title: Calcium-activated potassium channels in isolated presynaptic nerve terminals from rat brain.
abstract: 86Rb efflux was examined in isolated presynaptic nerve terminals (synaptosomes) from rat brain in a study designed to assess K permeability (PK) changes sensitive to alterations in internal Ca activity. Rb efflux from 86Rb-loaded synaptosomes into nominally Ca-free physiological saline (PSS) containing 5 mM-K was about 0.3-0.4%/s. Raising extracellular K concentration [( K]o), to depolarize the synaptosomes, stimulated the 86Rb efflux. Addition of Ca to the 5 mM-K PSS had no effect, but Ca did further stimulate 86Rb efflux into K-rich solutions. The effect of Ca was graded, with apparent half-maximal activation, KA approximately equal to 0.5 mM-Ca. These data fit the view that, during depolarization, Ca enters the terminals through voltage-regulated Ca channels, and that the rise in intracellular Ca concentration opens certain (Ca-activated) K channels. The Ca-dependent stimulation of 86Rb efflux was greatest during the initial seconds of incubation (component CT), and then declined to a much lower rate (component CS). Much of this change in rate could be attributed to inactivation of voltage-regulated Ca channels and reduced entry of Ca. The Ca-dependent increase in 86Rb efflux was completely inhibited by 100 microM-La. In the presence of Ca, but not in its absence, the Ca ionophore A23187 stimulated 86Rb efflux both in 5 and 100 mM-K PSS. The effect in 100 mM-K was quantitatively greater, perhaps because of the increased outward driving force on Rb in depolarized synaptosomes. When synaptosomes were suspended in media containing the voltage-sensitive fluorescent dye, DiS-C3-(5) (1,1'-dipentyl-2,2'-thiocarbocyanine), the addition of Ca+ A23187 decreased the fluorescence intensity (= synaptosome hyperpolarization) when the media contained 5 mM-K but not 100 mM-K. This implies that in the presence of Ca + A23187, PK was increased, and the membrane potential moved closer to the K equilibrium potential, EK. Quinine sulphate, a blocker of Ca-activated K channels, reduced the Ca-stimulated 86Rb efflux with high affinity (apparent half-maximal inhibition, KI approximately equal to 1 microM). Tetraethylammonium chloride, another agent known to block Ca-activated K channels, was also a relatively potent inhibitor of Ca-stimulated 86Rb efflux (KI approximately equal to 0.2 mM). The K-channel blocker, 4-aminopyridine, partially inhibited Ca-stimulated 86Rb efflux at concentrations below 0.5 mM, but stimulated this efflux at concentrations greater than or equal to 1 mM.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.forename: D K
authors.initials: DK
authors.lastname: Bartschat
authors.forename: M P
authors.initials: MP
authors.lastname: Blaustein
mesh_terms: D015761:4-Aminopyridine; D000631:Aminopyridines / Q000494:pharmacology; D000818:Animals; D001921:Brain / Q000166:cytology*; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D005260:Female; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects*; D007811:Lanthanum / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D011188:Potassium / Q000378:metabolism* / Q000494:pharmacology; D011803:Quinine / Q000494:pharmacology; D011868:Radioisotopes; D051381:Rats; D012413:Rubidium; D013574:Synaptosomes / Q000378:metabolism*
chemical_list: D000631:Aminopyridines; D007473:Ion Channels; D011868:Radioisotopes; D000001:Calcimycin; D007811:Lanthanum; D011803:Quinine; D015761:4-Aminopyridine; D012413:Rubidium; D011188:Potassium; D002118:Calcium