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A time course study of active Ca transport in the duodenum and the terminal ileum was conducted using the everted gut sac technique during the last week of pregnancy and throughout lactation. A triphasic pattern was revealed in the proximal duodenum: a marked rise between 18 and 20 days of pregnancy, a plateau maintained during the last 3 days of pregnancy and the first 2-3 days of lactation, and a fall by day 4 of lactation. The late-pregnancy rise was significant also when expressed as milligrams Ca transported relative to tissue weight, indicating that intestinal hypertrophy was not the cause of the increase. The ratio of serosal to mucosal Ca concentration remained low until approximately day 11 of lactation, when it rose toward a new peak. There was no active Ca transport in the ileum until the third week of lactation. Serum prolactin levels increased 10-fold between 18 and 20 days of pregnancy and remained high until at least day 7 of lactation, but did not correlate significantly with duodenal Ca transport. Injected rat prolactin did not result in a precocious rise in Ca transport in pregnant rats. The fluctuations in duodenal Ca transport during lactation were reflected by a small, but statistically significant, decrease in net fractional Ca absorption at 6-9 days compared with either 2-4 days or 13-16 days. We suggest that duodenal active Ca transport plays only a small role in total intestinal Ca absorption in the lactating rat except when dietary Ca is greatly restricted.

The concentrations of 1,25-dihydroxyvitamin D [1,25-(OH)2D], calcium, and phosphorus were measured in the serum of rats during pregnancy and at various stages of lactation. The concentration of 1,25-(OH)2D hormone increased almost two-fold during pregnancy and the latter part of lactation, but decreased to control levels or very low values immediately after birth and weaning, respectively. Furthermore, the concentration of 1,25-(OH)2D was inversely correlated with the concentration of calcium, suggesting that circulating 1,25-(OH)2D fluctuates in concert with calcium demands during the reproductive cycle. Parathyroidectomy in lactating rats caused a 70 percent inhibition of the normally observed 1,25-(OH)2D increase, indicating that parathyroid hormone, in response to changes in serum calcium, is a primary modulator of 1,25-(OH)2D during lactation.

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