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-inos itol 1,3,4,5-tetrakisphospate (IP4) on small chloride (SCl) channels f rom rabbit skeletal muscle sarcoplasmic reticulum are reported. We fin d that the inositol polyphosphates (6-40 mu M) are potent reversible b lockers of SCl channels in lipid bilayers at -40 mV with > 10(-5) M ci s (cytoplasmic) Ca2+ when added to the cis, but not trans, chamber. IP 3 or IP4 at 20 mu M reduced the mean open time from 89 +/- 16 msec to 11 +/- 2 msec or to 8.0 +/- 1.0 msec respectively, by abolishing the l ongest time constant component in the open time distribution. Neither IP3 nor IP4 altered the six single-channel conductance levels. The fra ction of low conductance events increased similar to 4-fold and the dw ell time at the lower conductance levels increased similar to 3-fold. Channel gating was altered so that most transitions were between the c losed level and an open level, in contrast to control channels which r emained open for long periods with many transitions between the six op en levels. The actions of the inositol polyphosphates were: (1) not pr evented by 20 mu g/ml cis heparin (an IP3 receptor blocker); (2) mimic ked by 10 mu M 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 mu g/ml each) and vanadate (500 mu M). The results suggest that an interaction between polyanions and SCl channels would allow the channels to be modulated in vivo by inosi tol polyphosphates.