Negative feedback and stochastic behavior in the opening and closing of single L-type Ca channels

Calcium channels form a vital link in the control of essential processes like excitability, contraction and neurosecretion. To appreciate how Ca channels function in this capacity, a clear understanding of their opening and closing, or gating properties is fundamentally required. With the advent of the patch-clamp technique, which enables detection of gating events from single channels, there has been the hope over the past few years that Ca channel gating can be thoroughly understood at an elementary, stochastic level. Yet, major questions about Ca channel gating remain unanswered because of the complexities of Ca-facilitated inactivation, a prominent feature of many Ca channels whereby Ca entry through the channel pore appears to speed long-term closure, or inactivation of the channel. While the existence of this important physiological control feature is unmistakable, even some of the rudimentary properties of this negative feedback system remain unclear and present interesting theoretical challenges absent in many other ion channels. We have begun to explore the use of conditional open probability analysis (COPA) to probe for changes in single-channel gating properties as a function of prior Ca entry through cardiac L-type Ca channels. We calculate P_oo(t,t_j), defined as the chance that a channel is open at time t if it is known to be open at time t_j. The decay timecourse of P_oo(t,t_j) gauges the gating structure of the channel at time t_j, much as the impulse response of a dynamical process embodies the character of a system at a given time. COPA reveals that Ca entry during the openings of a single Ca channel produces alterations in gating that evolve over hundreds of milliseconds, consistent with an enzymatic basis for part of the negative feedback loop. We are currently using COPA to investigate the dynamic range of the feedback control, and the mechanisms underlying the modulation.