Modulation of potassium currents by angiotensin and oxidative stress in cardiac cells from the diabetic rat

Abstract

Diabetes induces oxidative stress and leads to attenuation of cardiac K + currents. We investigated the role of superoxide ions and angiotensin II (ANG II) in generating and linking oxidative stress to the modulation of K + currents under diabetic conditions. K + currents were measured using patch-clamp methods in ventricular myocytes from streptozotocin (STZ)-induced diabetic rats. Superoxide ion levels, indicating oxidative stress, were measured by fluorescent labelling with dihydroethidium (DHE). ANG II content was measured using enzyme-linked immunosorbent asssay (ELISA). The results showed DHE fluorescence to be significantly higher in cells from diabetic males, compared to controls. Relief of stress by the NADPH oxidase inhibitor apocynin or by superoxide dismutase (SOD) but not by catalase reversed the attenuation of K + currents and reduced DHE fluorescence. In cells from diabetic females, neither apocynin nor SOD augmented K + currents, ANG II was not elevated and DHE fluorescence was significantly weaker than in cells from males. Reduced glutathione (GSH) also augmented K + currents in cells from diabetic males but not females. In ovariectomized diabetic females K + currents were augmented by GSH and apocynin. Current augmentation and the attenuation of DHE fluorescence by apocynin were significantly blunted by excess ANG II (300 nM). Diabetic male rats pretreated with the angiotensin-converting enzyme (ACE) inhibitor quinapril were hyperglycaemic, but their cellular ANG II levels and DHE fluorescence were significantly decreased. In cells from these rats, K + currents were insensitive to apocynin. In conclusion, diabetes-related oxidative stress attenuates K + currents through ANG II-generated increased superoxide ion levels. When ANG II levels are lower, as in diabetic females or following ACE inhibition in males, oxidative stress is reduced, with blunted alterations in K + currents.

Sex-dependent impairment of cardiac action potential conduction in type 1 diabetic rats

“…Earlier work from our laboratory showed that cardiac angiotensin II levels and oxidative stress are increased in diabetic male hearts (41, 43). These changes are reduced or absent in diabetic females (41)(42)(43). Earlier work from our laboratory also showed that in vivo treatment of male rats with an angiotensin-converting enzyme (ACE) inhibitor reduces or prevents diabetes-induced increase in angiotensin II and the resulting oxidative stress (41).…”

“…Selective activation results in lower cellular angiotensin II levels in diabetic female hearts compared with diabetic male hearts (43). Concordantly, cells from diabetic females show less oxidative stress (41).…”

“…Isolated ventricular myocytes and tissue sections were prepared for immunofluorescence microscopy as described previously (33,41). Briefly, isolated cell suspensions or tissue sections on glass slides were fixed with 1% formaldehyde and permeabilized with 2% Triton X-100.…”

“…Ventricular myocytes were obtained by enzymatic dispersion, as described recently (41). Hearts were perfused for 5 to 6 min (at 37°C, bubbled with 100% O2) with a solution consisting of (in mM): 113 NaCl, 4.7 KCl, 1.2 MgSO4, 0.6 KH2PO4, 0.6 Na2HPO4, 12 NaHCO3, 12 KHCO3, 5.5 glucose, 10 HEPES, and 30 taurine.…”

NADPH oxidase signaling and cardiac myocyte function

“…In streptozotocin-induced diabetes in rats, membrane K+ currents were attenuated secondary to increased angiotensin II-induced NADPH oxidase activation [47]. Angiotensin II-induced swelling-activated Cl− current in rabbit ventricular myocytes involved NADPH oxidase activation, and was inhibited by a range of inhibitors including gp91ds-tat [48].…”