1360 AJVR, Vol 64, No. 11, November 2003 D iarrhea has been reported as an adverse effect of antibiotic administration, including penicillin, in people and horses. 1-3 Anecdotally, horses frequently defecate within minutes after IV injection of penicillin G potassium (KPEN). Erythromycin and other macrolide antibiotics have been shown to have proki- netic effects on the gastrointestinal tract of several species, including horses. 4-9 Although the macrolides share a similar spectrum of antimicrobial activity with the penicillins, their molecular structure is quite dif- ferent, and there is no reason to suspect that the mech- anism of action on the motility of the gastrointestinal tract would be similar. The antibiotic drugs strepto- mycin, chloramphenicol, and tetracycline have been shown to have prokinetic effects on smooth muscle tis- sue in vitro, but sodium penicillin was without effect in the same study. 10 Chlortetracycline had a stimulatory effect on motility in dogs in vivo and rabbits in vitro. 11 Other antibiotics, including clindamycin, gentamicin, pivmecillinam, kanamycin, and trimethoprim, have been shown to decrease the peristaltic response in vitro. 12 In vitro, excess extracellular potassium can lower the resting membrane potential of smooth muscle cells and result in lowering the threshold of initiation of an action potential and subsequent muscular contrac- tion. 13 Bolus administration of K + could potentially have the same effect in vivo, resulting in intestinal smooth muscle contraction. Thus, effects of KPEN on intestinal motility could be attributed to bolus admin- istration of the potassium ion itself. The purpose of the study reported here was to evaluate the effect of KPEN or potassium chloride (KCl) administration on defe- cation and motility of the cecum and pelvic flexure in horses. Materials and Methods Experimental protocol—The experimental protocol was reviewed and approved by the Texas A&M University Laboratory Animal Care Committee. Four mature geldings and 1 mare (2 Thoroughbreds and 3 Quarter Horses) weighing between 461 and 547 kg were used. Feed and water were with- held for 24 hours, and horses were then anesthetized with xylazine hydrochloride (1.1 mg/kg, IV), ketamine hydrochlo- ride (2.2 mg/kg, IV), and diazepam (0.015 mg/kg, IV). Anesthesia was maintained with inhalation of sevoflurane (3.5 to 5.5%) in oxygen. A ventral midline celiotomy was per- formed, and 16 bipolar electrodes were introduced into the smooth muscle of the ileum, cecum, and pelvic flexure, as pre- viously described. 9 The 4 ileal electrodes were used in a differ- ent experiment. The surgical protocol involved induction of postoperative ileus by vigorous rubbing of the small intestine. 9 Amikacin (16 mg/kg) or gentamicin (6.6 mg/kg) was administered IV immediately prior to surgery and once daily for 4 to 5 days after surgery. Several horses also received metronidazole (10 mg/kg, PO, q 12 h) for 3 to 10 days after surgery. After surgery, horses were gradually returned to their regular ration, which consisted of free-choice coastal Bermuda grass hay and grain mix twice daily. Received March 14, 2003. Accepted May 19, 2003. From the Department of Large Animal Medicine and Surgery, College of Veterinary Medicine, Texas A&M University, College Station, TX 77843-4475. Dr. Bye’s present address is Central Houston Veterinary Hospital, 1638 Westheimer Rd, Houston, TX 77006. Dr. Hicks’ present address is Hope Veterinary Clinic, 4015 Hwy 281 S, Marble Falls, TX 78654. Supported by the Link Equine Research Endowment and the Department of Large Animal Medicine and Surgery, Texas A&M University. The authors thank Dr. Gwendolyn Carroll for technical assistance. Address correspondence to Dr. Roussel. Evaluation of the effects of penicillin G potassium and potassium chloride on the motility of the large intestine in horses Allen J. Roussel, DVM, MS; R. Neil Hooper, DVM, MS; Noah D. Cohen, VMD, MPH, PhD; Alicia D. Bye, DVM; Russell J. Hicks, DVM; Joseph L. Schulze, BS Objective—To evaluate effects of IV administration of penicillin G potassium (KPEN) or potassium chloride (KCl) on defecation and myoelectric activity of the cecum and pelvic flexure of horses. Animals—5 healthy horses. Procedure—Horses with 12 bipolar electrodes on the cecum and pelvic flexure received KPEN or KCl solu- tion by IV bolus 4 hours apart. Each horse received the following: 2 X 10 7 U of KPEN (high-dose KPEN) fol- lowed by 34 mEq of KCl (high-dose KCl), 1 X 10 7 U of KPEN (low-dose KPEN) followed by 17 mEq of KCl (low-dose KCl), high-dose KCl followed by high-dose KPEN, and low-dose KCl followed by low-dose KPEN. Number of defecations and myoelectric activity were recorded for 60 minutes. The first three 5-minute seg- ments and first four 15-minute segments of myoelec- tric activity were analyzed. Results—Number of defecations during the first 15- minute segment was greater after high-dose KPEN treatment than after high-dose or low-dose KCl treat- ment. Compared with reference indexes, myoelectric activity was greater in the pelvic flexure for the first 5- minute segment after high-dose KCl treatment, in the cecum and pelvic flexure for the first 5-minute seg- ment and in the pelvic flexure for the first 15-minute segment after low-dose KPEN treatment, and in the pelvic flexure for the first and second 5-minute seg- ments and the first three 15-minute segments after high-dose KPEN treatment. Conclusions and Clinical Relevance—IV administra- tion of KPEN stimulates defecation and myoelectric activity of the cecum and pelvic flexure in horses. Effects of KPEN may be beneficial during episodes of ileus. (Am J Vet Res 2003;64:1360–1363)