Separate Effects of 1,25-Dihydroxyvitamin D and Calcium on Renal Calbindin-D28k and Intestinal Calbindin-D9k.

The effects of normal, low and high plasma concentrations of 1,25-(OH)[sub 2]vitamin D (1,25-(OH)[sub 2]D) combined with normal, low and high concentrations of plasma calcium on renal calbindin-D28k and intestinal calbindin-Dgk were examined in rats. We found that the expression of renal calbindin-D28k was significantly (P<0.05) increased by high levels of 1,25-(OH)[sub 2]D, but was not affected by a 50% reduction in 1,25-(OH)[sub 2]D. In contrast the intestinal calbindin-Dgk responded significantly (P<0.005) to both high and low 1,25-(OH)[sub 2]D levels. The effect of 1,25-(OH)[sub 2]D on intestinal calbindin-Dgk was modulated by plasma calcium concentrations. Increased plasma calcium levels did not affect the renal calbindin-D28k concentrations, but suppressed intestinal calbindin-D9k (P<0.05). The effect of calcium was not mediated by calciotropic hormones. This suggests the existence of a calcium-sensing mechanism in the proximal duodenum. It is concluded that intestinal calbindin-D9k is more sensitive than renal calbindin-D28k to changes in 1,25-(OH)[sub 2]D and that intestinal calbindin-Dgk in contrast to renal calbindin-D28k is sensitive to changes in plasma calcium concentrations. [ABSTRACT FROM AUTHOR]