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

Abstract

1. Noradrenaline (NA) slows the activation kinetics of N‐type calcium channels, via G proteins. It has been suggested that the G proteins act by binding directly to the calcium channels. If the slow kinetics reflect binding and unbinding of G proteins, the rates should depend on the concentration of activated G protein. 2. We used different concentrations of NA, and increasing durations of intracellular dialysis with GTP‐gamma‐S, to vary the concentration of activated G protein. 3. At depolarized potentials (‐20 or ‐10 mV), the slow activation kinetics showed no detectable concentration dependence. This analysis required correction for effects of inactivation on the measured time constants. 4. At ‐80 mV, reinhibition of calcium channel current was more rapid for larger responses. Thus, the effect appears to be concentration dependent at ‐80 mV, but not at more depolarized voltages. 5. This voltage dependence is actually expected from kinetic principles: the binding step is rate limiting when the position of equilibrium is toward the bound state (at ‐80 mV), but not when equilibrium favours unbinding (when the channel is open). 6. During inhibition, the channel appears to ‘sense’ directly the concentration of the modulator, possibly active G proteins.

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

“…These authors measured the kinetics ofreinhibition during a prolonged depolarization to 0 mV following a strong depolarizing prepulse and found a good correlation between the kinetics of reblock and the extent of G-protein activation, supporting Bean's model. However, Elmslie and Jones (1992), in bullfrog sympathetic neurons, found that reblock kinetics at 0 mV did not change with the extent of inhibition in response t3 different concentrations of leutinizing hormonereleasing hormone, in agreement with the model of Kasai and Aosaki.…”

“…Kasai's model was based on the finding that the slow activation kinetics of calcium current during a moderate depolarization, presumably reflecting unblock, were independent of transmitter concentration. In support of Kasai's model, Elmslie and Jones (1992) report that the kinetics of reblock at -10 mV following a strong depolarizing prepulse also are independent of transmitter concentration. It more than one inhibitor to the channel does not seem unreasonable.…”

“…By measuring kinetics of reblock at a negative holding potential of -80 mV, we avoid possible complications from Ca2+ channel activation and inactivation kinetics that complicate measurements of reblock kinetics at more positive potentials (cf. Kasai and Aosaki, 1989;Lopez and Brown, 199 1;Elmslie and Jones, 1992). We have used the normal desensitization of the response to prolonged application of these transmitters as a means to vary the extent of inhibition.…”

Facilitation of rabbit α1B calcium channels: involvement of endogenous Gβγ subunits

“…The present results provide evidence that actual dissociation of Gâã, rather than just a reduction in its effectiveness, occurs during depolarization. The rate of calcium current reinhibition has been shown previously to be dependent on the concentration of agonist (Elmslie & Jones, 1994;Zhou, Shapiro & Hille, 1997). In oocytes, reinhibition of á1B following a depolarizing prepulse during modulation by a receptor agonist had a time constant of about 77 ms (Zhang, Ellinor, Aldrich & Tsien, 1996).…”

“…depolarizing prepulses to reverse G protein-mediated inhibition (Elmslie & Jones, 1994). Enhancement of calcium currents in response to prior depolarization is termed facilitation, but it does not necessarily involve reversal of G protein modulation (for review, see Dolphin, 1996).…”