Markovian models of low and high activity levels of cardiac ryanodine receptors

The modal gating behavior of single sheep cardiac sarcoplasmic reticulum (S R) Ca2+-release/ryanodine receptor (RyR) channels was assessed. We find tha t the gating of RyR channels spontaneously shifts between high (H) and low (L) levels of activity and inactive periods where no channel openings are d etected (l). Moreover, we find that there is evidence for multiple gating m odes within H activity, which we term H1 and H2 mode. Our results demonstra te that the underlying mechanisms regulating gating are similar in native a nd purified channels. Dwell-time distributions of L activity were best fitt ed by three open and five closed significant exponential components whereas dwell-time distributions of H1 activity were best fitted by two to three o pen and four closed significant exponential components. Increases in cytoso lic [Ca2+] cause an increase in open probability (Po) within L activity and an increase in the probability of occurrence of H activity. Open lifetime distributions within L activity were Ca2+ independent whereas open lifetime distributions within H activity were Ca2+ dependent. This study is the fir st attempt to estimate RyR single-channel kinetic parameters from sequences of idealized dwell-times and to develop kinetic models of RyR gating using the criterion of maximum likelihood. We propose distinct kinetic schemes f or L, H1, and H2 activity that describe the major features of sheep cardiac RyR channel gating at these levels of activity.