Flecainide blocks the stimulatory effect of veratridine on slowly adapting pulmonary stretch receptors in anaesthetized rabbits without changing lung mechanics

Flecainide inhibits the stimulatory effect of veratridine on the response of airway mechanoreceptors to maintained inflations in rats

AIMS

the purpose of the present study was to investigate (a) whether maintained inflations result in the inhibition of slowly adapting pulmonary stretch receptor (SAR) discharge to elicit an abrupt change in receptor activity and (b) whether pretreatment with veratridine, a Na(+) channel opener, and/or flecainide, a Na(+) channel blocker, alters the responses of SAR properties to maintained inflations.

MAIN METHODS

we measured the properties of SAR activity during maintained inflations at different pressures in 31 anesthetized, artificially ventilated rats with unilateral vagotomy.

KEY FINDINGS

During maintained inflations (approximately 5, 10 and 15 cmH(2)O) for about 5s, the procedures did not produce the induction of inhibition of either 16 low-threshold SARs (firing during both inflation and deflation) or 15 high-threshold SARs (firing during inflation only). In these preparations, the excitatory responses of SARs to maintained inflations at three different pressures were markedly enhanced after administration of veratridine (50 μg/kg), but under such conditions, the inhibition of SAR discharges was not observed. In the same SAR preparations, after flecainide treatment (9 mg/kg) sufficient for the blockade of veratridine (50 μg/kg)-induced SAR stimulation, maintained inflations at higher pressures (10 and 15 cmH(2)O) greatly inhibited SAR discharges. Under these conditions, the inhibition of SAR discharges was not observed during maintained inflations at 5 cmH(2)O.

SIGNIFICANCE

These results suggest that neither low-threshold SARs nor high-threshold SARs in the rat lung are deactivated during maintained inflations at higher pressures.

Effects of potassium channel and Na+-Ca2+ exchange blockers on the responses of slowly adapting pulmonary stretch receptors to hyperinflation in flecainide-treated rats

1. The effects of K(+) channel blockers, such as 4-aminopyridine (4-AP) and tetraethylammonium (TEA), and a reverse-mode Na(+)-Ca(2+) exchange blocker, 2-[2-[4-(4-nitrobenzyloxyl) phenyl] ethyl] isothiourea methanesulphonate (KB-R7943), on the responses of slowly adapting pulmonary stretch receptor activity to hyperinflation (inflation volume=3 tidal volumes) were investigated in anaesthetized, artificially ventilated, unilaterally vagotomized rats after pretreatment with a Na(+) channel blocker flecainide. The administration of flecainide (9 mg kg(-1)) at a dose greater than that which abolished 50 microg kg(-1) veratridine-induced SAR stimulation also inhibited hyperinflation-induced stimulation of SARs. 2. In flecainide-treated animals, administration of 4-AP (0.7 and 2 mg kg(-1)) stimulated SAR activity during normal inflation and also caused a partial blockade of hyperinflation-induced SAR inhibition. 3. The discharges of SARs during normal inflation in flecainide-treated animals were not significantly altered by administration of either TEA (2 and 7 mg kg(-1)) or KB-R7943 (1 and 3 mg kg(-1)), but both K(+) channel and Na(+)-Ca(2+) exchange blockers partially attenuated hyperinflation-induced SAR inhibition. 4. These results suggest that hyperinflation-induced SAR inhibition in the presence of flecainide (9 mg kg(-1)) involves the activation of several K(+) conductance pathways.

Activation of sympathoadrenomedullary system increases pulmonary nitric oxide production in the rabbit

Nitric oxide (NO) is continuously produced in the lung and can be measured in exhaled gas of different species. To investigate a possible neuro-humoral regulation of pulmonary NO production in vivo we injected veratrine, an activator of Na(+) channels known to activate the sympathoadrenal system, in anaesthetized, mechanically ventilated and laparotomized rabbits. Exhaled NO concentration increased by 38+/-3% when plasma adrenaline rose from 12.3+/-3.1 to 49.5+/-10.7 pmol ml(-1) in response to veratrine (500 microg kg(-1), i.v.). Pretreatment with atenolol, a beta(1)-adrenoceptor antagonist (1 mg kg(-1)), or bilateral ligation of adrenal blood vessels inhibited the increase in exhaled NO in response to veratrine. Atenolol also decreased basal NO, suggesting an endogenous regulation of pulmonary NO by adrenaline. Neither phentolamine (1 mg kg(-1)), atropine (1 mg kg(-1)) nor vagotomy inhibited the veratrine-induced pulmonary NO production. These results suggest a role of the sympathoadrenal system in the regulation of pulmonary NO production.

Effects of ouabain and flecainide on CO(2)-induced slowly adapting pulmonary stretch receptor inhibition in the rabbit

The inhibitory effect of CO2 on slowly adapting pulmonary stretch receptors (SARs) was examined before and after administration of ouabain, a Na+-K+ ATPase inhibitor, and flecainide, a Na+ channel blocker. The experiments were performed in anesthetized, artificially ventilated rabbits after vagus nerve section. CO2 inhalation (maximal tracheal CO2 concentration ranging from 9.2 % to 10.4%) for about 60 sec decreased the receptor activity during both inflation and deflation. The magnitude of decreased SAR activity during deflation was greater than that seen during inflation. Administration of ouabain (25 microg/kg) initially stimulated SAR activities during inflation and deflation, and after 20 min, the SAR response was still kept excitatory in both inflation and deflation phases. Under these conditions, CO2 inhalation inhibited SAR activities during inflation and deflation. Flecainide treatment (3 mg/kg) that abolished veratridine (30 microg/kg)-induced SAR excitation had no significant effect on the inhibitory responses of SAR activity to CO2. These results suggest that the inhibitory effect of CO2 occurs when ouabain results in intracellular Na+ concentration ([Na+]i) increases in the SAR endings, and that CO2-induced SAR inhibition may not be related to the reduction of influx of Na+ through voltage-gated Na+ channels.

Excitatory mechanism of veratridine on slowly adapting pulmonary stretch receptors in anesthetized rabbits

The excitatory effects of veratridine on slowly adapting pulmonary stretch receptors (SARs) were studied before and after administration of ouabain (a Na+-K+ ATPase inhibitor) in anesthetized, artificially ventilated rabbits after vagus nerve section. Administration of veratridine (40 microg/kg) stimulated SAR activity but did not significantly alter tracheal pressure. Administration of ouabain (50 microg/kg) initially stimulated SAR activity during both inflation and deflation, but after 20 min, two different types of SAR responses were observed; one became silent at the peak, of inflation only, and the other maintained excitatory activity during both inflation and deflation phases. Veratridine usually inhibited SAR activity in ouabain-treated animals, irrespective of the difference of ouabain effects. These results suggest that veratridine-induced stimulation of SARs is closely related to the change in the Na+ ion gradient, which is regulated by Na+ pump activity.