Inositol polyphosphates modify the kinetics of a small chloride channel in skeletal muscle sarcoplasmic reticulum

The actions of D-myo-inositol 1,4,5-trisphosphate (IP3) and D-myo-inositol 1,3,4,5-tetrakisphospate (IP4) on small chloride (SCl) channels from rabbit skeletal muscle sarcoplasmic reticulum are reported. We find that the inositol polyphosphates (6-40 microM) are potent reversible blockers of SCl channels in lipid bilayers at -40 mV with >10(-5) m cis (cytoplasmic) Ca2+ when added to the cis, but not trans, chamber. IP3 or IP4 at 20 microM reduced the mean open time from 89 +/- 16 msec to 11 +/- 2 msec or to 8.0 +/- 1.0 msec respectively, by abolishing the longest time constant component in the open time distribution. Neither IP3 nor IP4 altered the six single-channel conductance levels. The fraction of low conductance events increased approximately 4-fold and the dwell time at the lower conductance levels increased approximately 3-fold. Channel gating was altered so that most transitions were between the closed level and an open level, in contrast to control channels which remained open for long periods with many transitions between the six open levels. The actions of the inositol polyphosphates were: (1) not prevented by 20 microg/ml cis heparin (an IP3 receptor blocker); (2) mimicked by 10 microM cis synthetic inositol polyphosphates, L-chiro-inositol 1,4,6-trisphosphate and L-chiro-inositol 1,4,6-trisphosphorothionate (which do not bind to IP3 receptors); (3) mimicked by cis additions of the polyanions heparin or hepran (20 microg/ml each) and vanadate (500 microM). The results suggest that an interaction between polyanions and SCl channels would allow the channels to be modulated in vivo by inositol polyphosphates.