Can [125I]-Iodocyanopindolol Label β3-Adrenoceptors in Rat Urinary Bladder?

β(3)-Adrenoceptors have been demonstrated to mediate urinary bladder smooth muscle relaxation but proof of their expression at the protein level has been missing because of lack of suitable antibodies or radioligands. As among various available radioligands [(125)I]-iodocyanopindolol ([(125)I]-ICYP) exhibited the smallest problems in labeling cloned human β(3)-adrenoceptors in previous studies, we have explored its suitability to label β(3)-adrenoceptors in rat urinary bladder in saturation and competition radioligand binding experiments. Rat lung was used as an internal control and exhibited all characteristics expected from this tissue with regard to β1/β2-adrenoceptor labeling. Saturation and competition binding studies with [(125)I]-ICYP in rat bladder yielded saturable binding sites with an affinity compatible with β(3)-adrenoceptors. In competition experiments various agonists and antagonists largely exhibited a profile compatible with a population consisting largely of β(3)-adrenoceptors. However, the binding competition properties of ICI 118,551 and SR 59,230A were not easily explained by the idea of labeling a homogeneous β(3)-adrenoceptor population but interpretation of the data was limited by a high degree of non-specific binding in [(125)I]-ICYP concentrations required to label the receptors. We conclude that [(125)I]-ICYP can be used to label tissue β(3)-adrenoceptors but results obtained with this ligand have to be interpreted with caution.