EFFECT OF CHRONIC RESPIRATORY-ACIDOSIS ON CALCIUM-METABOLISM IN THE RAT

Chronic metabolic acidosis typically results in hypercalciuria and negative calcium balance. The impact of chronic respiratory acidosis on calcium metabolism has been less well studied. To address this issue, metabolic balance and static bone histomorphometric data were obtained during a 14-day exposure of rats to 10% CO2 (blood pH 7.33, Paco(2) 83 mm Hg) and were compared with pair-fed controls. All rats were fed a 0.8% calcium diet. Urinary calcium excretion (mg/period, mean +/- SEM) was increased during both week 1 and week 2 (16 +/- 3 vs 9 +/- 1 and 16 +/- 2 VS 9 +/- 1, CO2 group vs controls, respectively [p < 0.05]). Net intestinal calcium absorption (intake minus fecal excretion) was increased throughout the period of hypercapnia (week 1, 213 +/- 19 mg vs 135 +/- 15 mg; week 2, 135 +/- 16 mg vs 43 +/- 14 mg; and cumulatively, 344 +/- 27 mg vs 178 +/- 20 mg, CO2 group vs controls [p < 0.01]). As a consequence of the marked increment in intestinal calcium absorption during hypercapnia, mean net calcium balance was more positive than that of controls throughout the study (week 1, 197 +/- 18 mg vs 126 +/- 15 mg; week 2, 120 +/- 15 mg vs 34 +/- 15 mg; and cumulatively, 317 +/- 25 mg vs 159 +/- 20 mg, CO2 group vs controls, respectively [p < 0.01]). There were no significant differences in calcium intake, plasma total calcium, immunoreactive parathyroid hormone, 25-hydroxyvitamin D, or creatinine clearance between the two groups. 1,25 dihydroxyvitamin D, however, was decreased in the CO2 group versus controls, 45 +/- 6 pg/ml versus 82 +/- 9 pg/ml, respectively (p < 0.05). In the CO2 group, no significant changes in endosteal bone cell activity were present to suggest increased bone resorption or decreased bone formation. These results indicate that gut net absorption of calcium is enhanced during chronic respiratory acidosis in the rat and results in significant calcium retention. Hypercalciuria also occurs, which might reflect, at least in part, hyperabsorption of dietary calcium or diminished renal tubular calcium absorption. The mechanism of these changes in calcium balance during chronic respiratory acidosis and their impact on dynamic bone metabolism require further study.