The Young Mouse Heart Is Composed of Myocytes Heterogeneous in Age and Function

Abstract

Abstract-The recognition that the adult heart continuously renews its myocyte compartment raises the possibility that the age and lifespan of myocytes does not coincide with the age and lifespan of the organ and organism. If this were the case, myocyte turnover would result at any age in a myocardium composed by a heterogeneous population of parenchymal cells which are structurally integrated but may contribute differently to myocardial performance. To test this hypothesis, left ventricular myocytes were isolated from mice at 3 months of age and the contractile, electrical, and calcium cycling characteristics of these cells were determined together with the expression of the senescence-associated protein p16 INK4a and telomere length. The heart was characterized by the coexistence of young, aged, and senescent myocytes. Old nonreplicating, p16INK4a -positive, hypertrophied myocytes with severe telomeric shortening were present together with young, dividing, p16INK4a -negative, small myocytes with long telomeres. A class of myocytes with intermediate properties was also found. Physiologically, evidence was obtained in favor of the critical role that action potential (AP) duration and I CaL play in potentiating Ca 2ϩ cycling and the mechanical behavior of young myocytes or in decreasing Ca 2ϩ transients and the performance of senescent hypertrophied cells. The characteristics of the AP appeared to be modulated by the transient outward K ϩ current I to which was influenced by the different expression of the K ϩ channels subunits. Collectively, these observations at the physiological and structural cellular level document that by necessity the heart has to constantly repopulate its myocyte compartment to replace senescent poorly contracting myocytes with younger more efficient cells. Thus, cardiac homeostasis and myocyte turnover regulate cardiac function. Key Words: action potential profile Ⅲ excitation-contraction coupling Ⅲ myocyte volume Ⅲ telomere length Ⅲ senescence-associated proteins T he recent identification of a population of resident progenitor cells that controls cardiomyogenesis imposes a reinterpretation of the fundamental mechanisms of growth and senescence of the heart. 1 Traditionally, the heart was viewed as an organ characterized by a predetermined number of myocytes which is defined shortly after birth and is preserved throughout life. However, BrdU administration has documented that myocyte renewal occurs in the adult heart and that the rate of cell regeneration is slow, heterogeneous, and involves only a small percentage of cells at any given time. 2,3 In mammals, ventricular myocytes are replaced several times throughout life. 2,3 These findings were obtained by pulse-chase BrdU labeling assay 2 and by the rate of growth and commitment of cardiac progenitor cells. 3 Collectively, they challenge the notion that the age and lifespan of myocytes coincide with the age and lifespan of the organ and organism. According to the new paradigm, the continuous turnover of myocytes results in a heterogen...

Reduction in Kv Current Enhances the Temporal Dispersion of the Action Potential in Diabetic Myocytes: Insights From a Novel Repolarization Algorithm

“…Isolated LV myocytes were placed in a bath on the stage of an IX51, IX53, or IX71 (Olympus, Tokyo, Japan) microscopes for patch‐clamp measurements 19, 25, 30, 31. Experiments were conducted at 37°C.…”

“…Electrical signals were digitized using 250‐kHz 16‐bit resolution A/D converters (Digidata 1322, 1440A, and 1550; Molecular Devices) and recorded using pCLAMP 9.0 and 10 software (Molecular Devices) with low‐pass filtering at 2 kHz. Membrane capacitance (C m ) was measured in voltage‐clamp mode using a 5‐mV voltage step and pCLAMP software algorithm; this parameter was employed to normalize transmembrane currents 19, 20, 25, 30. Pipettes were pulled by means of a vertical (PB‐7; Narishige International Inc., East Meadow, NY), or horizontal (P‐1000; Sutter Instrument Co, Novato, CA) glass microelectrode pullers.…”

“…For AP measurements, current‐clamp mode was employed 19, 20, 30, 31. Cells were stimulated with current pulses ≈1.5 times threshold.…”

“…The composition of the pipette solution was (in mmol/L): NaCl 10, KCl 113, MgCl 2 0.5, K 2 ‐ATP 5, glucose 5.5, HEPES 10, EGTA 10, and CaCl 2 1 (pH 7.2 with KOH). To test the effects of inhibition of Kv currents, APs were continuously recorded for the same cells before and after exposure to 4‐aminopyridine (4‐AP; 0.1 mmol/L) 30, 32, 33, 34, 35. At this concentration level, 4‐AP has been previously reported to abolish the large, slowly inactivating phase of the Kv‐based I K,slow 32, 34, 35 leading to prolongation of AP 33.…”

“…Test steps were preceded by a prestep to −30 mV, 20 ms in duration, from holding potential (V h ) of −70 mV. Amplitude of Kv currents was measured as the difference between the peak outward current at the beginning of the test step and I ss , the current at the end of the 500‐ms step 30. Currents were normalized by C m .…”