Zinc Supplementation Sustains Diaphragm Contractility and Preserves SERCA2a Expression in Aged Female Rats with Type 2 Diabetes

Diabetes mellitus (DM) is a chronic metabolic disease characterized by hyperglycemia, and the diaphragm-the primary respiratory muscle-is adversely affected by this diabetic process. The aim of this study is to investigate the effects of zinc sulfate (ZnSO₄) treatment on diaphragm muscle contractile dynamics, calcium homeostasis, apoptosis, and fibrosis in an 18-month-old female Type 2 diabetic rat model. Thirty-two 18-month-old female Wistar rats were randomly divided into four groups: Control (CON), CON + ZnSO₄, Diabetes Mellitus (DM), and DM + ZnSO₄. The DM model was induced by a high-fat diet and administration of 30 mg/kg streptozotocin (STZ); after the disease was confirmed, ZnSO₄ was administered intraperitoneally at a daily dose of 10 mg/kg to the treatment groups. The mechanical functions of the diaphragm muscle were evaluated using a post-rest potentiation protocol in an isolated organ bath; qPCR analyses (Caspase-3, TGF-β1, SERCA2a) were performed to investigate cellular apoptosis, fibrosis, and calcium regulation. Compared with the CON group, the DM group exhibited a severe ~90% reduction in diaphragmatic contraction force (CF) and a ~97% decline in maximal contraction/relaxation velocities (±dF/dt_max) (p < 0.0001), which strongly correlated with a 30% suppression of SERCA2a gene expression (p < 0.01). Concomitantly, apoptotic Caspase-3 (~2.6-fold) and profibrotic TGF-β1 (~3.1-fold) mRNA levels were significantly elevated (p < 0.0001). In the DM + ZnSO₄ group, daily zinc treatment (10 mg/kg/day, i.p. for 6 weeks, initiated 4 weeks after diabetes confirmation) did not reverse the elevated Caspase-3 and TGF-β1 expressions (p > 0.05). However, SERCA2a expression was fully preserved back to control levels (p < 0.05 vs. DM), leading to a substantial ~3-fold improvement in CF and velocities (p < 0.05 to p < 0.0001 vs. DM). On the other hand, the healthy CON + ZnSO₄ group exhibited a physiological slowing of contractility (~53% decrease in CF), without histological damage, likely due to a competitive antagonism between excess divalent zinc (Zn²⁺) and calcium (Ca²⁺) on myofilaments. Although zinc cannot reverse the structural apoptotic and fibrotic remodeling in the aged diabetic diaphragm, it successfully rescues functional contractility by preserving SERCA2a transcriptional expression.