Effects of deletion of muscle LIM protein on myocyte function

Muscle LIM protein (MLP) may serve as a scaffold protein on the actin-based cytoskeleton, and mice deficient in this protein (MLPKO) have been recentl y reported to develop dilated cardiomyopathy. To determine the causes of de pressed contractility in this model, we measured intracellular Ca2+ concent ration ([Ca2+](i)) transients (fluo 3), cell shortening, L-type Ca2+ channe l current (I-Ca,I-L), Na/Ca exchanger current (I-Na/Ca), and sarcoplasmic r eticulum (SR) Ca content in left ventricular MLPKO myocytes. I-Ca,I-L-volta ge relationships, I-Na/Ca density, and membrane capacitance did not differ between wildtype (WT) and MLPKO myocytes. The peak systolic [Ca2+]i was sig nificantly increased in MLPKO myocytes (603 +/- 54 vs. 349 +/- 18 nM in WT myocytes). The decline of [Ca2+](i) transients was accelerated in MLPKO myo cytes, and SR Ca2+ content was increased by 21%, indicating that SR Ca2+ AT Pase function is normal or enhanced in MLPKO myocytes. Confocal imaging of actin filaments stained with tetramethylrhodamine isothiocyanate-labeled ph alloidin showed disorganization of myofibrils and abnormal alignment of Z b ands, and fractional shortening was significantly diminished in MLPKO myocy tes compared with that in WT myocytes at comparable peak [Ca2+](i). Thus a reduced [Ca2+]-induced shortening may be involved in the pathogenesis of my ocardial dysfunction in this genetic model of heart failure.