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/dtmax) (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.