Prof. Michael Kotlikoff and his collaborators have created mice with calcium sensing GFP in their heart smooth muscle. The two videos on this page show the calcium levels increasing when the heart muscles contract and decreasing when they relax. The one on the left was shot with a digital camcorder (cost ~$250) and the one on the right with a CMOS (complimentary metal-oxide semiconductor) camera ( cost ~150,000).
Besides creating a variety of genetically modified mice expressing fluorescing calcium sensors in different tissues, the Kotlikoff lab has also determined the structural basis for the action of the calcium sensing fluorescent protein known as GCaMP2. In the bright calcium bound state, the calmodulin stabilizes the chromophore in an ionized state similar to that observed in enhanced GFP.
Crystal structure of GCaMP2 with calcium bound to it.