The cation selectivity of the sarcoball Ca2+ channel in frog muscle fibres

We have measured single-channel currents from sarcoplasmic reticulum (SR) blebs (sarcoballs) of frog skeletal muscle fibres using conventional patch-clamp electrodes with excised patches. With both the pipette and bath solutions containing 50 mM Ca(gluconate)2 the slope conductance of the single channels was 39.2 pS for the most commonly seen state, with a reversal potential of -0.4 mV. The cation selectivity of this channel was investigated by replacing the bathing solution with either gluconate or HEPES salts of selected cations. The Goldman permeability ratios, calculated from the reversal potentials, were found to be P(Ca2+)/P(K+)=2.4, P(Ca2+)/ P(Na+)=2.7, P(Ca2+)/P(Tris+)=3.1, P(Ca2+)/P(Mg2+)=1.0 and P(Ca2+)/P(Ba2+)=1.1. Each value for the monovalent ions was found to be less than the corresponding value reported for the SR ryanodine receptor channel from skeletal and cardiac muscle. Single-channel activity could be recorded when the preparation was bathed in symmetrical 50 mM Mg(gluconate)2 solutions, and these channels had a similar conductance and open probability to that measured when the preparation was bathed in symmetrical Ca(gluconate)2 solution. The channel activity in symmetrical 50 mM Ca(gluconate)2 solution was insensitive to bath-applied caffeine (5 mM) and ryanodine (10 microM). The results are in agreement with the conclusion that the sarcoball Ca2+ channel is not the ryanodine receptor release channel, but possibly a form of the SR Ca2+-ATPase which is uncoupled from the catalytic events of the pump and acts as a passive ion channel.