Concentration dependence of neurotransmitter effects on calcium current kinetics in frog sympathetic neurones.
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.
Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH 44106.
K S
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Elmslie
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Jones
D000818:Animals; D015220:Calcium Channels / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D019204:GTP-Binding Proteins / Q000502:physiology; D006160:Guanosine Triphosphate / Q000502:physiology; D007700:Kinetics; D009638:Norepinephrine / Q000494:pharmacology*; D011892:Rana catesbeiana; D013564:Sympathetic Nervous System / Q000187:drug effects* / Q000378:metabolism
D015220:Calcium Channels; D006160:Guanosine Triphosphate; D019204:GTP-Binding Proteins; D009638:Norepinephrine