Irritant Inhalation Evokes P Wave Morphological Changes in Spontaneously Hypertensive Rats via Reflex Modulation of the Autonomic Nervous System

Abstract

Irritant inhalation is associated with increased incidence of atrial fibrillation (AF) and stroke. Irritant inhalation acutely regulates cardiac function via autonomic reflexes. Increases in parasympathetic and sympathetic reflexes may increase atrial susceptibility to ectopic activity and the initiation of arrhythmia such as AF. Both age and hypertension are risk factors for AF. We have shown that irritant-evoked pulmonary-cardiac reflexes are remodeled in spontaneously hypertensive (SH) rats to include a sympathetic component in addition to the parasympathetic reflex observed in normotensive Wistar-Kyoto (WKY) rats. Here, we analyzed P wave morphology in 15-week old WKY and SH rats during inhalation of the transient receptor potential ankyrin 1 agonist allyl isothiocyanate (AITC). P Wave morphology was normal during vehicle inhalation but was variably modulated by AITC. AITC increased RR intervals (RRi), PR intervals, and the P Wave duration. In SH rats only, AITC inhalation increased the occurrence of negative P waves. The incidence of AITC-evoked negative P waves in SH rats was dependent on RRi, increasing during bradycardic and tachycardic cardiac cycles. Inhibition of both parasympathetic (using atropine) and sympathetic (using atenolol) components of the pulmonary-cardiac reflex decreased the incidence of negative P waves. Lastly, the probability of evoking a negative P Wave was increased by the occurrence of preceding negative P waves. We conclude that the remodeled irritant-evoked pulmonary-cardiac reflex in SH rats provides a substrate for altered P Wave morphologies. These are likely ectopic atrial beats that could provide a trigger for AF initiation in structurally remodeled atria.

Keywords: ECG; P wave; TRPA1; atrial fibrillation; autonomic (vegetative) nervous system; ectopic beat; hypertension; irritant.

FIGURE 1

Effect of allyl isothiocyanate (AITC) inhalation on ECG in conscious Wistar-Kyoto (WKY) and spontaneously hypertensive (SH) rats. Representative ECG for a single WKY rat (A) and two separate SH rats (B,C) during exposure to vehicle (4% ethanol) and AITC (4.3 mg/ml). Note the prominent and reproducible P Wave prior to the QRS complex in all three animals during vehicle inhalation. The trace during AITC inhalation by the WKY rat (A) was interrupted by a period of poor ECG recording (not shown). Horizontal scale bars denote 100 ms, vertical scale bars denote 0.1 mV.

FIGURE 2

Effect of AITC inhalation on P Wave morphology in conscious WKY and SH rats. Representative consecutive P waves for vehicle (4% ethanol) and AITC (4.3 mg/ml) inhalation shown in Figure 1: a single WKY rat (A) and two separate SH rats (B,C). The consecutive P waves shown for AITC inhalation by the WKY rat (A) was interrupted by a period of poor ECG recording (not shown). In some cases, the P Wave could not be discerned (N.D.). Each P Wave is colored by a rainbow color range denoting the particular RRi of that cardiac cycle. Horizontal scale bars denote 10 ms, vertical scale bars denote 0.05 mV, dotted lines denote the location of 0 mV.

FIGURE 3

Effect of AITC inhalation on heart rate and P Wave parameters in conscious WKY and SH rats. (A,C,E,G,I) Mean ± SEM of RRi (A), PRi (C), Pwidth (E), P-H (G), and the % of cardiac cycles with a negative P Wave (I) during inhalation of vehicle (open squares) and AITC (4.3 mg/ml, closed squares) in WKY rats (gray, n = 9) and SH rats (red, n = 12). The mean ± SEM of RRi (A) and PRi (C) were previously published (Hooper et al., 2019). (B,D,F,H,J) Mean ± SEM of the difference between vehicle- and AITC-evoked RRi (B), PRi (D), Pwidth (F), P-H (H), and the % of cardiac cycles with a negative P Wave (J) in WKY rats (gray, n = 9) and SH rats (red, n = 12). In (A,C,E,G), * denotes significant effect of AITC compared to vehicle (p < 0.05) and $ denotes significant difference between WKY and SH rats (p < 0.05), both assessed by ANOVA with Sidak’s multiple comparisons. In (B,D,F,H), $ denotes significant difference between WKY and SH rats (p < 0.05, unpaired T-test). In (I), * denotes significant effect of AITC compared to vehicle (p < 0.05, Kruskal–Wallis ANOVA with Dunn’s multiple comparisons). In (J), $ denotes significant difference between WKY and SH rats (p < 0.05, unpaired Mann–Whitney test).

