Fig. 9. Effect of fentanyl on displaced α1-adrenoceptor nonselective radiolabeled antagonist 125I-HEAT binding sites in competition binding assays. The negative logarithm of the inhibitory constant for the α1B-adrenoceptor subtype was fivefold higher ( *P < 0.05) than the α1D-adrenoceptor subtype but not significantly different at the α1A-adrenoceptor subtype. B/Bo = % specific binding .