alpha(1D)-Adrenergic receptors, key regulators of cardiovascular system function, are organized as a multi-protein complex in the plasma membrane. Using a Type-I PDZ-binding motif in their distal C-terminal domain, alpha(1D)-ARs associate with syntrophins and dystrophin-associated protein complex (DAPC) members utrophin, dystrobrevin and alpha-catulin. Three of the five syntrophin isoforms (alpha, beta(1) and beta(2)) interact with alpha(1D)-ARs and our previous studies suggest multiple isoforms are required for proper alpha(1D)-AR function in vivo. This study determined the contribution of each specific syntrophin isoform to alpha(1D)-AR function. Radioligand binding experiments reveal alpha-syntrophin enhances alpha(1D)-AR binding site density, while phosphoinositol and ERK1/2 signaling assays indicate beta(2)-syntrophin augments full and partial agonist efficacy for coupling to downstream signaling mechanisms. The results of this study provide clear evidence that the cytosolic components within the alpha(1D)-AR/DAPC signalosome significantly alter the pharmacological properties of alpha(1)-AR ligands in vitro. (C) 2011 Elsevier Inc. All rights reserved.