Implications of the finding of calcium carbonate in rabbit urine with respect to renal acidification.
Formation of calcium carbonate or carbamino compounds in a bicarbonate solution should generate hydrogen ions and thereby elevate the PCO2 of that solution. The presence of these substances in rabbit urine was established by demonstrating a significantly lower value for urine total carbon dioxide content measured by microgasometry (number of moles) than with the titration technique (number of equivalents). As there was a significant correlation between the calcium and the carbonate contents in the urine, we surmised that most of the carbonate was in the form of a suspension of calcium carbonate. Direct analysis of precipitates from rabbit bladders confirmed this impression. The formation of calcium carbonate in a solution should raise the PCO2 by the reaction: Ca2+ + 2HCO3- leads to CaCO3 + H2CO3 with subsequent dehydration of the H2CO3. In vitro studies demonstrated that the addition of 1 mmol calcium to 1 L of a 200 mM bicarbonate solution raised the PCO2 by approximately 50 mmHg (1 mmHg = 133.322 Pa). However, in vivo measurements of the PCO2 of rabbit urine containing a similar quantity of calcium carbonate revealed that there was no such rise in  vivo (urine values = blood values). Therefore, the formation of calcium carbonate should have occurred at an earlier site in the nephron, thereby allowing the PCO2 to diffuse into peritubular capillaries during transit through the nephron.
D000693:Anaerobiosis; D000818:Animals; D002119:Calcium Carbonate / Q000652:urine*; D006863:Hydrogen-Ion Concentration; D007668:Kidney / Q000502:physiology*; D009399:Nephrons / Q000378:metabolism; D011817:Rabbits; D001743:Urinary Bladder / Q000378:metabolism; D014556:Urine / Q000032:analysis*
D002119:Calcium Carbonate