The pancreatic B cell is equipped with specific receptors for 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) and contains vitamin D-dependent calcium binding proteins (calbindin-D). Insulin secretion is impaired by vitamin D deficiency and is restored by 1,25-(OH)2D3 (concomitantly with an improved calcium handling within B cells) but the effect of 1,25-(OH)2D3 on the pancreatic B cell via calbindin-D is unclear. Therefore we examined the relationship between calbindin-D28K or calbindin-D9K and the activity of the endocrine pancreas in normal (N), four week vitamin D-deficient (−D) and one week 1,25-(OH)2D3-replete (+D) rats. Calbindin-D9K was not found in the pancreas, neither in the islets nor in the exocrine part, of any of the groups of rats (N, −D, or +D). Surprisingly, total islet calbindin-D28K content was increased by vitamin D deficiency and partly restored by 1,25-(OH)2D3. Calbindin-D28K immunostaining was observed only on A and B cells in the endocrine part of the pancreas, the greatest staining being found in A cells. This difference in staining density was increased by vitamin D deficiency and decreased by 1,25-(OH)2D3 treatment. Invitro, 1,25-(OH)2D3 also produced a negative influence on calbindin-D28K staining in A cells, as demonstrated using pieces of pancreas incubated with the steroid for 2 h. No significant influence on labeling intensity of B cell calbindin-D28K could be shown. Plasma insulin and islet insulin release in response to 10 mm arginine stimulation were decreased in −D rats and enhanced in +D rats towards N values. In contrast, plasma glucagon and the amount of glucagon secretion, stimulated in vitro by 10 mm arginine or by low (1·7 mm) glucose concentration, was increased in −D rats and attenuated by 1,25-(OH)2D3.
Thus there appears to be no relationship between the steady state level of B cell calbindin-D28K and the regulation of insulin secretion by 1,25-(OH)2D3 in vitamin D-deficient rats. However there is a correlation between A cell calbindin-D28K and glucagon secretion, which are both negatively regulated by 1,25-(OH)2D3. The predominance of calbindin-D28K in A cells raises the question as to how A and B cells interact and the role of calbindin-D28K in calcium handling.