KRAJNIAK, K.G.; CORELY, C.; Southern Illinois Univ. Edwardsville: Effects of Alpha and Beta Adrenergic Agonists on the Earthworm Crop-gizzard

Epinephrine and norepinephrine cause a dose-dependent increase in contraction amplitude of the isolated crop-gizzard of the earthworm, Lumbricus terrestris. To determine whether the receptor that mediates this response is similar to any of the mammalian adrenergic receptors we challenged the isolated crop-gizzard with a series of agonists. Alpha adrenergic agonists included methoxamine, naphazoline, phenylephrine, synephrine, tetrahydrozoline, xylometazoline, while the beta adrenergic agonists were clonidine, isoproterenol, dobutamine, albuterol, tulobuterol, and clenbuterol. Epinephrine had an EC₅₀ of 1.2 x 10^-7 M and a relative efficacy (RE) of 1. The other agonists which excited the crop-gizzard included norepinephrine with an EC₅₀ of 3.5 x 10^-7 M and an RE of 0.739; phenylephrine with an EC₅₀ of 2.8 x 10^-7 M and an RE of 0.15; methoxamine with an EC₅₀ of 8.1 x 10^-8 M and an RE of 0.144; and tetrahydrozoline with an EC₅₀ of 2.2 x 10^-6 M and an RE of 0.099. The alpha agonists clonidine, naphazoline, and xylometazoline and the beta agonists albuterol, clenbuterol, isoproterenol, and tulobuterol did not affect the spontaneous rhythmicity of the earthworm crop-gizzard. Surprisingly synephrine and dobutamine both caused a dose-dependent decrease in amplitude which mimicked the effect of octopamine. Octopamine had an EC₅₀ of 3.3 x 10^-7 M, while synephrine had an EC₅₀ of 1.2 x 10^-8 M and dobutamine had an EC₅₀ of 1.8 x 10^-7 M. These data suggest that the adrenergic receptor in the Lumbricus crop-gizzard may have some similarity to the alpha receptor since the only alpha agonists were excitatory, although with a far lower efficacy.