Bolus intravenous injections of phosphorothioate oligonucleotides (PS-ODN) into primates cause profound hypotension, which has been attributed to complement activation, the biochemical pathway leading to acute inflammatory response. Because the hypotension was not accompanied by peripheral or pulmonary edema and epinephrine was not effective, but administration of 200 ml Ringer's lactate was effective, we examined the possibility that the 15-base PS-ODN interferes with sympathetic tone. We administered doses ranging from 3.3 to 10 mg/kg of a 15-base PS-ODN as a 30-60 s iv bolus into the right atrium of conscious Macaca mulatta. Blood pressure fell to 27 mm Hg following a 5.0 mg/kg dose, but no hypotension was observed after a 3.3 mg/kg dose; 10 mg/kg was lethal. Adrenergic receptor binding was evaluated in radioligand binding assays using rat cerebral cortex membranes with radiolabeled prazosin. The 15-base PS-ODN competes with prazosin for the alpha(1)-adrenergic receptor with an IC50 of 14 microM, which favors binding over serum albumin (K(d) = 37 to 48 microM). Admixing serum albumin with 5.0 mg/kg 15-base PS-ODN prior to injection prevented hypotension, suggesting that unbound PS-ODN interferes with sympathetic tone before binding to plasma proteins. Interactions of the 15-base PS-ODN with the alpha(1)-adrenergic receptor in vivo were confirmed by a decreased response to phenylephrine. Reducing the length from 15 to 9 or 5 bases abolished alpha(1)-adrenergic receptor binding in vitro and bolus infusion of 5.0 mg/kg of 9-base PS-ODN no longer produced hypotension. In conclusion, the 15-base PS-ODN shows cooperative binding to the alpha(1)-adrenergic receptor, which produces cardiovascular effects that are oligomer length, dose, and formulation dependent.