Possible relationship between the B-cell threshold for glucose-induced insulin secretion and blood glucose concentrations in the normal toad

Glucosensing and glucose homeostasis: from fish to mammals

This review is focused on two topics related to glucose in vertebrates. In a first section devoted to glucose homeostasis we describe how glucose levels fluctuate and are regulated in different classes of vertebrates. The detection of these fluctuations is essential for homeostasis and for other physiological processes such as regulation of food intake. The capacity of that detection is known as glucosensing, and the different mechanisms through which it occurs are known as glucosensors. Different glucosensor mechanisms have been demonstrated in different tissues and organs of rodents and humans whereas the information obtained for other vertebrates is scarce. In the second section of the review we describe the present knowledge regarding glucosensor mechanisms in different groups of vertebrates, with special emphasis in fish.

Effect of an acute glucose overload on Islet cell morphology and secretory function in the toad

The aim of this work was to study the effect of induced hyperglycemia on islet cell mass and insulin secretion in normal toads. Immunolabeled beta cell area, replication (bromodeoxyuridine) and apoptosis (propidium iodide) rate, islet neogenesis (cytokeratin), and insulin secretion in vitro were measured in adult male specimens of Bufo arenarum during and after interruption of the injection of either a 50% glucose solution (2 g/100 g) or its vehicle for 4 days. Glucose administration caused hyperglycemia (122.6 +/- 16.7 and 508.3 +/- 115.9 mg/dl vs 23.5 +/- 1.26 and 22.8 +/- 1.8 mg/dl, at days 3 and 5, respectively, P < 0.05) and a significant decrease in the number of islets/mm(2) (day 3: 9.7 +/- 0.9 vs 3.3 +/- 0.4, P < 0.05; day 5: 9.4 +/- 0.8 vs 7.4 +/- 0.6; day 9: 9.6 +/- 0.9 vs 6.2 +/- 0.4, P < 0.05) and in the percentage of immunolabeled beta cell area (day 3: 2.07 +/- 0.2 vs 0.5 +/- 0.1%, P < 0.05; day 5: 1.8 +/- 0.1 vs 0.6 +/- 0.1%; day 9: 1.7 +/- 0.1 vs 0.7 +/- 0.1%, P < 0.05). Glucose-injected animals had a simultaneous significantly higher percentage of BrdU-labeled beta cells (day 3: 0.46 +/- 0.02 vs 0.23 +/- 0.03%; day 5: 0.54 +/- 0.13 vs 0.22 +/- 0.02%; day 9: 0.61 +/- 0.0 vs 0.27 +/- 0.05%, P < 0.05) and cytokeratin-labeled endocrine cells (day 3: 0.21 +/- 0.06 vs 0.01 +/- 0.00%; day 5: 0.17 +/- 0.06 vs 0.01 +/- 0.01%; day 9: 1.25 +/- 0.2 vs 0.01 +/- 0.008%, P < 0.05) and a higher rate of apoptotic beta cells (day 3: 0.14 +/- 0.04 vs 0.05 +/- 0.02%; day 5: 0.4 +/- 0.06 vs 0.05 +/- 0.2, P < 0.05; day 9: 0.47 +/- 0.04 vs 0.06 +/- 0.03, P < 0.05). Comparable amounts of insulin were secreted in vitro by both groups in response to 2 mM glucose, whereas there was a significantly reduced response to 8 mM glucose in treated animals (day 3: 73 +/- 12 vs 165 +/- 20%; day 5: 74 +/- 11 vs 204 +/- 18%, P < 0.05). This decreased response to high glucose reverted to normal after removal of the glucose injection. These results show for the first time that short-term hyperglycemia triggers marked morphological and transient secretory changes in the toad pancreas similar in part to those elicited in the pancreas of several mammals. As with other results previously reported, these results support the usefulness of the toad as an alternative easily handled model to study the growth and secretory function of the endocrine pancreas.