Previous studies suggest that systemic arterial pressure is tonically regulated by the interaction of peripheral sympathetic nerves with vascular alpha-1A adrenoceptors in vivo. To explore this relationship further, the present study examined the inhibitory effect of selective alpha-1A [5-methylurapidil (5-MU) and nifedipine (NIF)] and alpha-1B [chloroethylclonidine (CEC)] antagonists on the pressor response to electrical stimulation (ES) of the spinal cord in pithed rats. Diastolic pressure changes were measured in the presence of 5-MU or CEC and compared with control responses. Pretreatment with 5-MU (0.5 mg/kg i.v.) significantly suppressed the ES pressor response (50-80% inhibition) at all stimulation frequencies. Likewise, NIF (inhibitor of calcium influx associated with alpha-1A adrenoceptor activation) selectively inhibited the pressor response to ES to the same degree as did 5-MU. CEC (25 mg/kg i.v.) also significantly shifted the ES response curve; however, this effect was mediated by activation of presynaptic alpha-2 receptors on sympathetic terminals because prior administration of idazoxan (5 mg/kg) prevented the inhibitory effect of CEC. Based on the potent inhibitory effects of 5-MU and NIF on the ES pressor response in the pithed rat, it was concluded that vascular alpha-1A adrenoceptors reside in the synaptic region of neurovascular junction where they are primarily activated by neuronal norepinephrine release.(ABSTRACT TRUNCATED AT 250 WORDS)