OBJECTIVES

Chronic renal failure (CRF) is a common disease in dogs that causes metabolism disorders. Calcium (Ca) status has been addressed to be important, because it is usually related to hyperphosphatemia, hypercalciuria and secondary hyperparathyroidism. Normocalcemia, hypocalcemia or hypercalcemia has been reported based on total serum Ca measurement. However, the biologically active fraction of Ca is represented by ionized Ca (± 50% of total Ca), but the percentages of each Ca fraction (protein-bound, complexed with substances, and ionized) from total is sensitive to changes mainly due to blood pH and total protein. The purpose of the present study was to evaluate serum ionized Ca (Ca2+) concentrations in healthy dogs and in dogs with CRF presenting metabolic acidosis.

MATERIALS

Control group - blood samples were collected from 40 clinically normal dogs, aging from 1 to 12 year-old, divided in 4 groups according to age ranges. Experimental group - 23 blood samples were collected from dogs with CRF that presented renal hyperazotemia (urea > 80mg/dL and creatinine > 2.0mg/dL), for at least 15 days, and also metabolic acidosis (base excess < -8 and bicarbonate < 18mmol/L). Serum Ca2+, Na+, K+, Cl- were measured by ion-selective electrodes method. Total Ca was determined by colorimetric method (corrected by serum albumin concentration). Blood pH and gases measurements were performed by analyzer method. Student t test and Spearman correlation were calculated.

RESULTS

No difference between total Ca serum concentrations in normal dogs (10.83±0.16mg/dL; mean±SEM) and in dogs with CRF (11.49±0.42mg/dL) was observed. However, Ca2+ fraction was significant (P<0.0001) lower in dogs with CRF (4.77±0.18mg/dL) than in normal dogs (5.41±0.03mg/dL). Estimated Ca2+ concentrations values (50% of total Ca) were similar to those values determined by ion-selective method in healthy dogs (50.9±0.8%); on the other hand, in dogs with CRF, Ca2+ represented less than 50% of total Ca (40.22±1.9%). Ionized Ca fraction was decreased (hypocalcemia) in 56.5% of the dogs, normal in 26.1% and increased in 17.2%. Besides, in dogs with CRF, low pH was observed in 82.6%, and serum K+ concentrations were higher (P<0.0007) than in normal dogs. No correlation between pH and Ca2+was observed.

CONCLUSION

Decreases in blood pH can cause calcium to dissociate from protein, resulting in decrease in protein-bound and increase in Ca2+ fraction. However, in the present study, even though 82.6% of the dogs with CRF presented acidemia, only 17.4% of the dogs showed high level of serum Ca2+ fraction, and 56.5% showed decrease of Ca2+. Those data can suggest that, even though there is no alteration on total Ca serum concentrations in dogs with CRF, Ca2+ fraction status should be monitored, because alterations in this active form could compromise even more body Ca metabolism in CRF. Thus, other possible mechanisms could be associated with changes of Ca2+ fraction, and not only be influenced by blood pH and total protein.

* Sponsored by FAPESP (process # 99/00854-0)