3.2 Hypokalemia-ischemia-induced changes of repolarization and arrhythmogenesis
In addition to having the specific border zone, the low-normal potassium rats demonstrated prolonged repolarization in all zones and attenuated shortening of repolarization in ischemia. Moreover, the inverse association between the blood [K+] and ARI duration was observed. It can be explained by the suppression of K+ channels conductance at lowering the extracellular [K+], specifically IK1 inhibition21 and reduced Na+/K+-ATPase currents22 resulting in the prolongation of the repolarization phase. As it has been already mentioned above, the ARI shortening in the ischemic zone might be expected to be determined not by the blood [K+] level but by the local rise of extracellular [K+]. However, the ARIs in the ischemic zone at lower blood [K+] were longer than at higher blood [K+]. This observation is in line with the previously discussed one that the low-normal potassium animals had less affected conduction characteristics in the ischemic zone and preserved activation in the border zone. The in vivo observations were supported by the in vitro patch-clamp experiments, where APD shortened at high-normal extracellular [K+] and did not change at low [K+] under hypoxic conditions. As expected, IK1 current decreased at low extracellular [K+] due to the hypokalemic reduction of the conductance for K+ channels 21. The less pronounced APD shortening in hypoxia could be explained by the found decreased IK(ATP) at low extracellular [K+].
In the present study, we observed the increased ESB in the low-normal potassium rats as compared to the normal-high rats. The prolongation of APD caused by the reduction of outward potassium currents, Na,K-ATPase inhibition, Na+ and Ca2+ intracellular overload at hypokalemic conditions promotes the occurrence of early afterdepolarization12, 23. This fact could explain the greater extrasystolic burden in low-normal potassium animals, with the possible origin of triggers from the border zone, where the ischemic changes overlapped the relatively prolonged APD. It is of note that IK(ATP) activation has been reported to abolish triggered activity24. We found that IK(ATP) was significantly suppressed in the low-potassium animals, which could further facilitate ectopic activity. Interestingly, the incidence of life-threatening arrhythmias (VT/VF) did not differ between the low-normal and normal-high potassium animals despite the difference in ESB, which usually serves as triggers for reentrant arrhythmias. The high ESB did not realize in an appropriately high incidence of lethal arrhythmias in this group possibly due to the preserved activation characteristics in the border zone of myocardium.
Hypokalemia has long been recognized as a proarrhythmic condition as it prolongs APD, promotes abnormal automaticity and afterdepolarizations, affects excitation thresholds and refractoriness 25. These effects are well documented in individual cardiomyocytes, and the present study extends this consideration into “spatial dimension”. Our findings demonstrated that the decrease in extracellular [K+] resulted in the formation of the border zone adjacent to the typical ischemic zone. It is noteworthy that the cut-off [K+] level (4.7 mM) is far above true hypokalemia, which implies that such a phenomenon may be quite common. Furthermore, this relatively high cut-off [K+] level implies that the observed increased ESB was due to the presence of the border zone itself rather than the hypokalemia-induced changes in individual cells.
Broadly speaking, the border zone can be considered as a mildly ischemic myocardial region with preserved activation characteristics and abnormal repolarization. Most likely, the role of the border zone in arrhythmogenesis could not be ascribed to the changes in dispersion of repolarization, but its critical arrhythmogenic property appears to be the prolonged APD 26-28, which may be caused not only by [K+] changes studied here, but also by early ischemic APD prolongation 29. The findings obtained in the present and previous studies cited above suggest that the border zone can present a source of ectopic activity, which might trigger VT/VF. Prolonged APD prone to early afterdepolarizations in combination with preserved excitability may underlie this activity. The chances of inducing the reentrant arrhythmias by these extrasystoles would probably depend on other myocardial properties, for example, characteristics of the ischemic region.
Limitations
There are some limitations of the present study, which warrant cautious interpretation of the findings obtained. The experimental model is far from clinical conditions concerning mostly the species (rats), which demonstrate specific electrophysiological myocardial properties, and the short duration of the ischemic episode. In vitro experiments could not technically reproduce the situation in the in vivo experiments. The mapping technique did not allow us to characterize the entire ventricular surface, and therefore the exact configuration of the ischemic/border/normal zones remained unknown.
CONCLUSIONS
The present study demonstrated that the formation of the border zone depends on the extracellular potassium concentration likely due to the attenuation of IK(ATP) response to hypoxia in low-normal [K+]. The border zone was characterized by the preserved excitation and altered repolarization processes and was associated with the increased extrasystolic burden.
METHODS
Effects of extracellular [K+] on electrophysiological parameters were investigated in vivo in a rat model of heart ischemia/reperfusion as well as in vitro in isolated cardiomyocytes. Experiments were performed in a total of 26 Wistar male rats (4-month-old, 323±8 g) obtained from the Scientific Collection of Experimental Animals of the Institute of Biology, Federal Research Center, Komi Scientific Center, Ural Branch of the Russian Academy of Sciences. The study conformed to the Guide for the Care and Use of Laboratory Animals, 8th Edition published by the National Academies Press (US), 2011, the guidelines from Directive 2010/63/EU of the European Parliament on the protection of animals used for scientific purposes, and was approved by the ethical committee of the Institute of Physiology of the Komi Science Centre, Ural Branch of Russian Academy of Sciences (approval 08 December 2022).