Functional properties of the native type 3 ryanodine receptor Ca2+-releasechannel from canine diaphragm

mRNA and protein analyses have previously shown that the diaphragm expresse s two ryanodine receptor isoforms: RyR1 and RyR3.RyR1 is the main Ca2+-rele asing pathway in this muscle type. We now report the conducting, gating, an d immunological properties of the native and purified forms of the less abu ndant RyR3 channel. The conductance of this native Ca2+-release channel was 330 pS in 50 mM/250 mM trans/cis CsCH3SO3. It was activated by Ca2+ concen trations of 1-1000 muM, and did not inactivate at mM concentrations of Ca2. Both isoforms were purified by either a sucrose density gradient or immun oprecipitation as > 450 kDa proteins on SDS-PAGE. Western blot analysis con firmed the presence of RyR1 and RyR3, which displayed conductances of 740 /- 30 and 800 +/- 25 pS, respectively, in 250 mM KCl. We thus provide evide nce that one form of the diaphragm SR Ca2+-release channels may be classifi ed as RyR3, with gating properties different from those of the well-charact erized RyR1 and RyR2 isoforms.