Relaxation of equine tracheal muscle in vitro by different adrenoceptor drugs

Abstract

Strips of tracheal smooth muscle from 12 horses were contracted by carbachol in tissue baths under isometric conditions. This contraction (approximately 50% of maximum: EC50) was relaxed completely with adrenoceptor drugs. The only exception was clenbuterol, where the degree of relaxation was approximately 90%. In all horses the EC50-value for isoprenaline (mean 1.6 x 10(-8) M) was less than that for adrenaline (mean 9.6 x 10(-8) M) and noradrenaline (mean 1.8 x 10(-6) M). The potency ratio was 1 < 6 < 110 which indicates that the beta 2-subtype dominates among the beta-adrenoceptors of equine airways. All preparations were also very sensitive to the specific and potent beta 2-receptor agonists clenbuterol (mean 5.7 x 10(-9) M) and procaterol (mean 3.6 x 10(-10) M). No differences in EC50-values due to age, sex and breed were observed in this material. The standard deviation of the mean EC50-values seems to be larger for the specific beta 2-adrenoceptor agonists than for the unspecific. A reason for this could be differences in the pattern of the beta-adrenoceptor population.

International Workshop on Equine Chronic Airway Disease Michigan State University 16–18 June 2000

“…Defective inhibitory control of airway smooth muscle could also participate in the aetiology of bronchospasm. Inhibition of equine airway smooth muscle contraction occurs via activation of 2 -adrenoceptors LeBlanc et al 1991;Torneke et al 1997Torneke et al , 1998, release of nitric oxide (NO) from inhibitory nonadrenerg i c noncholinergic (iNANC) nerves (Yu et al 1994b) and release of inhibitory prostanoids such as PGE 2 from the airway mucosa (Yu et al 1993). During acute 'heaves', iNANC-mediated relaxation is absent in bronchi Yu et al 1994a) and there is a shift in mucosal prostanoid production from inhibitory prostanoids such as PGE 2 to excitatory prostanoids (Yu et al 1994a).…”

Tracheal rupture following general anaesthesia in a horse

“…Clenbuterol, a beta 2 ‐adrenoceptor agonist, has been shown to relax tracheal muscle in vitro (Törneke et al . ). Relaxation of the trachea will reduce airway resistance and therefore the administration of clenbuterol to this horse may have contributed to the favourable outcome.…”

Pharmacokinetics of procaterol in thoroughbred horses

“…Intravenous administration of β 2 ‐adrenoreceptor agonists is known to cause excitement and increased HR in horses (Törneke et al ., ). Therefore, preliminary trials for i.v.…”

“…PCR administration. Similar effects in horses have been reported with other β 2 ‐adrenoreceptor agonists, such as terbutaline, and these phenomena are a direct response of stimulation of β ‐adrenoreceptors (Hasson et al ., ; Törneke et al ., ). As the change in HR had strong correlation with plasma PCR concentrations in this study, these adverse responses are likely due to high PCR concentrations in the plasma.…”

“…Comparisons of pharmacokinetics between these β 2 ‐adrenoreceptor agonists indicate that clenbuterol is the most suitable drug for oral administration in horses. However, in vitro studies showed that terbutaline and PCR had higher efficacy for the relaxation of equine smooth muscle than clenbuterol (Törneke et al ., , ). To confirm the clinical effects of PCR by p.o.…”