Primary culture of human atrial myocytes is associated with the appearance of structural and functional characteristics of immature myocardium.

We examined changes in the structural and physiological characteristics of human atrial myocytes during primary culture in the presence of serum. Action potentials and ionic currents were recorded in freshly dissociated (FM) and cultured (CM) whole-cell patch-clamped myocytes, alpha-smooth muscle actin, sarcomeric alpha-actinin and beta-myosin heavy chains (beta-MHC) were stained with monoclonal antibodies. From day 5 to day 21, myocytes lost their rod shape, spread and exhibited reorganized sarcomeres. These morphological changes were associated with a marked increase in membrane capacitance (+266%). Both beta-MHC and alpha-smooth muscle actin were expressed in CM but not in FM, indicating a dedifferentiation process. CM were characterized by a lower resting potential (-30 +/- 2 v -60 +/- 4 mV, P < 0.05) and, when repolarized, by a shorter action potential duration (APD) than FM (APD-60: 126.9 v 159.6 ms, P < 0.05). The inward rectifier K+ current was absent in CM, thus explaining the low resting potential. The density of the transient component of the voltage-activated K+ current Ito1 was not modified during culture, while that of the sustained component Isus was increased fourfold. The amplitude of ICa was increased, but its density was unchanged, indicating that CM maintained a normal density of functional calcium channels. Neither the voltage dependence nor the inactivation of ICa was modified in CM. The time constants of inactivation of ICa were unchanged, although the amplitude of the rapidly inactivating component of ICa was increased in CM compared to FM. Moreover, ICa was increased by the beta-adrenergic agonist isoproterenol (1 microM) throughout the culture period. Our results demonstrate that in long-term serum-supplemented culture, adaptation of human atrial myocytes to their new environment is associated with differential alterations of the main ionic currents and phenotypic changes characteristic of immature myocardium.