The rapid cooling (RC) response in muscle is an increase in cytoplasmic Ca2+ 
concentration ([Ca2+]i) that is probably caused by Ca2+ release from the 
sarcoplasmic reticulum (SR). However, the molecular bases of this response have 
not been completely elucidated. Three different isoforms of the SR Ca2+ release 
channels, or ryanodine receptors (RyRs), have been isolated (RyR1, RyR2, and 
RyR3). In the current investigation, the RC response was studied in RyR-null 
muscle cells (1B5) before and after transduction with HSV-1 virions containing 
the cDNAs encoding for RyR1, RyR2, or RyR3. Cells were loaded with fluo 4-AM to 
monitor changes in [Ca2+]i and perfused with either cold ( approximately 0 
degrees C), room temperature (RT), or RT buffer containing 40 mM caffeine. 
Control cells showed no significant response to cold or caffeine, whereas robust 
Ca2+ transients were recorded in response to both RC and caffeine in transduced 
cells expressing any one of the three RyR isoforms. Our data demonstrate 
directly that RyRs are responsible for the RC response and that all three 
isoforms respond in a similar manner. Ca2+ release from RyRs is likely caused by 
a RC-induced conformational change of the channel from the closed to the open 
state.