Genetic analysis of alpha 2-adrenergic receptors and blood pressure using Dahl salt-sensitive rats.

Objective: To evaluate the role of alpha 2-adrenergic receptors in genetic hypertension by cosegregation analysis using Dahl rats.

Design: Inbred Dahl salt-sensitive (SS/Jr) rats were crossed with inbred Dahl salt-resistant (SR/Jr) rats; also, SS/Jr rats were crossed with several control strains, and large F2 populations were subsequently produced from each cross. All F2 populations were raised on a high-salt diet. The rats were genotyped, where possible, at the loci for three different subtypes of alpha 2-adrenergic receptors designated as classes I, II and III. The blood pressures of the rats classified by genotype at each alpha 2-adrenergic receptor subtype locus were compared using analysis of variance.

Methods: Genomic clones of three classes of alpha 2-adrenergic receptors were isolated from genomic lambda-phage libraries of SS/Jr or SR/Jr rat strains, or both, by screening with complementary DNA for human alpha 2-adrenergic receptors. Fragments of the rat genomic clones obtained were used for genotyping by restriction fragment length polymorphism. Also, cloned genomic DNA flanking the alpha 2-adrenergic receptors and containing microsatellites was sequenced; genotyping at informative microsatellite markers was performed using the polymerase chain reaction. Two of the three classes of rat alpha 2-adrenergic receptors were localized to rat chromosomes by linkage analysis or using a panel of mouse-rat hybrid somatic cell lines.

Results: Rat alpha 2-adrenergic receptor classes I and III genes were assigned to rat chromosomes 14 and 3, respectively. These correspond to alpha 2-adrenergic receptor genes on human chromosomes 4 and 2, respectively. Extensive cosegregation analysis, involving five alleles in six segregating populations for class I alpha 2-adrenergic receptors, yielded no evidence of an effect of these loci on blood pressure. Classes II and III alpha 2-adrenergic receptors could each be tested in only one population and there was no evidence for an effect of either receptor gene on genetic differences in blood pressure. The dopamine-1B receptor was closely linked to the class I alpha 2-adrenergic receptor on rat chromosome 14. Thus, the negative cosegregation of the class I receptor with blood pressure applies equally to the dopamine-1B receptor.

Conclusions: Genetic analysis in segregating populations involving crosses of inbred Dahl salt-sensitive rats with five other strains provides no evidence for a genetic effect of class I alpha 2-adrenergic receptors, or of the dopamine-1B receptor, on blood pressure. Classes II and III alpha 2-adrenergic receptors also failed to cosegregate with blood pressure but, because only limited testing was possible with the classes II and III receptors, this negative result is not definitive.