Hepatocytes from α1B-adrenoceptor knockout mice reveal compensatory adrenoceptor subtype substitution

1 alpha(1)-Adrenoceptors (ARs) play an important functional role in the liver; yet little is known about their cellular location. We identified the subtypes present in wild-type (WT) and alpha(1B)-AR knockout (KO) mice livers at 3 and 4 months of age, and investigated their distribution in hepatocytes. 2 The fluorescent alpha(1)-AR antagonist quinazolinyl piperazine borate-dipyrromethene (QAPB) was used to visualise hepatic alpha(1)-ARs and radioligand binding with [H-3]-prazosin was used to quantify the alpha(1)-AR population. 3 QAPB and [H-3]-prazosin bound specifically to hepatic alpha(1)-ARs with nanomolar affinity. The cellular distribution of alpha(1)-ARs was similar in WT and alpha(1B)-AR KO hepatocytes; QAPB binding was distributed diffusely throughout the cell with no binding evident on the plasma membrane. Radioligand binding produced B-max values as follows: 3-month WT - 76 +/- 3.3 fmol mg(-1); 4-month WT - 50 +/- 3.1 fmol mg(-1); 3-month alpha(1B)-AR KO - 7.4 +/- 0.73 fmol mg(-1); 4-month alpha(1B)-AR KO 30 +/- 2.0 fmol mg(-1). 4 In 3- and 4-month WT liver, all antagonists acted competitively. RS100329 (alpha(1A)-selective) and BMY7378 (alpha(1D)-selective) bound with low affinities, indicating the presence alpha(1B)-ARs. In 4-month a1B-AR KO liver prazosin produced a biphasic curve, whereas RS100329 and BMY7378 produced monophasic curves of high and low affinity, respectively, indicating the presence Of alpha(1A)-ARs. 5 In conclusion, we have made the novel observation that alpha(1)-ARs can compensate for one another in the absence of the endogenously expressed receptor; yet there appears to be no subtype-specific subcellular location of alpha(1)-ARs; the WT livers express alpha(1B)-ARs, while alpha(1B)-AR KO livers express alpha(1A)-ARs. This study provides new insights into both hepatocyte and alpha(1)-AR biology.