EXPRESSION OF PLASMA-MEMBRANE CALCIUM-PUMP MESSENGER-RNA IN RAT INTESTINE - EFFECT OF AGE AND 1,25-DIHYDROXYVITAMIN-D

The capacity of the small intestine to actively transport Ca declines markedly with increasing age in the rat. The basal-lateral plasma memb rane Ca pump is thought to be an important component of the active tra nsport mechanism. Therefore, the purpose of this study was to determin e if there are changes in the expression of the intestinal Ca pump wit h age, mRNA levels were quantitated by Northern and dot blot analysis using a cDNA probe based on the sequence of the plasma membrane Ca pum p expressed in the rat intestine (PMCA1). In the duodenum, Ca pump mRN A levels were 3-4-times higher in young (2 months) rats compared to ad ult (12 months) and old (27 months) rats. In the ileum, Ca pump mRNA L evels were one third those of the duodenum, and ileal levels were high er in young rats compared to adult rats. These changes in mRNA levels with age and segment were significantly correlated with Ca pump activi ty as measured in basal-lateral membrane vesicles in vitro. To determi ne intestinal responsiveness to 1,25(OH)(2)D, rats were fed a strontiu m diet to induce vitamin D deficiency. In young animals, 1,25(OH)(2)D significantly increased Ca pump mRNA levels 4-fold in the duodenum. 1, 25(OH)(2)D had a similar effect in the adult duodenum. These studies d emonstrate that there are changes in Ca pump mRNA levels with age and intestinal segment. Since there was no change in the capacity of 1,25( OH)(2)D to increase Ca pump mRNA levels, the decline in Ca pump expres sion may be due to the age-related decrease in serum 1,25(OH)(2)D rath er than to decreased responsiveness to 1,25(OH)(2)D.