Gentamicin tissue concentrations in equine small intestine and large colon

Gentamicin sulfate (2.2 mg/kg of body weight, IV) was given to anesthetized horses. Jejunal and large colon tissue samples (1 g), serum, and urine were collected over a 4-hour period. Maximum gentamicin concentrations in serum (10.06 +/- 2.85 micrograms/ml) occurred at 0.25 hours after injection. Maximum gentamicin concentrations in the large colon (4.13 +/- 1.80 micrograms/ml) and jejunum (2.26 +/- 1.35 micrograms/ml) occurred in horses at 0.5 and 0.33 hours, respectively. Tissue concentrations decreased in parallel with serum concentrations and were still detectable at the end of the 4-hour period. During the time that samples were collected, the total amount of gentamicin excreted in the urine ranged from 7.21 +/- 3.11 mg to 11.91 +/- 7.12 mg, with a mean urinary concentration of 57.01 +/- 5.37 micrograms/ml. Over the 4-hour collection period, the fraction of dose that was excreted unchanged in the urine was 4.8 +/- 1.9%. Pharmacokinetic analyses of the serum concentration-time data gave a serum half-life of 2.52 +/- 1.29 hours, volume of distribution of 227 +/- 83 ml/kg, and body clearance of 1.12 +/- 0.26 ml/min/kg. The half-lives of the antibiotic in the jejunum and large colon were 1.32 and 1.33 hours, respectively.