This paper reports that in the rat coadministration of calcium (calcium chloride, CAS 10043-52-4, Ca2+) enhances intestinal absorption and bioavailability of monofluorophosphate (sodium monofluorophosphate, CAS 10163-15-2, MFP). Evidence obtained with two different experimental models is presented indicating that the latter effects are indirect consequences of the inhibitory effect of Ca2+ on alkaline phosphatase. Pharmacokinetic experiments in previous studies showed that fluorine bound to plasma proteins is the variable that determines the bioavailability of MFP. The area under the curve of fluorine bound to plasma proteins in rats receiving MFP + Ca2+ was significantly greater than in controls. In isolated duodenal loops in situ, Ca2+ increased the intestinal rate constant of MFP absorption and decreased the rate constant of MFP hydrolysis. Inhibition of hydrolysis increased the concentration of MFP at the intestinal lumen. This fact, however, is not only the cause of increased MFP absorption. Inhibition of alkaline phosphatase with L-phenylalanine, to the same extent as with Ca2+, increased MFP absorption with respect to controls but to a lower degree than with Ca2+. The rate constant of MFP hydrolysis by purified rat intestinal alkaline phosphatase was significantly inhibited by 50 mmol/l Ca2+ in comparison to control levels. Ca2+ decreased significantly Vmax of the enzyme (p-nitrophenyl phosphate as substrate) and had no effect on Km value. Lineweaver-Burk plots suggested a noncompetitive inhibition mechanism.

The paper reports that high fluoride concentrations in the intestinal lumen hinders the absorption of this anion. This conclusion was verified with three different experimental models. Pharmacokinetic experiments done in human volunteers revealed that the bioavailability of fluoride from sodium fluoride (NaF, CAS 7681-49-4) enteric coated tablets was 33% of that of plain (immediate release) tablets. The latter findings were confirmed in rats receiving 1 ml of NaF solutions (40, 80 or 160 mmol/l) by gavage. The greatest AUC (area under the curve of fluoremia as a function of time) was obtained with an oral dose of 80 mumol of NaF. This parameter was significantly greater (p < 0.01) with 80 mumol than with 40 mumol NaF, but similar to that observed with 160 mumol. Fecal fluoride excretions (in the 24 h following a single dose of NaF) and the bone fluoride contents (found at the end of 30 days of treatment with 40, 80 or 160 mumol NaF/day), agreed with the AUC values. The rate of fluoride absorption (v, mumol/10 min) through the intestinal wall was investigated with perfused, isolated rat duodenum in vivo. Fluoride absorption increased between 0 and 10 mmol/l luminal fluoride and decreased with higher concentrations. Oxygen consumption of duodenal-tissue decreased exponentially between zero (1.12 mumol O2 min-1 g-1) and 10 mmol/l fluoride (0.45 mumol O2 min-1 g-1).