Differential blocking effects of tetrodotoxin on double-peaked vasoconstrictor responses to periarterial nerve stimulation in canine isolated, perfused splenic artery.

1. In the present study, we investigated the effects of progressive inhibition of neuronal sodium channels by increasing concentrations of tetrodotoxin (TTX; 1-30 nmol/L) on the double-peaked vasoconstrictor responses to electrical periarterial nerve stimulation in the canine isolated and perfused splenic artery. 2. Double-peaked vasoconstrictions (biphasic vasoconstrictor responses) were consistently observed in following electrical stimulation with 30 s trains of pulses at 1-10 Hz. At low frequencies of stimulation (1-3 Hz), a submaximal concentration of 3 nmol/L TTX had no effect on the first phase of the contractile response, but almost completely inhibited the second-phase response. At high frequencies (6-10 Hz), the two vasoconstrictor phases were almost equally inhibited by 50% by 3 nmol/L TTX. A three-fold increase in the concentration of TTX used (10 nmol/L) abolished the second-phase vasoconstriction at all stimulation frequencies tested, whereas this concentration of TTX failed to block the first-phase response. Further increasing the concentration of TTX to 30 nmol/L completely blocked the remaining first-phase response. 3. Treatment with 0.1 mumol/L prazosin did not modify the first-phase response to any of the stimulation frequencies in the presence of 3 nmol/L TTX. Moreover, 0.1 mumol/L prazosin had no affect on the second-phase response at low frequencies (1-3 Hz), while at high frequencies (6-10 Hz) it slightly, but significantly inhibited the second-phase response. The vasoconstrictor responses that persisted after 3 nmol/L TTX and 0.1 mumol/L prazosin were completely suppressed by subsequent application of 1 mumol/L alpha, beta-methylene ATP at all stimulation frequencies (1-10 Hz). 4. In conclusion, progressive inhibition of sodium channels by increasing the concentration of TTX may exert a more preferential inhibition on adrenergic rather than purinergic components, suggesting that TTX-sensitive sodium channels may have a more important role in determining the adrenergic rather than purinergic transmission of sympathetic nerves.