Mechanisms of [Ca2+]i transient decrease in cardiomyopathy of db/db type 2 diabetic mice

Cardiovascular disease is the leading cause of death in the diabetic population. However, molecular mechanisms underlying diabetic cardiomyopathy remain unclear. We analyzed Ca2+-induced Ca2+ release and excitation-contraction coupling in db/db obese type 2 diabetic mice and their control littermates. Echocardiography showed a systolic dysfunction in db/db mice. Two-photon microscopy identified intracellular calcium concentration ([Ca2+](i)) transient decrease in cardiomyocytes within the whole heart, which was also found in isolated myocytes by confocal microscopy. Global [Ca2+](i) transients are constituted of individual sparks. Ca2+ sparks in db/db cardiomyocytes were less frequent than in +/+ myocytes, partly because of a depression in sarcoplasmic reticulum Ca2+ load but also because of a reduced expression of ryanodine receptor Ca2+ channels (RyRs), revealed by [H-3]ryanodine binding assay. Ca2+ efflux through Na+/Ca2+ exchanger was increased in db/db myocytes. Calcium current, I-Ca, triggers sarcoplasmic reticulum Ca2+ release and is also involved in sarcoplasmic reticulum Ca2+ refilling. Macroscopic I-Ca was reduced in db/db cells, but single Ca2+ channel activity was similar, suggesting that diabetic myocytes express fewer functional Ca2+ channels, which was confirmed by Western blots. These results demonstrate that db/db mice show depressed cardiac function, at least in part, because of a general reduction in the membrane permeability to Ca. As less Ca2+ enters the cell through I-Ca, less Ca2+ is released through RyRs.