Modulation of ryanodine receptors (RyRs) by Ca2+. (A) Ensemble currents of single RyR2 show the effect of fast Ca2+ pulses on the Po of a non-phosphorylated channel ('control') or the same channel after phosphorylation with the catalytic subunit of protein kinase A ('+PKA'). Laser photolysis of 'caged Ca2+' (o-nitrophenyl EGTA, tetrapotassium salt; NP-EGTA) increased [Ca2+] from 0.1 to 10 μmol/l at the point labeled as 'Ca2+ pulse' (modified from . (B) Ca2+-Po curves of RyR1 and RyR2. Channel activity was measured at the indicated stationary concentrations of Ca2+ ('RyR1' and 'RyR2 steady-state') and right after a fast Ca2+ pulse, as in (A) ('RyR2 (peak Po)') (unpublished results) (see text for details).