Characterization of dog fat cell adrenoceptors: variations in alpha-2 and beta adrenergic receptors distribution according to the extent of the fat deposits and the anatomical location.

In the search of the most accurate animal model for studies on fat cell adrenoceptor regulation, the characterization of dog fat cell beta and alpha-2 adrenoceptor properties was carried out using selected radioligands and biological measurements of fat cell function. Moreover, the distribution of dog fat cell adrenoceptors and the functional consequences were explored in fat deposits from different anatomical locations (perirenal, omental and s.c. adipose tissue). Beta and alpha-2 adrenoceptors involved in the control of the lipolytic activity of dog fat cells exhibit pharmacological characteristics which are very similar to those defined in human adipocytes. However, in contrast with the results obtained in human adipocytes, there is a preponderance of beta over alpha-2 adrenergic binding sites, whatever the location of the fat deposits, in the dog. The higher values of the ratio beta-sites over alpha-2-sites were found in perirenal adipocytes whereas the lowest values of this ratio were found in s.c. deposits. A close positive relationship exist between alpha-2 adrenoceptor number, alpha-2 adrenergic responsiveness and fat cell volume. To conclude, beta and alpha-2 adrenoceptors exhibit similar pharmacological properties in dog and human fat cells. Variations according to the anatomical location of the fat deposits exist in both species. However, as opposed to human fat cells, it is evident that the balance between beta and alpha-2 adrenergic responsiveness is predominantly under the control of beta adrenoceptors in dog fat cells.