Agonist-independent effect of an allosteric enhancer of the A1 adenosine receptor in CHO cells stably expressing the recombinant human A1 receptor.

The allosteric enhancer PD 81,723, a 2-amino-3-benzoylthiophene derivative, has been shown to potentiate agonist binding to A1 adenosine receptors (A1AdoRs) and to enhance the functional effects of adenosine and adenosine analogs. The objective of this study was to determine whether the apparent agonist-independent effect of PD 81,723 observed in CHO cells stably expressing the recombinant human A1AdoR was due to the potentiation of the action of endogenous adenosine, to the presence of constitutive receptor activity and/or to the binding of PD 81,723 to the agonist binding site of the A1AdoR. The allosteric enhancer PD 81,723, the A1AdoR agonist (R)-N6-(2-phenylisopropyl)adenosine and adenosine all significantly inhibited forskolin-stimulated cAMP accumulation in intact cells and increased [35S]-5'-(gamma-thio)triphosphate binding to cell membranes. The effects of adenosine on cAMP formation and [35S]-5'-(gamma-thio)triphosphate binding were attenuated by adenosine deaminase, but the effects of PD 81,723 were not. In the presence of ADA, the A1AdoR antagonist 8-cyclopentyl-1,3-dipropylxanthine increased forskolin-stimulated cAMP accumulation in cells expressing the recombinant human A1AdoR but not in nontransfected CHO cells. In binding experiments, the agonist (R)-N6-(2-phenylisopropyl)adenosine, but not PD 81,723, significantly displaced the specific binding of the A1AdoR agonist [3H]-N6-cyclohexyladenosine and the antagonist [3H]-8-cyclopentyl-1,3-dipropylxanthine. The results of this study demonstrate that in CHO cells stably expressing the recombinant human A1AdoR, the agonist-independent effect of PD 81,723 is not due to potentiation of the action of endogenous adenosine or mediated by the binding of the allosteric enhancer to the agonist binding site of the recombinant human A1AdoR. It is possible that these effects are due to potentiation of constitutive receptor activity by PD 81,723.

Influence of yohimbine and tolazoline on the cardiovascular, respiratory, and sedative effects of xylazine in the horse.

To determine the effects of yohimbine and tolazoline on the cardiovascular, respiratory and sedative effects of xylazine, four horses were sedated with xylazine and treated with either yohimbine, tolazoline or saline. Xylazine was administered as an intravenous (i.v.) bolus (1.0 mg/kg) followed by a continuous infusion at the rate of 12 micrograms/kg/min. Heart rate, respiratory rate, mean arterial pressure, arterial blood gases, and the chin-to-floor distance were recorded throughout the experiment. After 60 min, either yohimbine or tolazoline was administered i.v. in incremental doses until reversal of sedation (defined as the return of the chin-to-floor distance to baseline values) was achieved. A control group in which a saline bolus was administered instead of an antagonist drug was included for comparison. The average dose of yohimbine administered was 0.12 +/- 0.02 (SEM) mg/kg. While the average dose of tolazoline was 7.5 +/- 1.1 mg/kg. Both tolazoline and yohimbine antagonized the ventricular bradycardia and A-V conduction disturbances observed with xylazine administration. No change in mean arterial pressure was observed with xylazine or yohimbine administration, but tolazoline caused persistent mild systemic hypertension. There were no clinically significant changes in respiratory rate or arterial blood gas values with administration of either xylazine, yohimbine or tolazoline. The chin-to-floor distance decreased significantly with xylazine administration and increased significantly with administration of either yohimbine or tolazoline. In conclusion, both yohimbine and tolazoline successfully antagonized the cardiovascular and CNS depression associated with xylazine administration.

Pulmonary edema associated with transient airway obstruction in three horses.

Pulmonary edema associated with transient airway obstruction was detected in 3 horses. The cause of obstruction was different in each horse, but after relief of the obstruction, clinical signs and radiographic abnormalities were indicative of pulmonary edema. In 2 of the 3 horses, pink frothy fluid was evident in the airways. The horses were treated with furosemide, nasal insufflation of O2, anti-inflammatory agents, and anti-biotics. Of the 3 horses examined, 1 horse died acutely, 1 horse recovered fully, and 1 developed pleuritis and was subsequently euthanatized.