In vivo imaging of intracellular Ca2+ after muscle contractions and direct Ca2+ injection in rat skeletal muscle in diabetes

The effects of muscle contractions on the profile of postcontraction resting intracellular Ca2+ ([Ca2+](i)) accumulation in Type 1 diabetes are unclear. We tested the hypothesis that, following repeated bouts of muscle contractions, the rise in resting [Ca2+](i) evident in healthy rats would be increased in diabetic rats and that these changes would be associated with a decreased cytoplasmic Ca2+-buffering capacity. Adult male Wistar rats were divided randomly into diabetic (DIA; streptozotocin, ip) and healthy control (CONT) groups. Four weeks later, animals were anesthetized and spinotrapezius muscle contractions (10 sets of 50 contractions) were elicited by electrical stimulation (100 Hz). Ca2+ imaging was achieved using Fura-2 AM in the spinotrapezius muscle in vivo (i. e., circulation intact). The ratio (340/380 nm) was determined from fluorescence images following each set of contractions for estimation of [Ca2+](i). Also, muscle Ca2+ buffering was studied in individual myocytes microinjected with 2 mM Ca2+ solution. After muscle contractions, resting [Ca2+](i) in DIA increased earlier and more rapidly than in CONT (P < 0.05 vs. precontraction). Peak [Ca2+](i) in response to the Ca2+ injection was significantly higher in CONT (25.8 +/- 6.0% above baseline) than DIA (10.2 +/- 1.1% above baseline). Subsequently, CONT [Ca2+]i decreased rapidly (< 15 s) to plateau 9-10% above baseline, whereas DIA remained elevated throughout the 60-s measurement window. No differences in SERCA1 and SERCA2 (Ca2+ uptake) protein levels were evident between CONT and DIA, whereas ryanodine receptor (Ca2+ release) protein level and mitochondrial oxidative enzyme activity (succinate dehydrogenase) were decreased in DIA (P < 0.05). In conclusion, diabetes impairs resting [Ca2+](i) homeostasis following muscle contractions. Markedly different responses to Ca2+ injection in DIA vs. CONT suggest fundamentally deranged Ca2+ handling.