Sub-chronic stimulation of glucocorticoid receptor impairs and mineralocorticoid receptor protects cytosolic Ca2+ responses to glucose in pancreatic β-cells

The development of diabetes associated with stress, obesity, and metabolic syndrome involves elevated plasma glucocorticoid levels. It has been shown that short-term (<1 day) exposure to glucocorticoids reduces insulin secretion from pancreatic islets by affecting several steps of glucose signaling in beta-cells. However, longer term direct effects of glucocorticoids on beta-cells remain to be established. In this study, single beta-cells isolated from rat islets were treated with glucocorticoids, mineralocorticoids, and their receptor agonists/antagonists for 3 days in culture, followed by assessment of the beta-cell responsiveness to glucose by measuring cytosolic Ca2+ concentration ([Ca2+]i) using fura-2. Following treatment with corticosterone at 10-500 ng/ml for 3 days, the first-phase [Ca2+]i response to 8.3 mM glucose in beta-cells was suppressed. Simultaneous administration of RU-486, a glucocorticoid receptor (GR) antagonist, prevented this suppression. RU-486 by itself promoted the beta-cell [Ca2+]i response to glucose. Conversely, dexamethasone (1000 ng/ml), a highly selective GR agonist, impaired beta-cell [Ca2+]i responses to glucose. A mineralocorticoid receptor (MR) antagonist spironolactone, co-administered with corticosterone, further depressed [Ca2+]i responses to glucose, while an MR ligand aldosterone attenuated the corticosterone inhibition of [Ca2+]i responses. Neither spironolactone nor aldosterone by itself affected [Ca2+]i responses. These results indicate that long-term treatment with corticosterone impairs beta-cell [Ca2+)]i responses to glucose. This effect is mediated by GR and attenuated partially by simultaneous MR stimulation by corticosterone. The results show a novel function of MR to protect islet beta-cells against deteriorating glucocorticoid action via GR.