L-type Calcium Currents of Human Myocytes from Ventricle of Non-failing and Failing Hearts and from Atrium

Abstract

L-type calcium currents were studied in ventricular myocytes isolated from non-failing hearts, i.e. donor hearts not suitable for transplantation, and from severely failing hearts, i.e. explanted hearts of organ recipients, in order to identify possible alternations of the currents in cardiomyopathy. Human atrial myocytes were investigated for comparative purposes. As deficient production of cyclic AMP might contribute to the development of cardiac failure, the response to forskolin, a direct stimulator of adenylyl cyclase, were also studied. The patch-clamp technique was applied in the single electrode whole-cell mode. Calcium currents were similar in myocytes from non-failing and failing hearts: Maximum current-densities were 3.8 v 3.1 pA/pF, and 2.2 pA/pF in atrial cells. In human ventricular cells, threshold was at -33 mV. maximum at +6 mV and reversal potential at about +50 mV, potential of half-maximum steady-state inactivation -24 mV and -18 mV. The slopes of steady-state inactivation curves were +4.1 in myopathic and +5.5 mV in non-failing cells. In all myocytes the current inactivated with two time constant, a fast one with weak and a slow one with pronounced potential dependency. Ventricular or atrial myocytes from patients pretreated with calcium antagonists and untreated did not differ in current density or steady-state inactivation. Forskolin (0.5 μM) increases calcium currents in myocytes from non-failing and failing hearts to the same extent (by 143 and 150%). While β-adrenoceptor numbers are reported to decline in severely failing myocardium, our data do not suggest that alterations of the properties of calcium currents contribute to the pathophysiology of heart failure, though the number of investigated hearts is limited due to restricted access to non-failing cardiac tissue. No evidence for important of the signal transduction cascade beyond the level of GTP binding proteins was found.