Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

CHAPTER 4 THE PHARMACOLOGY OF THE AUTONOMIC NERVOUS SYSTEM
of α-adrenoceptors on the smooth muscle of the bladder
sphincter and pelvic urethra results in increased tone of
smooth muscle cells and improvement of the sphincter
function.
Formulations and dose rates
Oral formulations of phenylpropanolamine for veterinary use are available
as capsules or as syrup.
DOGS
• 1 mg/kg q.8 h
CATS
• 1–1.5 mg/kg q.12 h
Adverse effects
A number of studies have reported aggression, anorexia,
cardiac arrhythmia, hypertension and diarrhea as a
result of using phenylpropanolamine in animals.
Contraindications and precautions
Because phenylpropanolamine is not a selective α-
adrenoceptor agonist but shows some residual potency
at β-adrenoceptors, its use is contraindicated in pregnant
and lactating animals.
Known drug interactions
When used in combination with β-adrenoceptor blockers,
especially nonselective β-adrenoceptor blockers,
phenylpropanolamine can cause a strong increase in
peripheral vascular resistance.
During anesthesia with inhalation narcotics such as
halothane and isoflurane, phenylpropanolamine can
induce cardiac arrhythmia.
Dobutamine
Clinical applications
Dobutamine is an apparently β 1 -selective adrenoceptor
agonist which clinically is used as a positive inotrope in
the treatment of heart failure and as an emergency treatment
for cardiogenic shock. It is also used as a diagnostic
treatment for the early detection of myocardial
systolic dysfunction.
Mechanism of action
The apparent β 1 -selective effects of dobutamine, showing
mainly positive inotropic actions in the heart, are based
on partly opposing actions of the constituents of naturally
occurring racemic dobutamine. (+)Dobutamine is
a nonselective β-adrenoceptor agonist and also a potent
antagonist at α 1 -adrenoceptors, whilst (−)dobutamine
shows very low potency at β-adrenoceptors but has
strong α 1 mimetic effects. This results in balancing out
of the α 1 -adrenoceptor actions of the isomers whilst
some local vascular α 1 effects result in physiological
antagonism of β 2 actions, leading to an apparently β 1 -
selective pharmacodynamic profile of dobutamine.
As a result, the predominant effect of dobutamine
treatment is a strong increase in cardiac contraction
force which, compared to the effects of adrenaline and
isoprenaline, is accompanied by only very sparse positive
chronotropic actions.
Pharmacokinetics
The plasma half-life of dobutamine is extremely low in
dogs (∼2min) and the drug therefore needs to be infused
intravenously at a rate of 2–20 µg/kg/min. The plasma
half-life in cats is higher and the recommended infusion
rate is