Increased Availability and Open Probability of Single L-Type Calcium Channels From Failing Compared With Nonfailing Human Ventricle

Abstract

Background —The role of the L-type calcium channel in human heart failure is unclear, on the basis of previous whole-cell recordings. Methods and Results —We investigated the properties of L-type calcium channels in left ventricular myocytes isolated from nonfailing donor hearts (n=16 cells) or failing hearts of transplant recipients with dilated (n=9) or ischemic (n=7) cardiomyopathy. The single-channel recording technique was used (70 mmol/L Ba 2+ ). Peak average currents were significantly enhanced in heart failure (38.2±9.3 fA) versus nonfailing control hearts (13.2±4.5 fA, P =0.02) because of an elevation of channel availability (55.9±6.7% versus 26.4±5.3%, P =0.001) and open probability within active sweeps (7.36±1.51% versus 3.18±1.33%, P =0.04). These differences closely resembled the effects of a cAMP-dependent stimulation with 8-Br-cAMP (n=11). Kinetic analysis of the slow gating shows that channels from failing hearts remain available for a longer time, suggesting a defect in the dephosphorylation. Indeed, the phosphatase inhibitor okadaic acid was unable to stimulate channel activity in myocytes from failing hearts (n=5). Expression of calcium channel subunits was measured by Northern blot analysis. Expression of α 1C - and β-subunits was unaltered. Whole-cell current measurements did not reveal an increase of current density in heart failure. Conclusions —Individual L-type calcium channels are fundamentally affected in severe human heart failure. This is probably important for the impairment of cardiac excitation-contraction coupling.