Altered [Ca2+]i mobilization in diabetic cardiomyocytes: responses to caffeine, KCl, ouabain, and ATP

To study the mechanisms mediating intracellular calcium transients involved in diabetic cardiac dysfunction, changes in intracellular calcium concentration ([Ca2+]i) in response to stimulation by caffeine, ouabain, KCl and ATP were studied in single cardiomyocytes (quiescent or electrically-stimulated) isolated from streptozotocin (STZ) diabetic rats. [Ca2+]i was measured by fluorescence microscopy using fura-2. Peak [Ca2+]i response to caffeine (20 mM) and decline of [Ca2+]i (-peak d[Ca2+]i/dt) were decreased in diabetic myocytes. Insulin treatment corrected these depressed [Ca2+]i responses. The data suggest a reduced sarcoplasmic reticulum (SR) calcium content and a depressed Na-Ca exchange activity in diabetic myocytes. Insulin deficiency may play a causal role in these changes. The maximum [Ca2+]i increase in response to ouabain was reduced in diabetic cells while the sensitivity of diabetic myocytes to ouabain was increased. This may be a result of depressed Na-K ATPase and elevated [Na+]i as previously reported. The KCl (12.5-50 mM)-induced [Ca2+]i increase was enhanced in diabetic cells. Caffeine (20 mM) and dichlorobenzamil (DCB, 10 microM) blocked this [Ca2+]i transient to a smaller degree in diabetic cells, but nitrendipine effects were similar in diabetic and control cells. These effects may be due to the increased L-channel activity and altered features, such as different responses to Ca-channel blockers, in diabetes which has previously been reported. The maximum response of [Ca2+]i to exogenous ATP was increased in diabetic cells while the sensitivity remained unchanged. The mechanisms underlying this enhanced response may be similar to the KCl-induced [Ca2+]i changes in diabetes.