Hypothyroidism modulates mitochondrial dynamics and mitophagy in the heart of rats under fed and fasting conditions.

Aims: Investigate the impact of hypothyroidism on mitochondrial dynamics and mitophagy in the heart under fed and fasting conditions.

Methods: Hypothyroidism was induced in male Wistar rats with methimazole (0.03 %) for 21 days. Half of the euthyroid and hypothyroid groups underwent a 48-h fasting. Mitochondrial number and ultrastructure were evaluated by transmission electron microscopy. Fusion, fission, mitophagy, oxidative stress, and mitochondrial oxidative phosphorylation system (OXPHOS) components were analyzed by Western Blot and qPCR.

Results: Hypothyroidism increased DRP1 activation and the p-DRP1/OPA1 ratio, indicating a shift toward mitochondrial fission over fusion. Under fasting, hypothyroidism prevented the increases in mitochondrial size, elongation, OPA1, and OXPHOS seen in euthyroid fasted rats. Hypothyroidism also raised 4-HNE content, an oxidative stress product, increased mitochondrial injury, and exacerbated fasting-related mitochondrial damage. This was accompanied by elevated Parkin levels in both fed and fasted hypothyroid groups, but without changes in PINK1 levels or Parkin activation. While fasting upregulated Bnip3l and Map1lc3b expression in euthyroid rats, hypothyroidism suppressed this response, though it did not prevent fasting-induced Bnip3 increases.

Conclusions: Hypothyroidism increases the activation of mitochondrial fission machinery and oxidative stress, and induces mitochondrial damage without activation of mitophagy proteins, suggesting disrupted mitophagy signaling. It also interferes with fasting-induced mitochondrial dynamics adaptations, highlighting the essential role of thyroid hormones in metabolic adaptation to fasting.