Hyperthyroidism is associated with preserved preconditioning capacity but intensified and accelerated ischaemic contracture in rat heart

Background: The present study was undertaken to define the effects of thyro xine administration on ischaemic preconditioning (PC) and the ischaemic con tracture. Methods: Hyperthyroidism was induced by administration of L-thyro xine in rats (THYR) while normal animals served as controls (NORMa). Isolat ed rat hearts were perfused in a Langendorff preparation. NORMa control (n = 16) and THYR control (n = 9) hearts underwent 20 min of ischaemia and 45 min reperfusion while NORMa PC (n = 16) and THYR PC (n = 14) were subjected to PC before ischaemia. Additional normal hearts were subjected to 30 min of ischaemia with and without PC, NORMb control, n = 8 and NORMb PC, n = 6. Postischaemic recoveries of left ventricular (LV) developed pressure were expressed as % of the initial value (LVDP%). Severity of contracture was me asured by the time (Tmax) and magnitude (Cmax) of peak contracture. Results : LVDP% was significantly higher after PC, both in NORMa and THYR rats. In NORMa control hearts, ischaemic contracture had not yet reached a plateau a t 20 min of ischaemia. Contracture appeared earlier in THYR control and PC than in NORMa control and PC groups. Tmax was 22.1 (0.9) vs 16.8 (1.4) min for NORMb control and PC, p < 0.05 and 12.5 (1.0) vs 9.3 (1.1) min for THYR control and PC hearts, p < 0.05. Tmax was earlier in both THYR groups comp ared to NORMb groups, p < 0.05. Cmax was significantly higher in both THYR groups compared to both NORMb groups. Conclusion: Ischaemic contracture is both accelerated and accentuated in thyroxine treated hearts while precondi tioning capacity is preserved. Preconditioning and thyroxine administration shorten Tmax in an additive way, whereas Cmax in hyperthyroid hearts did n ot further increase by preconditioning.