FIGURE 4

Correlation of RRi with P Wave parameters for individual cardiac cycles during AITC and vehicle inhalation. Each cardiac cycle is represented by either a blue dot (during vehicle inhalation) or a red dot (during AITC inhalation). (A,D) Correlation of RRi with P-H. (B,E) Correlation of RRi with P width. (C,F) Correlation of RRi with PRi. (A–C) Events in WKY rats (n = 9 animals, 8220 cycles during vehicle, 4224 cycles during AITC). (D–F) Events in SH rats (n = 12 animals, 11,745 cycles during vehicle, 9558 cycles during AITC).

FIGURE 5

Correlation of RRi with incidence of negative P waves during AITC and vehicle inhalation. Each cardiac cycle from WKY rats (A, n = 9 animals, 8220 cycles during vehicle, 4224 cycles during AITC), SH rats (B, n = 12 animals, 11,745 cycles during vehicle, 9558 cycles during AITC), WKY rats pretreated with 1 mg/kg atropine (C, 6 animals, 6771 cycles during vehicle, 6363 cycles during AITC), SH rats pretreated with 1 mg/kg atropine (D, 9 animals, 10,104 cycles during vehicle, 11,511 cycles during AITC), and SH rats pretreated with 0.5 mg/kg atenolol (E, n = 4 animals, 3185 cycles during vehicle, 2601 cycles during AITC) was grouped by RRi into 25 ms bins. For each RRi bin during vehicle (blue) and AITC (red) inhalation the number of cardiac cycles recorded (top), the number of cycles with negative P waves (middle) and the probability of a given cycle having a negative P Wave (bottom) is shown.

FIGURE 6

The effect of AITC inhalation on the incidence of negative P waves during bradycardia and tachycardia in conscious WKY and SH rats. (A) Mean ± SEM of the % of cardiac cycles which were classified as bradycardic. (B) Mean ± SEM of the % of bradycardic cardiac cycles with a negative P Wave. (C) Mean ± SEM of the % of cardiac cycles which were classified as tachycardic. (D) Mean ± SEM of the % of tachycardic cardiac cycles with a negative P Wave. Data was recorded during vehicle (open squares) and AITC (4.3 mg/ml, closed squares) inhalation in control WKY and SH rats (gray, n = 9 and 12, respectively), WKY and SH rats pretreated with 1 mg/kg atropine (blue, n = 6 and 9, respectively), and SH rats pretreated with 0.5 mg/kg atenolol (green, n = 4). The symbol * denotes significant effect of AITC compared to vehicle (p < 0.05), $ denotes significant difference between WKY and SH rats (p < 0.05), and # denotes significant effect of autonomic drugs compared to control (p < 0.05), each assessed in Kruskal–Wallis ANOVA with Dunn’s multiple comparisons.

FIGURE 7

Allyl isothiocyanate inhalation increases the probability of consecutive negative P waves in SH rats. (A) Mean ± SEM of the average number of negative P waves in each consecutive “train” of negative P waves during vehicle (open squares) and AITC (4.3 mg/ml, closed squares) inhalation in control WKY and SH rats (gray, n = 9 and 12, respectively), WKY and SH rats pretreated with 1 mg/kg atropine (blue, n = 6 and 9, respectively), and SH rats pretreated with 0.5 mg/kg atenolol (green, n = 4). The symbol * denotes significant effect of AITC compared to vehicle (p < 0.05) and $ denotes significant difference between WKY and SH rats (p < 0.05), both assessed in Kruskal–Wallis ANOVA with Dunn’s multiple comparisons. (B) Mean ± SEM the probability of a given cycle having a negative P Wave in control SH rats (n = 12) during vehicle (open squares) and AITC (4.3 mg/ml, closed squares) as a function of the prior number of consecutive negative P waves.