Pharmacological analysis of the double peaked vasoconstrictor responses to periarterial electric stimulation

Vasoconstrictor responses to periarterial nerve stimulation were studied with the use of prazosin (an alpha1-adrenoceptor antagonist) and alpha,beta-methylene ATP (a P2X receptor desensitizer) in canine isolated, perfused splenic arterial preparations. Double peaked responses (two phases of the constriction) were readily induced with the conditions of 30 s trains of pulses at 10 V amplitude, 1 ms duration in a frequency-related manner. At low frequencies (1-3 Hz), the 1st phase might contain only a purinergic component which was mostly inhibited by treatment with alpha,beta-methylene ATP. At high frequencies (4-10 Hz), the 1st phase was in part inhibited by prazosin and the remaining component of this phase was abolished by alpha,beta-methylene ATP. The 2nd phase response was markedly inhibited by prazosin, and remaining component of this phase was abolished by alpha,beta-methylene ATP at all frequencies used. It is concluded that (1) the 1st phase is probably induced via an activation of P2X receptors at low frequencies, and at high frequencies via both P2X and alpha1-adrenoceptors, and (2) the 2nd phase probably involves mostly alpha1-adrenoceptors and partially P2X receptors.