Calsequestrin is a calcium-binding protein known to sequester calcium 
accumulated in the sarcoplasmic reticulum (SR) of muscle cells during 
relaxation. In the present study, we used affinity-purified antibodies to 
chicken cardiac calsequestrin to identify a 60,000-Da calsequestrin in frog 
myocardium. Like previously identified cardiac calsequestrins, it is enriched in 
cardiac microsomes, it is enriched by biochemical procedures previously used to 
purify cardiac and skeletal calsequestrins, and it exhibits a pH-dependent shift 
in its apparent Mr on a two-dimensional gel system. Finally, the NH2-terminal 
amino acid sequence of this 60,000-Da immunoreactive protein purified by fast 
protein liquid chromatography was identical to that of rabbit skeletal and 
canine cardiac calsequestrin. Thus, we conclude that this protein corresponds to 
the calsequestrin isoform in frog ventricular muscle. Frog calsequestrin was 
localized in discrete foci present at the periphery but absent from the central 
regions of frog ventricular myocytes as determined by immunofluorescence 
labeling. Immunoelectron microscopic labeling demonstrated that calsequestrin 
was confined to the lumen of two structurally distinct regions of the SR, where 
it was localized in the subsarcolemmal region of the myofibers. One of these 
appeared to correspond to the terminal SR previously reported to be closely 
apposed to the sarcolemma of frog myofibers. The other region, although close to 
the sarcolemma, was not physically joined to it and appeared to correspond to 
corbular SR. It generally is believed that frog cardiac SR does not provide 
activator Ca2+ required for excitation-contraction coupling. However, the 
identification of a calsequestrin isoform very similar to mammalian cardiac 
calsequestrin that is confined to specialized regions of frog cardiac SR lends 
support to the idea that frog cardiac SR has the ability to store Ca2+ and thus 
function in some capacity in frog cardiac muscle contraction.