Urethral sphincter mechanism incompetence (USMI) is the most common cause of urinary incontinence in spayed adult dogs. Urethral tone in dogs is primarily mediated by an alpha1-adrenergic-receptor subtype. Thus, alpha1-adrenergic-receptor agonists (usually phenylpropanolamine or ephedrine) have been used to treat USMI in dogs. Although adverse effects associated with these drugs have been reported in humans, no important side effects have been reported in dogs and their success rate in treating USMI has been high. There is a lack of information, however, on the urodynamic and hemodynamic effects of these drugs as well as the duration of oral administration required to achieve maximum urodynamic effects.

To this end, 20 sexually intact adult female beagles were divided into 4 groups and given phenylpropanolamine (1.5 mg/kg PO) once, twice, or three times daily, or ephedrine (1.5 mg/kg PO) twice daily for 14 days. Urethral pressure profiles were measured with dogs under propofol anesthesia on days 0, 7, and 14. Arterial and central venous pressures were measured before anesthetic induction and 10 and 35 minutes after induction. The authors found that phenylpropanolamine administered once daily and ephedrine administered twice daily significantly increased urethral resistance. However, administering phenylpropanolamine Q 8 or 12 H did not significantly increase urethral pressure above baseline values and did not increase urethral resistance over measurements obtained with once-daily dosing. In addition, maximum urodynamic effects were achieved by 7 days of treatment and did not change significantly from day 7 to day 14. Significant increases in diastolic and mean arterial blood pressures with a compensatory decrease in heart rate were observed in all dogs during the treatment period. The authors concluded that phenylpropanolamine at 1.5 mg/kg PO Q 24 H or ephedrine 1.5 mg/kg PO Q 12 H may be recommended for treating USMI in dogs. Treatment efficacy can be assessed after 7 days. Due to their effects on blood pressure, however, caution is in order and blood pressure should be monitored when these drugs are used in dogs that have concurrent cardiovascular disease.

COMMENTARY: Although it was conducted in normal dogs, this study of urethral and hemodynamic effects of phenylpropanolamine (PPN) and ephedrine produced more than one point of possible clinical relevance. While both drugs significantly increased maximal urethral pressure (MUP) and could be evaluated for efficacy after 1 week of administration, PPN appeared to offer the advantage of once-daily dosing. In addition, more frequent administration of PPN (every 8-12 hours) did not significantly increase the MUP over that of once-daily dosing. Based on this finding it may be more efficacious to use a combination of drugs (eg, PPN and hormonal therapy) rather than continuing to increase the dosing frequency of a single agent.

Evaluation of the urodynamic and hemodynamic effects of orally administered phenylpropanolamine and ephedrine in female dogs. Carofiglio F, Hamaide AJ, Farnir F, et al. AM J VET RES 67:723-730, 2006.