Atropine and glycopyrrolate significantly raise the heart rate of Sonoran Desert toads (Incilius alvarius).

Glycopyrrolate offers longer HR elevation than atropine in conscious toads, but efficacy is reduced under alfaxalone sedation.

This prospective, randomized crossover study evaluated the effects of atropine (0.1 mg/kg IM) and two doses of glycopyrrolate (0.025 and 0.05 mg/kg IM) on heart rate (HR) in Sonoran Desert toads (Incilius alvarius). Part 1 (n=19 conscious toads) demonstrated that all three anticholinergic protocols significantly increased HR compared to both saline and negative controls between 3 and 15 minutes post-administration. Atropine's effect was significant versus saline from 3 to 30 minutes, while both glycopyrrolate doses maintained significance at 60 minutes, suggesting longer duration of action. Part 2 (n=10) assessed glycopyrrolate (0.05 mg/kg) following alfaxalone sedation (10 mg/kg IM), revealing that glycopyrrolate produced a statistically significant HR increase versus saline only at 10 minutes (mean difference 6.8 beats/min), indicating a markedly reduced and shorter efficacy window in sedated animals. These findings are clinically relevant given the challenges of amphibian anesthesia, particularly their propensity for bradycardia and unique anuran physiology, including vagal dominance. Both drugs demonstrated utility in conscious toads, with glycopyrrolate offering a longer effect. However, clinicians should be aware that alfaxalone sedation substantially attenuates glycopyrrolate's chronotropic efficacy. This study provides foundational pharmacological data to guide anticholinergic use in amphibian patients, though further research is needed to establish full clinical dosing protocols and safety profiles.

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