Truncated isoforms inhibit [H-3]prazosin binding and cellular trafficking of native human alpha(1A)-adrenoceptors

We have identified from human liver eight alpha(1A)-adrenoceptor (alpha(1A) -AR) splice variants that were also expressed in human heart, prostate and hippocampus. Three of these alpha(1A)-AR isoforms (alpha(1A-1)-AR, alpha(1A -2a)-AR and alpha(1A-3a)-AR) gave rise to receptors with seven transmembran e domains (7TM alpha(1A)-AR). The other five (alpha(1A-2b)-AR, alpha(1A-2c) -AR, alpha(1A-3c)-AR, alpha(1A-5)-AR and alpha(1A-6)-AR) led to truncated r eceptors lacking transmembrane domain VII (6TM alpha(1A)-AR). The 7TM alpha (1A)-AR isoforms transiently expressed in COS-7 cells bound [H-3]prazosin w ith high affinity (K-d 0.2 nM) and mediated a noradrenaline (norepinephrine )-induced increase in cytoplasmic free Ca2+ concentration, whereas the 6TM alpha(1A)-AR isoforms were incapable of ligand binding and signal transduct ion. Immunocytochemical studies with N-terminal epitope-tagged alpha(1A)-AR isoforms showed that the 7TM alpha(1A)-AR isoforms were present both at th e cell surface and in intracellular compartments, whereas the 6TM alpha(1A) -AR isoforms were exclusively localized within the cell. Interestingly, in co-transfected cells, each truncated alpha(1A)-AR isoform inhibited [H-3]pr azosin binding and cell-surface trafficking of the co-expressed 'original' 7TM alpha(1A-1)-AR. However, there was no modification of either the [H-3]p razosin-binding affinity or the pharmacological properties of alpha(1A-1)-A R. Immunoblotting experiments revealed that coexpression of the alpha(1A-1) -AR with 6TM alpha(1A)-AR isoforms did not impair alpha(1A-1)-AR expression . Therefore the expression in human tissues of many truncated isoforms cons titutes a new regulation pathway of biological properties of alpha(1A)-AR.