The effect of prolactin on calcium metabolism in wistar rats

Abstract

The effect of prolactin on intestinal absorption of calcium was investigated in sexually mature weaned and aged female Wistar rats. An electrolyte solution containing 5 mMCaCl2 or 0.5 mMCacl2 and 0.1 uCi (45)Ca was given intragastrically to the control normal saline-treated and prolactin-treated animals which received intraperitoneal administration of 0.02 mg PRL/100 g BW 1 hr before the experiment. Subsequent plasma (45)Ca contents during 1 hour were used as an index of intestinal calcium absorption and tibial (45)Ca contents at the end of 1 hour provided an index of bone deposition of absorbed calcium. Comtrol experiments showed that calcium absorption was highest in weaned rats while aged rats exhibited the lowest rate of calcium absorption. Prolactin enhanced calcium absorption in sexually mature female rats but not in the weaned or aged rats. The plasma (45)Ca contents of control versus prolatin-treated sexually mature rats were 3.91±0.20 vs 6.13±0.11 (p<0.001) 4.13±0.17 vs 5.86±0.33 (p<0.001) 4.22±0.18 vs 5.39±0.46 (p,0.05) and 3.84±0.023 vs 3.74±0.30 (p>0.05) at 5,15,30 and 60 minutes respectively. Tibial (45)Ca contents in weaned rats were significantly higher than those found in sexually mature and aged rats. The in vivo experiments demonstrated that the presence of other electrolytes in the luminal test solution enhanced calcium absorption when compared to Cavcl(,2) alone. However, calcium absorption was further enhanced when sodium in the electrolyte solution was replaced with choline. The action of prolactin on calcium absorption was evaluated in two conditions: in the passive calcium transport predominance (luminal calcium concentration 5 mM) and active calcium transport predominance (luminal calcium concentration 0.5 mM). It was found that when passive transport predominated prolactin markedly stimulated calcium absorption. This action of prolactin was absent when luminal sodium was replaced by choline. These results together with data from previous reports suggest that prolactin stimulated passive calcium absorption secondary to stimulating sodium transport. On the other hand, the action of prolactiin on active calcium absorption was evident both in the presence and absence of luminal sodium. In another wore the enhancing effect of prolaction on active transport of calcium was sodium-independent. With active transport predominance the plasma (45)Ca contents of control versus prolactin-treated rats given electrolyte solutions were 5.41±0.13 vs 6.55±0.13 (p<0.001) 4.99±0.32 vs 6.32±0.24 (p<0.01) and 4.32±0.24 vs 4.92±0.10 (p<0.05) at 15,30 and 60 minutes respectively. When the luminal test solution was a Na-free-electroltye solution the plasma 45Ca contents of control versus prolactin-treated rats were significantly different i.e., 4.13±0.38 vs 6.72±0.31 (p<0.001) and 5.11±0.35 vs 6.52±0.34 (p<0.05) at 5 and 15 minutes respectively. The in situ duodenal loop experiment was performed to demonstrate the action of prolaction on calcium absorption by the duodenum. It was found that prolactin markedly increased both the passive and active transport of calcium in the duodenum. The 5 minutes values of plasma (45)Ca contents were enhanced by 135.6% and 38.4% in the presence of luminal calcium concentration of 0.5 and 5 mM respectively.