Differential Blocking Effects of Tetrodotoxin on Double‐peaked Vasoconstrictor Responses to Periarterial Nerve Stimulation in Canine Isolated, Perfused Splenic Artery

Abstract

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.

PURINERGIC and ADRENERGIC COTRANSMISSION IN CANINE ISOLATED and PERFUSED GASTROEPIPLOIC ARTERIES

“…Studies on canine splenic artery also showed that the proportion of ATP and noradrenaline released from sympathetic nerve terminals is dependent upon the stimulation frequency 2,4 , 5,7 . It was therefore claimed that ATP and noradrenaline are possibly released from two separate populations of exocytotic vesicles within the periarterial sympathetic nerve in the canine splenic artery 19–22 . This is also likely to be the case in the canine gastroepiploic artery, because frequency dependent release of ATP and noradrenaline was also demonstrated.…”

“…Recently, studies performed in the canine splenic artery showed that guanethidine, an adrenergic neuron blocker, exerted a dominant inhibitory effect on adrenergic rather than purinergic components of sympathetic nerve cotransmission, indicating that guanethidine‐sensitive mechanisms may contribute mainly to determine noradrenaline secretion from neurosecretory vesicles rather than ATP secretion 19 . In addition, progressive inhibition of sodium channels by increasing the concentration of tetrodotoxin exerted a more preferential inhibition of adrenergic rather than purinergic components, suggesting that tetrodotoxin‐sensitive sodium channels may have a more important role in determining the adrenergic rather than purinergic transmission of sympathetic nerves 20 . It was further suggested that prolonged cold storage, which causes irreversible degeneration of adrenergic nerve fibres, preferentially depresses cotransmission of the purinergic component, whereas the adrenergic component is largely unaffected 21,22 .…”

Effects of ω‐conotoxin GVIA and diltiazem on double peaked vasoconstrictor responses to periarterial electric nerve stimulation in isolated canine splenic artery

“…Recently, Yang & Chiba (1999b,1999c) observed that the function of adrenergic component largely remained even in the cold stored preparation for 4–7 days but purinergic component of response to nerve stimulation was markedly damaged, because the first peaked responses became clearly smaller but second ones were only slightly depressed. Moreover, Yang & Chiba (1999d) also demonstrated that a smaller dose of tetrodotoxin, a selective sodium channel blocker inhibited only an adrenergic component but not purinergic component, showing that tetrodotoxin blocks second peaked vasoconstrictions induced by periarterial nerve stimulation in canine splenic artery but not first peaked ones. More recently, Yang & Chiba (1999e) demonstrated that guanethidine preferentially inhibited the second peaked adrenergic but not the first peaked purinergic responses.…”

Purinergic receptors in the splanchnic circulation

“…In the rabbit ileocolic artery, activation of presynaptic α 2 -adrenoceptors preferentially inhibits the release of noradrenaline [16]. In canine splenic arteries, adrenergic neuron blockade with guanethidine [189] or inhibition of sodium channels by tetrodotoxin [188] mainly depresses the adrenergic rather than the purinergic component. Opposing to this, the purinergic component is depressed, rather than the adrenergic component, after irreversible degeneration of adrenergic nerve fibres due to cold storage [190] or with dipyridamole [119].…”