Activation kinetics of calcium currents in bull‐frog sympathetic neurones.

Abstract

SUMMARY1. Calcium currents were recorded in dissociated bull-frog sympathetic neurones (BSNs) through patch pipettes using discontinuous voltage clamp. Activation kinetics were examined by analysing turn-on and turn-off currents.2. After short depolarizing pulses turn-off tail currents were fitted with the sum of two exponentials. The fast component (time constant, Tr t 240 jts at -40 mV) was undoubtedly due to the closure of calcium channels. The significance of a small and slower component is discussed.3. Neither activation nor deactivation time courses changed as channels inactivated during progressively longer pulses or when the holding potential was less negative. No specific component was selectively suppressed by these manipulations.4. Steady-state activation of the Ca2+ current was described by the Boltzmann distribution raised to the second power. Currents had an apparent threshold at -30 mV and were half-activated at + 5 mV.5. Calcium current turned on following m2 kinetics throughout the range of activation. The slowest time constant was around 1-2 ms between 0 and + 10 mV. Turn-on was faster at negative or more positive potentials.6. The time course of decay of tail currents became progressively faster at more negative potentials.7. The instantaneous current-voltages (I-V) curve was obtained from tail current measurements and fitted by a modified constant-field equation.8. The measured peak I-V curve could be reconstructed from the activation curve and the instantaneous I-V curve.9. The activation kinetics of the calcium current in BSNs were consistent with the existence of a single kinetic class of channels and can be described with a simple m2 Hodgkin-Huxley model.

Concentration dependence of neurotransmitter effects on calcium current kinetics in frog sympathetic neurones.

“…5 of Elmslie et al 1993). Boland & Bean (1993) Sala, 1991), or as simple exponential kinetics following a delay (Jones & Marks, 1989). With their parameters, all of the channels become unmodulated at extreme positive voltages, which is not observed in these cells (Elmslie et al 1990Boland & Bean, 1993 Elmslie et al 1990).…”

Differential Gating and Recruitment of P/Q-, N-, and R-Type Ca2+Channels in Hippocampal Mossy Fiber Boutons

“…where P is an amplitude factor, C and D determine current rectification and reversal potential (Sala, 1991;Bischofberger et al, 2002), V mid is the midpoint potential of the steady-state activation curve, and k is the corresponding slope factor. For nonstationary fluctuation analysis (Sigworth, 1980), 15-138 Ca 2ϩ current traces were analyzed.…”

Characteristics of Ca2+ release induced by Ca2+ influx in cultured bullfrog sympathetic neurones.

“…"ICa' was activated at voltages above -40 to -30 mV and inactivated with a slow time course (decay time to 90% of the maximum amplitude was 51+6 ms at 0 mV, n = 10). The characteristics of the voltage-dependent Ca2+ current in cultured bullfrog sympathetic neurones have been studied previously (Jones & Marks, 1989a, b;Sala, 1991) and therefore were not investigated in detail in this study.…”