Altered transarcolemmal Ca transport modifies the myofibrillar ultrastructure and protein metabolism in cultured adult ventricular cardiomyocytes

Structural and functional plasticity in long-term cultures of adult ventricular myocytes

Cultured heart cells have long been valuable for characterizing biological mechanism and disease pathogenesis. However, these preparations have limitations, relating to immaturity in key properties like excitation-contraction coupling and β-adrenergic stimulation. Progressive attenuation of the latter is intimately related to pathogenesis and therapy in heart failure. Highly valuable would be a long-term culture system that emulates the structural and functional changes that accompany disease and development, while concurrently permitting ready access to underlying molecular events. Accordingly, we here produce functional monolayers of adult guinea-pig ventricular myocytes (aGPVMs) that can be maintained in long-term culture for several weeks. At baseline, these monolayers exhibit considerable myofibrillar organization and a significant contribution of sarcoplasmic reticular (SR) Ca(2+) release to global Ca(2+) transients. In terms of electrical signaling, these monolayers support propagated electrical activity and manifest monophasic restitution of action-potential duration and conduction velocity. Intriguingly, β-adrenergic stimulation increases chronotropy but not inotropy, indicating selective maintenance of β-adrenergic signaling. It is interesting that this overall phenotypic profile is not fixed, but can be readily enhanced by chronic electrical stimulation of cultures. This simple environmental cue significantly enhances myofibrillar organization as well as β-adrenergic sensitivity. In particular, the chronotropic response increases, and an inotropic effect now emerges, mimicking a reversal of the progression seen in heart failure. Thus, these aGPVM monolayer cultures offer a valuable platform for clarifying long elusive features of β-adrenergic signaling and its plasticity.

Changes in myofibrils and cytoskeleton of neonatal hamster myocardial cells in culture: an immunofluorescence study

Myocardial cells in culture offer many possibilities for studying cellular and molecular biology of cardiac muscles. However, it is important to know how long these cells can be maintained in vitro without significant structural and biochemical changes. In this study, we have investigated the morphological changes of myofibril proteins and cytoskeletons by using immunofluorescent techniques in cultured neonatal hamster myocardial cells at different culture durations. Our results have demonstrated that these cultured cells still contain intact myofibrils and cytoskeletal proteins after 6 days in vitro incubation, however, the organization of some of these proteins is altered. The proteins most sensitive to these in vitro conditions are: myosin heavy chain, actin and desmin. The data indicate that the duration of the culture and the contractile activity of the myocardial cells in culture can influence organization of their contractile apparatus and cytoskeleton.

Na+/Ca2+ exchanger-deficient mice have disorganized myofibrils and swollen mitochondria in cardiomyocytes

The Na(+)/Ca(2+) exchanger (NCX1) plays a key role in maintaining Ca(2+) homeostasis in cardiomyocytes. Disruption of Ncx1 gene in mice results in embryonic lethality between embryonic day 9 and 10, with the mice lacking spontaneous heartbeats. We examined the mechanism of lack of heartbeats in Ncx1-deficient mice. Ultrastructual analysis demonstrated that Ncx1-deficient mice showed severe disorganization of myofibrils, a lack of Z-lines and swelling of mitochondria in cardiomyocytes. However, the expressions of cardiac-specific genes including transcription factor genes and contractile protein genes were not changed in Ncx1-deficient mice. Abnormal Ca(2+) handling itself or the lack of heartbeats due to the inactivation of Ncx1 gene may cause the disorganization of myofibrillogenesis. Although NCX1 protein levels were decreased in heterozygous mice, there were no changes in NCX2 and NCX3 protein levels between wild type and heterozygous mice.

Japanese herbal medicine Toki-shakuyaku-san (TJ-23) enhances cardiac contractile function in isolated ventricular cardiomyocytes

Toki-shakuyaku-san (TJ-23), a Japanese traditional herbal medicine, has a long history in Asia for the treatment of neurodegenerative, immune, and airway diseases. However, the effect of TJ-23 on heart function has not been elucidated. This study was designed to examine the effect of TJ-23 on ventricular contractile function at the single cardiomyocyte level. Ventricular cardiomyocytes from adult rat hearts were stimulated to contract at 0.5 Hz, and mechanical properties were evaluated using an IonOptix Myocam system. Contractile properties analyzed included peak shortening (PS), time-to-PS (TPS), time-to-90% relengthening (TR(90)), and maximal velocity of shortening/relengthening (+/-dL/dt). TJ-23 (10(-)(8) - 10(-)(5) mg/ml) exhibited significant augmentation in PS, with a maximal response of 27.2%. TJ-23 at 10(-)(7) - 10(-)(5) mg/ml also increased +/-dL/dt, shortened TR(90), while had no effect on TPS. Pretreatment with the Na(+)-K(+)-ATPase inhibitor ouabain (1 microM), removal of extracellular sodium from contractile buffer (which inhibits Na(+)/Ca(2+) exchanger), or both concurrently abolished the positive effect of TJ-23 in cell shortening without inhibiting the baseline cell shortening. This study demonstrated a direct cardiac stimulatory action of TJ-23 at the cardiomyocyte level, which may be related to, at least in part, a Na(+)/K(+)-ATPase and/or Na(+)/Ca(2+) exchanger-dependent mechanism.

Maintenance of Ca(i)(2+)transients during prolonged cardiac arrest aids rapid contractile recovery

We explored the effects of contractile arrest maintained for 24-72 h in the presence of 2,3-butanedione monoxime or a Ca(2+) channel blocker (nifedipine or verapamil) on contractile activity, Ca(i)(2+) transients, and myofibrillar protein content and ultrastructure in long-term cultures of spontaneously beating adult guinea-pig cardiomyocytes. The contractions were not affected by 5 mM 2, 3-butanedione monoxime, but they were strongly or fully suppressed by 10 and 18 mM 2,3-butanedione monoxime, respectively, while the Ca(i)(2+)transients triggered by the maintained spontaneous electrical activity were either not changed at all (5 and 10 mM 2, 3-butanedione monoxime) or decreased only slightly (18 mM 2, 3-butanedione monoxime). The uncoupling of excitation from contraction by 10-18 mM for 24-72 h did not affect the content of the myofibrillar proteins. Confocal laser microscopy showed that these exposures affected the assembly of myofilaments, giving an overall deranged appearance to the myofibrils. In spite of this effect, the cells' contractile activity was readily regained within 15-60 min upon the washout of 2,3-butanedione monoxime. The 24-72-h exposures to 5 microM nifedipine or 10 microM verapamil, which blocked fully both the Ca(i)(2+) transients and contractility, did not affect the myofibrillar protein content nor their assembly. However, the recovery of contractile activity after exposure to a Ca(2+)-channel blocker was significantly slower (several days) than after 2,3-butanedione monoxime exposure. Furthermore, cultures exposed to Ca(2+)-channel blockers also had significantly decreased sensitivity to beta-adrenergic stimulation. Altogether, these data indicate the importance of regular Ca(2+) influx for the maintenance of the functional integrity of adult cardiomyocytes during prolonged periods of contractile arrest.