An experimental investigation of rundown of the L-type calcium current

Background L-type calcium channels (LCCs) are multi-protein macro-molecular ion channel complexes that are involved in several critical functions in cardiac, skeletal, neuronal, smooth muscle, and endocrine cells. Like other ion channels, LCCs can be selectively over-expressed in a host cell line and studied using voltage-clamp patch-clamp experiments. However, L-type calcium current (ICaL) recordings commonly exhibit a reduction in current magnitude over time, commonly termed ‘rundown’. Previous studies have shown the effect of phosphorylation on rundown, here we provide evidence that accumulation of Ca2+ inside the cell also contributes towards ICaL rundown. Methods We generated experimental conditions that should promote the accumulation of sub-membrane Ca2+ in a CHO expression system, by increasing calcium import or decreasing export. These interventions took the form of: a decrease in inter-pulse duration between sweeps, block of the sodium-calcium exchanger, and increased temperature. Results On average, we found that current reduced to 63% of its initial value within 325 seconds. This reduction of current with time was found to follow two main patterns: linear or saturating decay. Additionally, current magnitude in some cells increased before stabilising or decaying. Conclusions This study shows that the rundown of ICaL in patch-clamp experiments can be reduced by modifying the experimental conditions, and implies that reduced accumulation of Ca2+ inside the cell membrane reduces calcium-dependent inactivation of ICaL.