RELAXATION OF EQUINE TRACHEAL MUSCLE IN-VITRO BY DIFFERENT ADRENOCEPTOR DRUGS

Strips of tracheal smooth muscle from 12 horses were contracted by car bachol in tissue baths under isometric conditions. This contraction (a pproximate to 50% of maximum: EC50) was relaxed completely with adreno ceptor drugs. The only exception was clenbuterol, where the degree of relaxation was approximate to 90%. In all horses the EC50-value for is oprenaline (mean 1.6 x 10(-8)M) was less than that for adrenaline (mea n 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 dominat es among the P-adrenoceptors of equine airways. All preparations were also very sensitive to the specific and potent beta(2)-receptor agonis ts clenbuterol (mean 5.7 x 10(-9)hr) and procaterol (mean 3.6 x 10(-10 )M). No differences in EC50-values due to age. sex and breed were obse rved 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 pat tern of the beta-adrenoceptor population.