ALTERED [CA2- RESPONSES TO CAFFEINE, KCL, OUABAIN, AND ATP(](I) MOBILIZATION IN DIABETIC CARDIOMYOCYTES )

To study the mechanisms mediating intracellular calcium transients inv olved in diabetic cardiac dysfunction, changes in intracellular calciu m concentration ([Ca2+](i)) in response to stimulation by caffeine, ou abain, 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) (-p eak d[Ca2+](i)/dt) were decreased in diabetic myocytes, Insulin treatm ent 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 pla y a causal role in these changes. The maximum [Ca2+](i) increase in re sponse 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, T he KCl(12.5-50 mM)-induced [Ca2+](i) increase was enhanced in diabetic cells. Caffeine (20 mM) and dichlorobenzamil (DCB, 10 mu M) blocked t his [Ca2+](i) transient to a smaller degree in diabetic cells, but nit rendipine effects were similar in diabetic and control cells. These ef fects may be due to the increased L-channel activity and altered featu res, such as different responses to Ca-channel blockers, in diabetes w hich has previously been reported, The maximum response of [Ca2+](i) t o exogenous ATP was increased in diabetic cells while the sensitivity remained unchanged, The mechanisms underlying this enhanced response m ay be similar to the KCl-induced [Ca2+](i) changes in diabetes.