CONTRACTILE ACTIVITY AND CELL-CELL CONTACT REGULATE MYOFIBRILLAR ORGANIZATION IN CULTURED CARDIAC MYOCYTES

Adult feline ventricular myocytes cultured on a laminin-coated substra tum reestablish intercellular junctions, yet disassemble their myofibr ils. Immunofluorescence microscopy reveals that these non-beating hear t cells lack vinculin-positive focal adhesions; moreover, intercellula r junctions are also devoid of vinculin. When these quiescent myocytes are stimulated to contract with the beta-adrenergic agonist, isoprote renol, extensive vinculin-positive focal adhesions and intercellular j unctions emerge. If solitary myocytes are stimulated to beat, an elabo rate series of vinculin-positive focal adhesions develop which appear to parallel the reassembly of myofibrils. In cultures where neighborin g myocytes reestablish cell-cell contact, myofibrils appear to reassem ble from the fascia adherens rather than focal contacts. Activation of beating is accompanied by a significant reduction in the rate of tota l and cytoskeletal protein synthesis; in fact, myofibrillar reassembly , redevelopment of focal adhesions and fascia adherens junctions requi re no protein synthesis for at least 24 h, implying the existence of a n assembly competent pool of cytoskeletal proteins. Maturation of the fasciae adherens and the appearance of vinculin within Z-line/costamer es, does require de novo synthesis of new cytoskeletal proteins. Chang es in cytoskeletal protein turnover appear dependent on agonist-induce d cAMP production, but myofibrillar reassembly is a cAMP-independent e vent. Such observations suggest that mechanical forces, in the guise o f contractile activity, regulate vinculin distribution and myofibrilla r order in cultured adult feline heart cells.