Calcium release from the sarcoplasmic reticulum

Calcium ion regulation in barnacle muscle fibers and its relation to force development

It is possible to use the photoproteins aequorin and obelin within the slowly responding striated muscle films to provide both qualitative and quantitive information as to the nature of the calcium-dependent reactions for force development. It seems that these reactions are not in equilibrium with the Ca2+ during a transient response. In addition, these experiments suggest that kinetically at least two calciums are required per functional unit for force development and that this active site is competed for by Mg2+, H+, and K+ ions.

Voltage-gated cation conductance channel from fragmented sarcoplasmic reticulum: steady-state electrical properties

The interaction of fragmented sarcoplasmic reticulum (SR) with an artificial planar phospholipid membrane under conditions known to induce fusion of phospholipid vesicles raises the conductance of the planar bilayer by several orders of magnitude. Measurements of steady-state electrical properties of bilayers thus modified by SR show that two types of conductance pathways are present. One is a voltage-independent pathway which may be somewhat anion-selective. The other is a voltage-gated ionophore showing selectivity to small monovalent cations. This latter ionophore is fully oriented within the artificial bilayer and is inhibited asymmetrically by divalent cations. It is also inhibited below pH6. The ionophore displays single-channel conductance fluctuations between two states. "open" and "closed", with an open-state conductance of 1.4 x 10-10 mho in 0.1 M K+. The physiological function of this ionophore is unknown.

A free calcium wave traverses the activating egg of the medaka, Oryzias latipes

Aequorin-injected eggs of the medaka (a fresh water fish) show an explosive rise in free calcium during fertilization, which is followed by a slow return to the resting level. Image intensification techniques now show a spreading wave of high free calcium during fertilization. The wave starts at the animal pole (where the sperm enters) and then traverses the egg as a shallow, roughly 20 degrees-wide band which vanishes at the antipode some minutes later. The peak free calcium concentration within this moving band is estimated to be about 30 microM (perhaps 100-1,000 times the resting level). Eggs activated by ionophore A23187 may show multiple initiation sites. The resulting multiple waves never spread through each other; rather, they fuse upon meeting so as to form spreading waves of compound origin. The fertilization wave is nearly independent of extracellular calcium because it is only slightly slowed (by perhaps 15%) in a medium containing 5 mM ethylene glycol-bis[beta-aminoethyl ether]N,N'-tetraacetic acid (EGTA) and no deliberately added calcium. It is also independent of the large cortical vesicles, which may be centrifugally displaced. Normally, however, it distinctly precedes the well-known wave of cortical vesicle exocytosis. We conclude that the fertilization wave in the medaka egg is propagated by calcium-stimulated calcium release, primarily from some internal sources other than the large cortical vesicles. A comparison of the characteristics of the exocytotic wave in the medaka with that in other eggs, particularly in echinoderm eggs, suggests that such a propagated calcium wave is a general feature of egg activation.