Ryanodine Receptor Dysfunction in Hearts of Streptozotocin-Induced Diabetic Rats

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Abstract
Studies have shown that evoked calcium release from sarcoplasmic reticulum is compromised in diabetic rat hearts. The present study was undertaken to determine whether this decrease might be ascribed to a reduction in expression and/or alteration in function of ryanodine receptor (RyR2) and whether changes could be minimized with insulin treatment. Hearts were isolated from 4- and 6-week streptozotocin (STZ)-induced diabetic, 4-week diabetic/2-week insulin-treated, and age-matched control rats. RyR2 mRNA and protein levels were determined using reverse transcription-polymerase chain reactions and polyacrylamide gel electrophoresis, respectively, whereas the functional integrity of RyR2 was assessed from their ability to bind [3H]ryanodine. RyR2 protein was unchanged with up to 6 weeks of untreated STZ-induced diabetes. Two weeks of insulin treatment initiated after 4 weeks of diabetes increased RyR2 mRNA levels by 42% and RyR2 protein levels by 45 to 61%. At equivalent amounts, RyR2 protein from 4-week STZ-induced diabetic rat hearts bound 9% less [3H]ryanodine than age-matched control rats (74.1 ± 3.9 versus 67.4 ± 3.4 fmol/μg RyR2), whereas that from 6-week STZ-diabetic rats bound 36% less than control rats (47.9 ± 4.8 versus 74.2 ± 4.5 fmol/μg RyR2, p < 0.05). RyR2 from insulin-treated animals bound significantly less [3H]ryanodine than control rats (65.2 ± 4.9 fmol/μg RyR2, p < 0.05). Apparent affinity of ryanodine for RyR2 was similar among all groups (Kd ≈ 1.04 ± 0.08 nM). Because expression did not change significantly but ryanodine binding decreased, these data suggest that the functional integrity of RyR2 is compromised in diabetic rat hearts, and these changes can be attenuated with 2 weeks of insulin treatment.

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