Activation of a potassium current by rapid photochemically generated step increases of intracellular calcium in rat sympathetic neurons.
Although Ca2+ is a well-established intracellular messenger, there are many questions concerning the kinetics and spatial localization of its effects. Such problems may now be approached with the photosensitive Ca2+ chelator nitr-5. The Ca2+ affinity of this molecule decreases by a factor of 40 after absorption of near-UV light; Ca2+ is liberated with a time constant of approximately equal to 300 microseconds. Nitr-5 or the related compounds nitr-2 and nitr-7, complexed with Ca2+, were introduced into rat sympathetic ganglion cells by dialysis from a patch pipette electrode operating in the whole-cell, voltage-clamp mode. Light flashes released Ca2+ and activated a K+ current. Flash-induced current relaxations followed a simple exponential time course with time constants as brief as 5 ms. Comparison of the kinetics among the chelators, which photolyze at different rates, suggests that release of Ca2+ from nitr-5 is too fast to limit the relaxation. Thus we confirm directly that Ca2+ can modulate membrane properties within a few milliseconds after entering a cell. A preliminary kinetic description of K+ current activation by Ca2+ in rat sympathetic neurons is presented; Ca2+ appears to bind to the channel with a rate constant of at least 2 X 10(7) M-1 X s-1.
D000818:Animals; D000831:Animals, Newborn; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D002614:Chelating Agents / Q000494:pharmacology*; D004533:Egtazic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D004553:Electric Conductivity; D005728:Ganglia, Sympathetic / Q000502:physiology*; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D008027:Light; D008564:Membrane Potentials / Q000187:drug effects; D009474:Neurons / Q000502:physiology*; D011188:Potassium / Q000378:metabolism*; D051381:Rats
D002614:Chelating Agents; D007473:Ion Channels; C051166:nitr 2; C052455:nitr 5; C052456:nitr 7; D004533:Egtazic Acid; D011188:Potassium; D002118:Calcium