Acute effects of prolactin on passive calcium absorption in the small intestine by in vivo perfusion technique.

The acute effect of intraperitoneally administered prolactin (0.2, 0.4, and 0.6 mg/kg body weight) on passive calcium transport in duodenum, proximal jejunum, and ileum of sexually mature female Wistar rats was investigated by using an in vivo perfusion technique. Test solution containing (in mM) NaCl, 100; KCl, 4.7; MgSO4, 1.2; CaCl2, 20; D-glucose, 11; sodium ferrocyanide (Na4Fe(CN)6), an index of net water transport, 20; and 0.7 microCi 45CaCl2 (1 Ci = 37 GBq) was perfused through the 10-cm intestinal loop for 60 min. Results showed that 0.4 mg prolactin/kg body weight significantly increased duodenal net Ca absorption (net Ca) from 23.81 +/- 1.84 to 30.56 +/- 1.57 mmol/g dry weight (p < 0.05) by stimulating the lumen to plasma calcium flux (CaL-P). The jejunum responded to 0.2, 0.4, and 0.6 mg prolactin/kg body weight by reversing from net Ca absorption of 18.60 +/- 1.70 mmol/g dry weight to net secretion of -3.30 +/- 1.56, -10.39 +/- 2.21, and -11.79 +/- 2.04 mmol/g dry weight (p < 0.01), respectively, as a result of a dose-dependent increase in plasma to lumen calcium flux (CaP-L). Calcium fluxes in the ileum on the other hand did not respond to prolactin. There was a close correlation between net water flux and net calcium flux in all three intestinal segments under basal condition regardless of the luminal sodium concentration. However, this correlation was lost after prolactin administration, which while having no effect on net water flux, altered the duodenal and jejunal calcium fluxes. By varying the luminal concentration of sodium, it was found that the stimulatory effect of 0.4 mg prolactin/kg body weight on the duodenal CaL-P was reduced when compared with control, i.e., 17.84 +/- 0.91 vs. 26.64 +/- 1.05 mmol/g dry weight at a sodium concentration of 180 mM, and 14.48 +/- 0.99 vs. 20.12 +/- 1.34 mmol/g dry weight at a sodium concentration of 140 mM. At a sodium concentration of 80 mM, the prolactin effect was absent. Since duodenal Na(+)-K+ ATPase activity was increased by prolactin from 3.77 +/- 0.16 to 4.95 +/- 0.30 mumol Pi.mg-1 protein.h-1 (p < 0.05), sodium dependency of the prolactin-enhanced lumen to plasma calcium flux may be related to both sodium-induced water flow and calcium-sodium exchange across the basolateral membrane. Thus, it was postulated that under basal condition, net calcium transport in the small intestine occurred with the sodium-induced water transport along the paracellular pathway. However, after prolactin administration, this association was lost. Prolactin-enhanced lumen to plasma calcium flux in the duodenum was sodium dependent and involved the Na(+)-K+ ATPase activity. In the proximal jejunum, prolactin stimulated plasma to lumen calcium flux, but the mechanism was not known.