ACTIVATION OF INOSITOL TRISPHOSPHATE-SENSITIVE CA2+ CHANNELS OF SARCOPLASMIC-RETICULUM FROM FROG SKELETAL-MUSCLE

1. The modulation by Ca2+ of the activation by inositol 1,4,5-trisphosphate (IP3) of Ca2+ channels present in native sarcoplasmic reticulum membranes from frog skeletal muscle was studied after channel incorporation into planar phospholipid bilayers in the presence of Ca2+ or Ba2+ as current carrier species. 2. Channel activity expressed as fractional open time (P(o)) was low (less-than-or-equal-to 0.15) in the presence of varying free Ca2+ concentrations bathing the myoplasmic face of the channel (cis side), and did not increase significantly between 0.01 and 30-mu-M-Ca2+. 3. Channel activation mediated by IP3 could be elicited from free Ca2+ levels similar to those of resting skeletal muscle (about 0.1-mu-M) and was found to be strongly regulated by the free Ca2+ concentration present at the myoplasmic moiety of the channel. 4. Channel activation by 10-mu-M-IP3 depended on the Ca2+ concentration on the cis side. P(o) reached a maximum between pCa 7.0 and 6.0, but decreased at higher concentrations of free Ca2+. Thus, Ca2+ exerted a modulatory influence on IP3-mediated activation in a concentration range where the channel was insensitive to Ca2+. 5. The results indicate that Ca2+ ions act as modulators of IP3 efficacy to open the channel. This could arise from an interaction of Ca2+ with the channel gating mechanism or with the agonist binding site.