Desensitization of the skeletal muscle ryanodine receptor: evidence for heterogeneity of calcium release channels.

Ca release channels from the junctional sarcoplasmic reticulum (SR) membranes of rabbit skeletal muscle were incorporated into the lipid bilayer membrane, and the inactivation kinetics of the channel were studied at large membrane potentials. The channels conducting Cs currents exhibited a characteristic desensitization that is both ligand and voltage dependent: 1) with a test pulse to -100 mV (myoplasmic minus luminal SR), the channel inactivated with a time constant of 3.9 s; 2) the inactivation had an asymmetric voltage dependence; it was only observed at voltages more negative than -80 mV; and 3) repetitive tests to -100 mV usually led to immobilization of the channel, which could be recovered by a conditioning pulse to positive voltages. The apparent desensitization was seen in approximately 50% of the experiments, with both the native Ca release channel (in the absence of ryanodine) and the ryanodine-activated channel (1 microM ryanodine). The native Ca release channels revealed heterogeneous gating with regard to activation by ATP and binding to ryanodine. Most channels had high affinity to ATP activation (average open probability (po) = 0.55, 2 mM ATP, 100 microM Ca), whereas a small portion of channels had low affinity to ATP activation (po = 0.11, 2 mM ATP, 100 microM Ca), and some channels bound ryanodine faster (< 2 min), whereas others bound much slower (> 20 min). The faster ryanodine-binding channels always desensitized at large negative voltages, whereas those that bound slowly did not show apparent desensitization. The heterogeneity of the reconstituted Ca release channels is likely due to the regulatory roles of other junctional SR membrane proteins on the Ca release channel.