Carteolol is a weak partial agonist on beta 3-adrenergic receptors in brown adipocytes

The ability of the beta 1/beta 2 partial agonist carteolol to act as an agonist on beta 3-adrenergic receptors was investigated by studying its ability to stimulate thermogenesis (oxygen consumption) in brown fat cells isolated from hamsters. Carteolol was able to induce thermogenesis, with an EC50 of 5 microM, but it was only a partial (40%) agonist. D,L-Propranolol had a pKB of 5.1 as an antagonist against carteolol in this system, indicating that the carteolol effect was probably mediated via beta 3-receptors. Also in mouse and rat cells, carteolol was a partial agonist with EC50 values around 1 microM. Being a partial agonist, carteolol acted as an antagonist against norepinephrine-, BRL-37344-, or CGP-12177-stimulated thermogenesis with a pKB of approximately 5. The partial agonist effects of carteolol are discussed in relation to the absence of agonist effect of this compound on guinea-pig taenia cecum beta 3-receptors and in relation to the possible plurality of beta 3-receptors.