Investigation of abnormal cardiac function in murine models of hypocontractility and hypercontractility

Heart failure has a significant impact on mortality and morbidity. Dilatedcardiomyopathy (DCM) is the third most common cause of heart failure and the mostcommon reason for heart transplantation. Familial DCM is known to be caused bymutations in the LMNA gene encoding lamins A and C. New methods to enhancecardiac contractility would be beneficial in the treatment or prevention of heart failure.The focus of this thesis was to evaluate the mechanisms of altered contractility in twomouse models: the LMNA knockout model (homozygous, Lmna-/-; heterozygous,Lmna+/-) generated by targeted deletion of the lmna gene, and the model of enhancedcontractility due to cardiac alpha1A-adrenergic receptor (1A-AR) overexpression(A1A1).Previous studies have found altered nuclear-desmin connections in lamin A/C deficientmice. It was proposed that these alterations result in defective force transmission,which leads to DCM. Studies in this thesis have supported this hypothesis. Studies ofisolated single cardiomyocytes from mice aged 4-6 weeks demonstrated abnormal cellmorphology and contractile dysfunction in Lmna-/- cardiomyocytes, while Lmna+/- cellsshowed no overt phenotype. Excitation-contraction coupling experiments and forcecalciumstudies in skinned fibers excluded altered calcium kinetics as a primary causeof DCM in this model, but there was evidence of reduced sarcomere numbers andreduced sarcomere lengths as a contributor to reduce force generation in Lmna-/- andLmna+/- mice.Previous in vivo studies showed that A1A1 mice had enhanced contractility with theabsence of hypertrophy. Studies on isolated single cardiomyocytes from A1A1 miceaged 8-12 weeks showed reduced contractility in the absence of 1A-AR stimulation,but an exaggerated response to 1A-AR stimulation. In contrast isolated isovolumicLangendorff perfused A1A1 hearts without 1A-AR stimulation replicated the enhancedcontractility observed in vivo. These studies are consistent with down-regulation ofcontractility due to the hyperactivity of the overexpressed 1A-AR in vivo, which onlybecomes evident in isolated cells without 1A-AR stimulation due to the loss offunctional receptor numbers during isolation. Sufficient spontaneously active 1A-ARsare preserved in the isolated Langendorff heart preparation to ensure maximumcontractility driven by increase calcium release