Splice isoforms of alpha(1a)-adrenoceptor in rabbit

1 Two splice isoforms of rabbit alpha(1a)-adrenergic receptor (AR), (named alpha(1a)-OCU.2-AR and alpha(1a)-OCU.3-AR) have been isolated from the live r cDNA library in addition to the previously reported isoform (alpha(1a)-OC U.1-AR). Although they have the identical splice position with human alpha( 1a)-AR isoforms, the C-terminal sequences are distinct from those of human isoforms. 2 Among these rabbit alpha(1a)-AR isoforms, there are no significant differ ences in pharmacological properties: high affinity for prazosin, WB4101, KM D-3213 and YM617 and low affinity for BMY7378, using COS-7 cells expressing each isoform by radioligand binding assay. 3 Competitive reverse transcription-polymerase chain reaction (RT-PCR) anal ysis revealed that mRNA of alpha(1a)-ARs was expressed in liver, thoracic a orta, brain stem and thalamus of rabbit. The splice isoforms exhibited a di stinct distribution pattern in rabbit; alpha(1a)-OCU.1-AR was expressed mos t abundantly in those tissues. 4 CHO clones, stably expressing each isoforms with receptor density 740 fmo l mg(-1) protein in alpha(1a)-OCU.1-AR, 1200 fmol mg(-1) in alpha(1a)-OCU.2 -AR and 570 fmol mg(-1) in alpha(1a)-OCU.3-AR, respectively, showed a norad renaline-induced increase in inositol trisphosphate which was suppressed by prazosin. 5 Noradrenaline elicited a concentration-dependent increase in extracellula r acidification rate (EAR) in the CHO clones with pEC(50) values of 6.19 fo r alpha(1a)-OCU.1-AR, 6.49 for alpha(1a)-OCU.2-AR and 6.58 for alpha(1a)-OC U.3-AR, respectively. 6 Noradrenaline caused a concentration-dependent increase in intracellular Ca2+ concentration ([Ca2+](i)) in the CHO clones with pEC(50) values of 6.1 4 for alpha(1a)-OCU.1-AR, 7.25 for alpha(1a)-OCU.2-AR and 7.70 for alpha(1a )-OCU.3-AR, respectively. 7 In conclusion, the present study shows the occurrence of three splice iso forms of rabbit alpha(1a)-AR, which an unique in C-terminal sequence and in tissue distribution. They show similar pharmacological profiles in binding studies but alpha(1a)-OCU.3-AR had the highest potency of noradrenaline in functional studies in spite of the lowest receptor density. These findings suggest that the structure of C-terminus of alpha(1a)-ARs may give the cha racteristic functional profile.