ENHANCED NONSATURABLE CALCIUM-TRANSPORT IN THE JEJUNUM OF RATS DURINGLACTATION, BUT NOT DURING PREGNANCY

The lactating (L) rat loses in excess of 100 mg of calcium (Ca) per da y to milk at peak lactation, Most of the Ca must be provided by increa sed intestinal absorption, In an effort to examine adaptation of intes tinal calcium absorption during lactation, nonsaturable absorption fro m the small intestine of rats was calculated from the disappearance of Ca from in situ ligated loops of jejunum during the last week of preg nancy and throughout lactation and weaning, Efficiency of absorption i s reflected by the slope of the regression line of Ca absorbed on Ca i ntroduced into the loop, Absorption of Ca in the jejunum was markedly enhanced starting at 5 days of lactation and for the remainder of lact ation. Two days after weaning, the efficiency of jejunal Ca absorption decreased to the nonmated (NM) control level, while the lactation-ass ociated intestinal hypertrophy persisted beyond 2 days postweaning. Th e percentages of water and Ca absorbed were positively and significant ly correlated in both L and NM rats, In contrast to Ca, magnesium (Mg) and strontium (Sr) transport from ligated loops were not enhanced dur ing lactation, Fifty millimolar glucose in the test solution increased the absorption of both water and Ca, but not Mg, from jejunal loops o f NM rats, Glucose increased Ca absorption in NM rats up to the level seen in L rats, Glucose did not increase Ca absorption further in L ra ts, perhaps because of the greater availability of glucose to the inte stine during lactation, We conclude that in rats the efficiency of non saturable Ca absorption from the jejunum is significantly increased du ring well established lactation, but not during pregnancy, The underly ing mechanism appears to be specific for Ca, may be dependent on gluco se, and is unrelated to intestinal hypertrophy.