Interstrain variation in cardiac and respiratory adaptation to repeated ozone and particulate matter exposures

Abstract

Increased ambient particulate matter (PM) is associated with adverse cardiovascular and respiratory outcomes, as demonstrated by epidemiology studies. Several studies have investigated the role of copollutants, such as ozone (O(3)), in this association. It is accepted that physiological adaptation involving the respiratory system occurs with repeated exposures to O(3). We hypothesize that adaptation to PM and O(3) varies among different inbred mouse strains, and cardiopulmonary adaptation to O(3) is a synchronized response between the cardiac and respiratory systems. Heart rate (HR), HR variability (HRV), and the magnitude and pattern of breathing were simultaneously measured by implanted telemeters and by plethysmography in three inbred mouse strains: C57Bl/6J (B6), C3H/HeJ (HeJ), and C3H/HeOuJ (OuJ). Physiological responses were assessed during dual exposures to filtered air (FA), O(3) (576 +/- 32 parts/billion), and/or carbon black (CB; 556 +/- 34 mug/m(3)). Exposures were repeated for 3 consecutive days. While each strain showed significant reductions in HR during CB with O(3) preexposure (O(3)CB) on day 1, prominent HRV responses were observed in only HeJ and OuJ mice. Each strain also differed in their adaptation profile in response to repeated O(3)CB exposures. Whereas B6 mice showed rapid adaptation in HR after day 1, HeJ mice generally showed more moderate HR and HRV adaptation after day 2 of exposure. Unlike either B6 or HeJ strains, OuJ mice showed little evidence of HR or HRV adaptation to repeated O(3)CB exposure. Adaptation profiles between HR regulation and breathing characteristics were strongly correlated, but these associations also varied significantly among strains. These findings suggest that genetic factors determine the responsivity and adaptation of the cardiac and respiratory systems to repeated copollutant exposures. During O(3)CB exposure, adaptation of cardiac and respiratory systems is markedly synchronized, which may explain a potential mechanism for adverse effects of PM on heart function.

Fig. 1.

Mice were exposed to 3 different weekly exposure protocols. Each daily exposure was repeated for 3 consecutive days, and the weekly order of protocols was standardized as follows: 1) 2 h of filtered air (FA) followed by 3 h of FA (FAFA); 2) 2 h of FA followed by 3 h of carbon black (CB) (FACB); and 3) 2 h of ozone (O₃) followed by 3 h of CB (O₃CB). The physiological parameters available at each stage of the protocol are also indicated. MB, morning before; MA, morning after; HR, heart rate; HRV, HR variability; V̇o₂, O₂ consumption; T_CO, core temperature; RR, respiratory responses.

Fig. 2.

Operational definition of physiological adaptation. Adaptation was assessed by both an interim response following either O₃ or FA preexposure (Pre; shaded region on left) and by the slope of the response curve during the subsequent 3-h exposures of CB or FA (shaded region on right). The linear model, as defined by the equation y = mx + b, indicates m as the slope and b as the intercept. The figure shows a typical mouse HR response curve, including the interim value and slope, which is based on a representative sample of five hourly averaged time points. Post, postexposure; bpm, beats/min.

Fig. 3.

HR responses were obtained from 3-min ECG samples in C57Bl/6J (B6; n = 8), C3H/HeJ (HeJ; n = 5), and C3H/HeOuJ (OuJ; n = 6) mice. A: average interim values obtained from five separate ECG samples following O₃ or FA preexposure. B: average slopes of the HR response curves during the second exposure to either FA or CB. The dashed line represents the minimum HR during a strain-specific circadian cycle. Time points represent the mean ± SE for each strain. *P < 0.05 and ***P < 0.001 vs. FAFA.

Fig. 4.

Total HRV [standard deviation of normal-to-normal intervals (SDNN)] responses were obtained from 3-min ECG samples in B6 (n = 8), HeJ (n = 5), and OuJ (n = 6) mice. A: average interim values obtained from five separate ECG samples following O₃ or FA preexposure. B: average slopes of the SDNN response curves during the second exposure to either FA or CB. Time points represent the mean ± SE for each strain. *P < 0.05, **P < 0.01, and ***P < 0.001 vs. FAFA.

Fig. 5.

Beat-to-beat HRV [root mean square of successive differences between adjacent R-R intervals (rMSSD)] responses were obtained from 3-min ECG samples in B6 (n = 8), HeJ (n = 5), and OuJ (n = 6) mice. A: average obtained from five separate ECG sample interim values following O₃ or FA preexposure. B: average slopes of the rMSSD response curves during the second exposure to either FA or CB. Time points represent the mean ± SE for each strain. *P < 0.05, **P < 0.01, and ***P < 0.001 vs. FAFA.

Fig. 6.

RR responses were obtained from 60-s breathing traces in B6 (n = 8), HeJ (n = 5), and OuJ (n = 6) mice. A: average interim values obtained from five separate breathing traces following O₃ or FA preexposure. B: average slopes of the RR response curves during the second exposure to either FA or CB. Time points represent the mean ± SE for each strain. **P < 0.01 and ***P < 0.001 vs. FAFA.

Fig. 7.

Tidal volume (Vt) responses were obtained from 60-s breathing traces in B6 (n = 8), HeJ (n = 5), and OuJ (n = 6) mice. A: average interim values obtained from five separate breathing traces following O₃ or FA preexposure. B: average slopes of the Vt response curves during the second exposure to either FA or CB. Time points represent the mean ± SE for each strain. *P < 0.05 vs. FAFA.

Fig. 8.

T_CO responses were obtained during the same 3-min period as the ECG samples in B6 (n = 8), HeJ (n = 5), and OuJ (n = 6) mice. A: average interim values following O₃ or FA preexposure. B: average slopes of the T_CO response curves during the second exposure to either FA or CB. The dashed line represents the minimum T_CO during a strain-specific circadian cycle. Time points represent the mean ± SE for each strain. *P < 0.05, **P < 0.01, and ***P < 0.001 vs. FAFA.

Fig. 9.

Echocardiography results are illustrated for separate groups of B6, HeJ, and OuJ mice (n = 4 mice/strain). Measurements were obtained before (Pre Ex) and after weekly exposures (Post Ex) for 3 consecutive days. The weekly sequence of protocols was standardized in the following order: 1) 3 h of FA; 2) 3 h of CB; and 3) 2 h of O₃. The average percent change in fractional shortening was computed from individual responses before and after weekly CB and O₃ exposures relative to FA exposure. Values represent the mean ± SE for each strain. *P < 0.05 vs. Pre Ex.