Mannitol and hyponatremia regulate cardiac ventricular conduction in the context of sodium channel loss of function

Figures

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  Figure 1.. Characterizing the Scn5a+/− mouse heart. A and B: relative to wild type (WT), the Scn5a+/− mouse expresses ∼60% reduced mRNA and ∼52% reduced voltage-gated sodium channel (Nav)1.5 protein. Nav1.5 protein expression is normalized to GAPDH. n = 2 males, 1 female WT; n = 2 males, 1 female Scn5a+/− hearts, significance determined by Student’s t test, *P < 0.05 compared with WT. C: no significant difference was measured in total connexin-43 (Cx43) expression (normalized to GAPDH) or Cx43 phosphorylation at Ser368 (normalized to total Cx43) between the genotypes. n = 2 males, 1 female WT; n = 2 males, 1 female Scn5a+/− hearts. P = not significant (n.s.) via one-way ANOVA. D: perinexal width, highlighted in yellow, was not significantly different between WT and Scn5a+/− hearts under baseline conditions. n = 2 males, 1 female WT; n = 2 males, 1 female Scn5a+/− hearts, n = 15 images/heart. Significance determined by nested t test; *P < 0.05 compared with WT.

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  Figure 2.. In vivo ventricular conduction is not significantly different between Scn5a+/− and wild-type (WT) mice. A: typical example of an ECG trace acquired with the ECGenie system for the WT (black) and Scn5a+/− (blue) mouse. B: QRS duration is not significantly different between the two genotypes, though QRSJ is significantly longer in the Scn5a+/− mouse (C). D: RR duration is not different between the WT and Scn5a+/− mouse. n = 5 males, 1 female WT; 5 males 1 female Scn5a+/−; significance was determined using Student’s t test, *P < 0.05 compared with WT.

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  Figure 3.. Conduction velocity (CV) of Scn5a+/− hearts is not significantly slower than wild type (WT) at baseline, but more sensitive to CV slowing due to mannitol challenge. A: representative activation time maps for Scn5a+/− and WT ventricles at normonatremia before and after mannitol challenge. Isochrone lines represent 2-ms intervals. Pulse symbol (П) indicates site of electrode placement for pacing. Arrows indicate vectors used for CV calculations. B: neither transverse CV (CVT) nor longitudinal CV (CVL) is significantly slower in the Scn5a+/− heart relative to WT. n = 8 males, 4 females WT; 6 males, 4 females Scn5a+/−, significance determined by Welch’s t test, *P < 0.05 compared with WT. C: perfusion with mannitol significantly slowed CVT in WT and CVT and CVL in Scn5a+/− hearts. D: CVT slows to a greater extent in Scn5a+/− relative to WT hearts after mannitol challenge. n = 4 males, 2 females WT; 4 males, 1 female Scn5a+/−. Significance determined by Welch’s t test, *P < 0.05 compared with CV of WT; #P < 0.05 compared with ΔCV of WT.

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  Figure 4.. Mannitol attenuates hyponatremia-induced conduction velocity (CV) slowing and resolves conduction block in Scn5a+/− hearts. A: representative activation time maps during baseline, hyponatremia, and combined hyponatremia and mannitol conditions. Isochrone lines represent 2 ms. Pulse symbol (П) indicates site of electrode placement for pacing. Arrows indicate vectors used for CV calculations. B: wild type (WT) does not exhibit severe conduction delay during hyponatremia, but 4/5 Scn5a+/− exhibit severe conduction delay under these conditions. Significance determined by Fisher’s exact test; *P < 0.05 compared with WT. C: WT and Scn5a+/− hearts have significantly slowed transverse CV (CVT) in response to hyponatremia, but no significant change in CVT after the addition of mannitol. n = 2 males, 4 females WT; 2 males, 3 females Scn5a+/−, significance determined by repeated-measures one-way ANOVA with the Geisser–Greenhouse correction (for unequal variability of differences), followed by the Tukey’s multiple comparisons test; *P < 0.05 compared with 152 mM Na+ condition.

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  Figure 5.. Attenuation of conduction velocity (CV) slowing during combined hyponatremia and mannitol challenge is not species specific. A and B: significant reductions in transverse CV (CVT) and longitudinal CV (CVL) are observed after perfusion with 120 mM Na+ and flecainide in ex vivo guinea pig hearts. Addition of mannitol does not show additive CV slowing effect in the context of hyponatremia and sodium channel inactivation. n = 6 males, significance determined by one-way ANOVA with Tukey’s multiple comparisons test; *P < 0.05 compared with 145 mM Na+ condition.