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Experimental Physiology 89.5 pp 629-636
DOI: 10.1113/expphysiol.2004.027607
© The Physiological Society 2004
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L-thyroxine increases susceptibility of adult rats to low K+-induced ventricular fibrillation, and sinus rhythm restoration in old rats

Narcis Tribulova1, Vladimir Knezl2, Ludmila Okruhlicova1, Jan Drimal2, Dalma Lamosova3, Jan Slezak1 and Jan Styk1

1 Institute for Heart Research2 Institute of Experimental Pharmacology3 Institute of Animal Biochemistry and Genetics, Slovak Academy of Sciences, Bratislava, Slovak Republic

Hypokalaemia increases the risk for life-threatening arrhythmias; however, data about interaction with thyroid status are lacking. The aim of this study was to investigate vulnerability of L-thyroxine (T4)-treated adult and old rats to low K+-induced ventricular fibrillation (VF) as well as the ability of the heart to recover sinus rhythm. The experiments were performed on isolated heart preparations using the heart of 4- and 20-month-old female Wistar rats without and with feeding with T4 50 µg (100 g day)–1 over a period of 2 weeks. Perfusion of the isolated heart with oxygenated Krebs–Henseleit solution at constant pressure was followed by perfusion with K+-deficient solution until occurrence of VF (< 10 min). After 2 min of sustained VF, the heart was perfused with normal solution for 10 min, during which sinus rhythm was restored. ECG, left ventricular pressure (LVP) and coronary flow were continuously monitored. The results showed that compared with untreated rats, the onset of low K+-induced ventricular premature beats was delayed and their number was significantly decreased in both T4-treated groups. Nevertheless, VF occurred earlier in T4-treated than in non-treated adult rats (6.78 ± 0.28 vs. 9.59 ± 0.55 min, P < 0.05), whereas the difference was not significant in aged animals. Furthermore, sinus rhythm appeared earlier in old T4-treated rats compared with non-treated rats (7.18 ± 0.57 vs. 8.94 ± 0.64 min, P < 0.05), whereas in adult hearts it set in at practically the same time regardless of treatment. In conclusion, our results indicate that administration of a pharmacological dose of T4 can increase the risk of low K+-induced VF in adult but not in old animals; in the latter it even facilitated restoration of sinus rhythm. Moreover, enhanced mechanical function was observed in both adult and old T4-treated hearts.

(Received 4 March 2004; accepted after revision 8 July 2004; first published online 15 July 2004)
Corresponding author N. Tribulova: Institute for Heart Research, Slovak Academy of Sciences, Dubravska cesta 9, 840 05 Bratislava 45, PO Box 104, Slovak Republic. Email: narcisa.tribulova{at}savba.sk






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