated exhibit diminished DA secretion during glucose stimulation, suggesting a new mechanism where D 2 -like receptors modify DA release to modulate GSIS. Lastly, β-cell-selective D2R knockout mice exhibit marked postprandial hyperinsulinemia in vivo . These results reveal that peripheral D2R and D3R receptors play important roles in metabolism through their inhibitory effects on GSIS. This opens the possibility that blockade of peripheral D 2 -like receptors by drugs including antipsychotic medications may significantly contribute to the metabolic disturbances observed clinically.

Type 2 vesicular monoamine transporter (VMAT2), found in the brain, is also expressed by β-cells of the pancreas in association with insulin. Preclinical experiments suggested that 11C-dihydrotetrabenazine PET–measured VMAT2 binding might serve as a biomarker of β-cell mass. We evaluated the feasibility of 11C-dihydrotetrabenazine PET quantification of pancreatic VMAT2 binding in healthy subjects and patients with long-standing type 1 diabetes. Methods 11C-Dihydrotetrabenazine PET was performed on 6 patients and 9 controls. VMAT2 binding potential (BPND) was estimated voxelwise by using the renal cortex as reference tissue. As an index of total pancreatic VMAT2, the functional binding capacity (the sum of voxel BPND × voxel volume) was calculated. Pancreatic BPND, functional binding capacity, and stimulated insulin secretion measurements were compared between groups. Results The pancreatic mean BPND was decreased in patients (1.86 ± 0.05) to 86% of control values (2.14 ± 0.08) (P = 0.01). In controls, but not in patients, BPND correlated with stimulated insulin secretion (r2 = 0.50, P = 0.03). The average functional binding capacity was decreased by at least 40% in patients (P = 0.001). The changes in functional binding capacity and BPND were less than the near-complete loss of stimulated insulin secretion observed in patients (P = 0.001). Conclusion These results suggest that 11C-dihydrotetrabenazine PET allows quantification of VMAT2 binding in the human pancreas. However, BPND and functional binding capacity appear to overestimate β-cell mass given the near-complete depletion of β-cell mass in long-standing type 1 diabetes, which may be due to higher nonspecific binding in the pancreas than in the renal cortex.