CONDUCTING AND VOLTAGE-DEPENDENT BEHAVIORS OF POTASSIUM-ION CHANNELS RECONSTITUTED FROM DIAPHRAGM SARCOPLASMIC-RETICULUM - COMPARISON WITH THE CARDIAC ISOFORM

Sarcoplasmic reticulum (SR) K+ channels from canine diaphragm were stu died upon fusion of longitudinal and junctional membrane vesicles into planar lipid bilayers (PLB). The large-conductance cation selective c hannel (gamma(max) = 250 pS; K-m = 33 mM) displays long-lasting open e vents which are much more frequent at positive than at negative voltag es. A major subconducting state about 45% of the fully-open state curr ent amplitude was occasionally observed at all voltages. The voltage-d ependence of the open probability displays a sigmoid relationship that was fitted by the Boltzmann equation and expressed in terms of thermo dynamic parameters, namely the free energy (Delta G(i)) and the effect ive gating charge(Z(s)): Delta G(i) = 0.27 kcal/mol and Z(s) = -1.19 i n 250 mM potassium gluconate (K-gluconate), Kinetic analyses also conf irmed the voltage-dependent gating behavior of this channel, and indic ate the implication of at least two open and three closed states. The diaphragm SR K+ channel shares several biophysical properties with the cardiac isoform: g = 180 pS, Delta G(i) = 0.75 kcal/mol, Z(s) = -1.45 in 150 mM K-gluconate, and a similar sigmoid P-o/voltage relationship . Little is known about the regulation of the diaphragm and cardiac SR K+ channels. The conductance and gating of these channels were not in fluenced by physiological concentrations of Ca2+ (0.1 mu M-1 mM) or Mg 2+ (0.25-1 mM), as well as by cGMP (25-100 mu M), lemakalim (1-100 mu M), glyburide (up to 10 mu M) or charybdotoxin (45-200 nM), added eith er to the cis or to the tuans chamber. The apparent lack of biochemica l or pharmacological modulation of these channels implies that they ar e not related to any of the well characterized surface membrane K+ cha nnels, On the other hand, their voltage sensitivity strongly suggests that their activity could be modulated by putative changes in SR membr ane potential that might occur during calcium fluxes.