[Type and content of adrenoceptors in toad heart]

The type and content of adrenoceptors in heart of normothermic and cold-acclimatized toad were studied by radioligand binding assay at different testing temperatures. The Bmax and Kd values of membrane from normal toad heart binding to 3H-DHA at 37 degrees C were respectively 55.11 +/- 6.22 fmol/mg protein and 2.15 +/- 0.42 nmol/L for the whole heart, 55.80 +/- 7.03 and 2.65 +/- 0.37 for sino-atrium and 54.27 +/- 3.06 and 1.84 +/- 0.14 for ventricle. While the binding to 3H-DHE was very low and nonspecific. When membrane from cold-acclimatized toad at 5-8 degrees C for 10 days was examined for its binding to 3H-DHA or 3H-DHE at 10 degrees C, the values of Bmax and Kd were as same as those of normothermic toad examined at 37 degrees C. The present results suggest that the toad heart is lacking of alpha-adrenoceptor, and the type and content of adrenoceptors are not influenced by temperature.