Multiple types of voltage-dependent Ca2+-activated K+ channels of large conductance in rat brain synaptosomal membranes

Abstract

K+-selective ion channels from a mammalian brain synaptosomal membrane preparation were inserted into planar phospholipid bilayers on the tips of patch-clamp pipettes, and single-channel currents were measured. Multiple distinct classes of K+ channels were observed. We have characterized and described the properties of several types of voltage-dependent, Ca2+-activated K+ channels of large single-channel conductance (greater than 50 pS in symmetrical KCl solutions). One class of channels (Type I) has a 200-250-pS single-channel conductance. It is activated by internal calcium concentrations greater than 10(-7) M, and its probability of opening is increased by membrane depolarization. This channel is blocked by 1-3 mM internal concentrations of tetraethylammonium (TEA). These channels are similar to the BK channel described in a variety of tissues. A second novel group of voltage-dependent, Ca2+-activated K+ channels was also studied. These channels were more sensitive to internal calcium, but less sensitive to voltage than the large (Type I) channel. These channels were minimally affected by internal TEA concentrations of 10 mM, but were blocked by a 50 mM concentration. In this class of channels we found a wide range of relatively large unitary channel conductances (65-140 pS). Within this group we have characterized two types (75-80 pS and 120-125 pS) that also differ in gating kinetics. The various types of voltage-dependent, Ca2+-activated K+ channels described here were blocked by charybdotoxin added to the external side of the channel. The activity of these channels was increased by exposure to nanomolar concentrations of the catalytic subunit of cAMP-dependent protein kinase. These results indicate that voltage-dependent, charybdotoxin-sensitive Ca2+-activated K+ channels comprise a class of related, but distinguishable channel types. Although the Ca2+-activated (Type I and II) K+ channels can be distinguished by their single-channel properties, both could contribute to the voltage-dependent Ca2+-activated macroscopic K+ current (IC) that has been observed in several neuronal somata preparations, as well as in other cells. Some of the properties reported here may serve to distinguish which type contributes in each case. A third class of smaller (40-50 pS) channels was also studied. These channels were independent of calcium over the concentration range examined (10(-7)-10(-3) M), and were also independent of voltage over the range of pipette potentials of -60 to +60 mV. Type III channels were unaffected by internal TEA concentrations <50 mM. Our results also indicate that the study of K+ channels in lipid bilayers may allow the identification and characterization of novel K+ channels from brain regions otherwise inaccessible to conventional recording techniques.