Furosemide administration results in a transient alteration in calcium balance in mature horses

Previous research documented that furosemide negatively impacted calcium balance for 3 days but did not determine when calcium balance returned to baseline. This study hypothesized that furosemide's impact on calcium would return to control values before 7 days post-administration. Ten mature geldings were assigned to either control (CON, n = 5) or treatment (FUR, n = 5) for the first of two 8-day total collections in crossover design. Treatment horses received one administration of furosemide (1 mg/kg, IV). A 10% sample of pooled faeces and urine from each day was kept. Calcium concentrations in hay, faeces and urine were determined by an atomic absorption spectrophotometer. Data were analysed using mixed-model-repeated measures ANOVA to determine influence of day and treatment. For urine output, FUR urinated twice as much during the 24 hr after administration than CON (p < .001). Horses in FUR excreted more urinary calcium 24-hr post-administration as compared to CON (9.3 ± 1.0 and 4.2 ± 1.0 g, respectively; p < .001). Calcium balance in FUR was more negative on day 1 than day 3 (p < .05). Faecal calcium concentrations remained the same from day 1 to day 7 in CON (6.3 ± 1.3 and 5.5 ± 1.3 g/kg, respectively; p > .10) but were lower in FUR on day 7 as compared to day 1 (4.8 ± 1.3 and 7.3 ± 1.3 g/kg, respectively; p < .001), indicating a potential mechanism to restore calcium balance. These findings corroborate previous studies on furosemide and calcium balance and provide evidence for a possible mechanism to recover net calcium losses after furosemide administration. Since calcium balance returns to baseline in 3 days and previous results have examined frequent, long-term use, furosemide may not negatively impact bone mineral content even if used over long periods.