Effect of luminal calcium on Ca2+ release channel activity of sarcoplasmic reticulum in situ.
Ca2+ influx into empty SR in the absence of Ca2+ pump activity was determined in skinned frog skeletal muscle fibers and compared with Ca2+ efflux from loaded SR (i.e., Ca2+ release) to deepen our understanding of the properties of the Ca2+ release channel (CRC). Calcium content in SR increased approximately in a first-order kinetics and finally reached the equilibrium level determined by cytoplasmic Ca2+ ([Ca2+]c). Because AMP caused an increase in the rate of Ca2+ influx, and procaine, Mg2+, and high concentrations of Ca2+ caused a characteristic decrease, the major Ca2+ influx pathway was concluded to be the CRC, as is true of Ca2+ release. The apparent rate constant (k(app)) of Ca2+ efflux did not significantly change when the loading level was decreased to one-third. At a given [Ca2+]c, the same equilibrium level of calcium in SR was attained with a similar k(app) by both Ca2+ influx and Ca2+ efflux. The relationship between [Ca2+]c and calcium in SR indicated the Ca2+ binding sites in SR. These results, together with the anticipated effects of these Ca2+ buffer sites on kinetics, are consistent with the idea that luminal Ca2+ inhibits the CRC.
Kurebayashi
N
N
Department of Pharmacology, Juntendo University School of Medicine, Tokyo, Japan. nagomik@med.juntendo.ac.jp
Ogawa
Y
Y
D000249:Adenosine Monophosphate / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D005456:Fluorescent Dyes; D016257:Fura-2; D066298:In Vitro Techniques; D017136:Ion Transport / Q000187:drug effects; D007700:Kinetics; D008954:Models, Biological; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism; D011898:Ranidae; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*
D015220:Calcium Channels; D005456:Fluorescent Dyes; D019837:Ryanodine Receptor Calcium Release Channel; D000249:Adenosine Monophosphate; D002118:Calcium; D016257:Fura-2