Cardiac contractile dysfunction in Lep/Lep obesity is accompanied by NADPH oxidase activation, oxidative modification of sarco(endo)plasmic reticulum Ca²⁺-ATPase and myosin heavy chain isozyme switch

Obesity is an independent risk factor for heart diseases but the underlying mechanism is not clear. This study examined cardiac contraction, oxidative stress, oxidative modification of sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) and the myosin heavy chain (MHC) isoform switch in obese mice.

Mechanical properties were evaluated in ventricular myocytes from C57BL/6J lean and Lep/Lep obese mice (formerly known as ob/ob mice), including peak shortening (PS), time to 50 or 90% PS, time to 50 or 90% relengthening (TR₅₀, TR₉₀), maximal velocity of shortening/relengthening (±dL/dt), intracellular Ca²⁺ and its decay (τ). Oxidative stress, lipid peroxidation, protein damage and SERCA activity were assessed by glutathione/glutathione disulfide, malondialdehyde, protein carbonyl and ⁴⁵Ca²⁺ uptake, respectively. NADPH oxidase was determined by immunoblotting.

Myocytes from Lep/Lep mice displayed depressed PS and ± dL/dt, prolonged TR₅₀, TR₉₀, elevated resting [Ca²⁺]_i, prolonged τ, reduced contractile capacity at high stimulus frequencies and diminished responsiveness to extracellular Ca²⁺ compared with lean controls. Cardiac glutathione/glutathione disulfide was decreased whereas malondialdehyde, protein carbonyl, membrane p47^phox and membrane gp91^phox were increased in the Lep/Lep group. SERCA isoenzyme 2a was markedly modified by oxidation in Lep/Lep hearts and associated with decreased ⁴⁵Ca²⁺ uptake. The MHC isozyme displayed a shift from the α to the β isoform in Lep/Lep hearts. Short-term incubation of angiotensin II with myocytes mimicked the mechanical defects, SERCA oxidation and ⁴⁵Ca²⁺ uptake seen in Lep/Lep myocytes. Incubation of the NADPH oxidase inhibitor apocynin with Lep/Lep myocytes alleviated contractile defects without reversing SERCA oxidation or activity.

These data indicate that obesity-related cardiac defects may be related to NADPH oxidase activation, oxidative damage to SERCA and the MHC isozyme switch.