A small influx of Ca2+ through voltage-gated Ca2+ channels (VGCC) located in the t-tubules is thought to initiate a much larger release of Ca2+ from the sarcoplasmic reticulum (SR) through ryanodine receptors. The key is that the Ca2+ sensitive ryanodine receptors are located in the SR membrane directly opposite the VGCC in the t-tubule membrane. A small influx of Ca2+ through VGCC is therefore translated into a much larger release of Ca2+ from the SR; this process is called calcium-induced calcium release. There is overwhelming support for this hypothesis, but other mechanisms of inducing calcium release from the SR have been proposed. All of the hypothesized mechanisms depend critically on a unique molecular architecture, but this architecture has seldom been directly examined. The images in this section of the gallery explore possible mechanisms of calcium-induced calcium release in rat ventricular myocytes.

Voltage Gated Ca Channels and Ryanodine Receptors
Voltage gated Ca2+ channels (green) and ryanodine receptors (red) show a large degree of coincidence, as was expected. This is the known path of calcium-induced calcium release in cardiac muscle.  and (small version - 191KB)(large version - 1.43 mb).

Calsequestrin and Ryanodine Receptors
It is hypothesized that these proteins are co-distributed and that a change in the conformation of the ryanodine receptors (red) initiates a change in the conformation of calsequestrin (green). The high degree of co-localisation is therefore expected and like the VGCC-RyR image pairs provides a positive control confirming the validity of the imaging protocols. and (127KB).

Na+/Ca2+ Exchanger and Ryanodine Receptors
Some groups have evidence which indicates that the Na+/Ca2+ exchanger (green) can, in reverse mode operation, bring sufficient amounts of Ca2+ into the cell to activate the ryanodine receptors (red) and initiate a contraction. Images of the distribution of these proteins indicates that there is almost no co-incidence between them and that they occupy different domains in the t-tubule membrane. They are therefore most likely not functionally coupled. and (small version - 239 KB)(large version - 1.95 mb).

Voltage-Gated Na+ Channel and Ryanodine Receptor
Is it possible that under some conditions voltage-gated Na+ channels (green) can conduct sufficient amounts of Ca2+ to activate ryanodine receptors (red)? Unlikely given that there is virtually no co-localisation of the two and they occupy different domains of the t-tubule membrane.  and (136 KB)

Voltage-Gated Na+ Channel and Na+/Ca2+ Exchanger
In smooth muscle cells the Na+/K+ ATPase was located adjacent to the Na+/Ca2+ exchanger, this allows small changes in the activity of either protein to be quickly and effectively translated into a change in the activity of the other protein. It was therefore reasonable to hypothesize that the voltage-gated Na+ channel (green) might be located adjacent to the Na+/Ca2+ exchanger (red) in the t-tubule membrane so that changes in the local Na+ concentration, in the vicinity of the channel, could affect changes in the activity of the Na+/Ca2+ exchanger. Since there is virtually no co-incidence of these proteins, or between either of them with ryanodine receptors, we conclude that the t-tubule membrane has at least three functional domains. One domain contains the Na+/Ca2+ exchanger, another contains the Na+ channel, and another contains voltage-gated Ca2+ channels which are located opposite ryanodine receptors in the SR membrane.
 and (176 KB)

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