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

Chronic metabolic acidosis typically results in hypercalciuria and neg ative calcium balance. The impact of chronic respiratory acidosis on c alcium metabolism has been less well studied. To address this issue, m etabolic 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 i ncreased throughout the period of hypercapnia (week 1, 213 +/- 19 mg v s 135 +/- 15 mg; week 2, 135 +/- 16 mg vs 43 +/- 14 mg; and cumulative ly, 344 +/- 27 mg vs 178 +/- 20 mg, CO2 group vs controls [p < 0.01]). As a consequence of the marked increment in intestinal calcium absorp tion during hypercapnia, mean net calcium balance was more positive th an 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, 3 17 +/- 25 mg vs 159 +/- 20 mg, CO2 group vs controls, respectively [p < 0.01]). There were no significant differences in calcium intake, pla sma total calcium, immunoreactive parathyroid hormone, 25-hydroxyvitam in D, or creatinine clearance between the two groups. 1,25 dihydroxyvi tamin 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 pre sent 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 signifi cant calcium retention. Hypercalciuria also occurs, which might reflec t, at least in part, hyperabsorption of dietary calcium or diminished renal tubular calcium absorption. The mechanism of these changes in ca lcium balance during chronic respiratory acidosis and their impact on dynamic bone metabolism require further study.