Abstract

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 Ca²⁺ 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 [³H]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 Ca²⁺. As less Ca²⁺ enters the cell through I_Ca, less Ca²⁺ is released through RyRs.