Ionic requirements for inositol trisphosphate- and thapsigargin-induced Ca2+ release from rat liver endoplasmic reticulum
Abstract
The Ca2+-transport and permeability properties of the endoplasmic reticulum (ER) from rat liver were studied by
manipulating the ionic composition of the external medium. The ATP-dependent Ca2+ loading into the ER vesicles was slow
in the absence of K+ and the amount of Ca2+ absorbed was also limited. However, the presence of gluconate in the medium
greatly enhanced the Ca2+-loading capacity of the vesicles. Ca2+ release from the vesicles was triggered by inositol (1,4,5)
trisphosphate in the presence of GTP or by thapsigargin. While the Ins(1,4,5)P3-induced Ca2+ release was dependent on the
presence of K+ in the external medium, thapsigargin released Ca2+ from the vesicles irrespective of the ionic composition of
the external medium. These findings suggest that a specific ion channel is not involved in the release of Ca2+ from ER
vesicles by thapsigargin.
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