Characterization of adrenoceptor types and subtypes in American bullfrogs acclimated to warm or cold temperature.

While indirect evidence suggested that the responsiveness of frog adrenoceptors changes in response to temperature, direct measurement of adrenoceptor binding following acclimation to warm and cold temperatures had not been done. In the present study, the radioligands [3H]prazosin, [3H]RX821002, and [125I]cyanopindolol were used to label and quantify alpha 1-, alpha 2-, and beta-adrenoceptors in bullfrogs acclimated to warm or cold environments. The number of alpha 1-, alpha 2-, and beta-adrenoceptors in atrium, ventricle, and kidney membranes was not significantly different between warm- and cold-acclimated frogs. Characterization of receptor subtypes using pharmacological antagonists demonstrated that alpha 2-adrenoceptors in frog spinal cord and kidney were of the same pharmacological subtype, which is similar to the mammalian alpha 2A-subtype. The beta-adrenoceptor in frog ventricle, atrium, and kidney was the beta 2-subtype. These results suggest that while the alpha 1-, alpha 2-, and beta-adrenoceptor types have evolved in the frog, multiple subtypes of adrenoceptors are not necessary for physiological regulation in this species.