Multiple modes of N-type calcium channel activity distinguished by differences in gating kinetics

Abstract

In many neurons, N-type Ca2+ channels are a major Ca2+ entry pathway and strongly influence neurotransmitter release. We carried out cell- attached patch recordings (110 mM Ba2+ as charge carrier) to characterize the rapid opening and closing kinetics of N-type Ca2+ channel gating in frog sympathetic neurons. Single channels display at least three distinct patterns of gating, characterized as low-, medium- , and high-rho o modes on the basis of channel open probability (rho o) during depolarizing pulses to -10 mV. Spontaneous transitions from one mode to another are infrequent, with an exponential distribution of dwell times and mean sojourns of approximately 10 sec in each mode. Thus, a channel typically undergoes hundreds or thousands of open- closed transitions in one mode before switching to a different mode. Transitions between modes during a depolarization were occasionally detected, but were rare, as expected for infrequent modal switching. Within each mode, the activation kinetics were well described by a simple scheme (C2-C1-O), as previously reported for other types of Ca2+ channels. Rate constants are strikingly different from one mode to another, giving each mode its own characteristic kinetic signature. The gating behavior at -10 mV ranges from brief openings (approximately 0.3 msec) and long closures (10–20 msec) for low-rho o gating to long openings (3 msec) and brief closures (approximately 1 msec) for high- rho o gating. Intermediate values for mean open durations (approximately 1.5 msec) and mean closed durations (approximately 3 msec) were found for medium-rho o gating. In addition to being kinetically distinct, channel openings in the low-rho o mode often exhibit a unitary current approximately 0.2 pA larger than in the medium- or high-rho o mode. Each mode is characterized by its own voltage dependence: activation occurs at relatively negative potentials and is most steeply voltage dependent in the high-rho o mode, while activation requires very strong depolarizations and is weakly voltage dependent in the low-rho o mode. The proportion of time spent in the individual modes varies greatly from one patch to another, suggesting that modal gating may be subject to cellular control.

Kinetic basis for the voltage-dependent inhibition of N-type calcium current by somatostatin and norepinephrine in chick sympathetic neurons

“…Thus, more than the inhibitory molecule (B,) showed little dependence on the one reluctant gating mode was required to achieve an adequate degree of desensitization while the forward rate constant for description of the data. This is in agreement with the singlebinding of inhibitory molecules (F,) showed a progressive dechannel results of Delcour et al (1993), who found three modcline as the extent of inhibition decreased during desensitization ulated gating modes based on measurements of single-channel (Table 1). However, the rate of dissociation of the second bound open time and open probability.…”

Emergence of ion channel modal gating from independent subunit kinetics

“…A detailed understanding of this important feature of IP 3 R behavior is therefore crucial for unraveling the complexity of Ca 2+ signaling and its role in cell function and disease. Modal gating has been observed in the kinetics of many other ion channels, such as K + (13)(14)(15)(16)(17)(18), Cl − (19), glutamate receptors (20,21), plasma membrane Ca 2+ (22)(23)(24), and ryanodine receptor Ca 2+ (25,26). However, the underlying biophysical basis of modal gating in the IP 3 R and most other channels remains unknown.…”

A single non-L-, non-N-type Ca2+ channel in rat insulin-secreting RINm5F cells

“…The channel is best characterized by its resistance to DHPs and -CTx-GVIA and is shown to coexist with the more common L-type channel observed in various -cells and -cell lines [27,28,35]. Few outside-out patch recordings also provide evidence for a -CTx-GVIA-sensitive Ca 2+ channel with single-channel properties close to the N-type channel described in peripheral neurons [8,24]. The non-L-, non-N-type channel described here has a high probability of being recorded in patches of -CTx-GVIA-treated cells bathed in nifedipine and, therefore, is associated with the Q-like current that contributes to most of the residual non-L, non-N-type current in RINm5F cells [18].…”

“…In peripheral neurons the channel activates above 910 mV in 100 mM [Ba 2+ ] o , exhibits a 20 pS singlechannel conductance (1.3 pA at + 10 mV), inactivates weakly during prolonged depolarizations and possesses marked sensitivity to holding potential [24]. Indeed, the neuronal N-type channel also exhibits multiple gating modes with low and high p o [8], small and large conductance and slow and fast inactivation [23] that make any further comparison more complicated and arbitrary.…”