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 (Po) 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 microM-Ca2+. 3. Channel activation mediated by IP3 could be elicited from free Ca2+ levels similar to those of resting skeletal muscle (about 0.1 microM) 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 microM-IP3 depended on the Ca2+ concentration on the cis side. Po 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.
Suárez-Isla
B A
BA
Departamento de Fisiología y Biofísica, Facultad de Medicina, Universidad de Chile.
Alcayaga
C
C
Marengo
J J
JJ
Bull
R
R
D000818:Animals; D001001:Anura; D001464:Barium / Q000494:pharmacology; D001665:Binding Sites; D002118:Calcium / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D015640:Ion Channel Gating / Q000187:drug effects*; D007700:Kinetics; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
D015220:Calcium Channels; D001464:Barium; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium