STREAMING POTENTIALS REVEAL A SHORT RYANODINE-SENSITIVE SELECTIVITY FILTER IN CARDIAC CA2+ RELEASE CHANNEL

Single cardiac sarcoplasmic reticulum Ca2+ release channels were recon stituted into planar bilayer membranes. Streaming potentials were meas ured in osmotically asymmetric solutions as a shift in the reversal po tential. Potential changes induced by water movement through the bilay er (concentration polarization) and reduced ion activity in the concen trated nonelectrolyte solutions were determined using valinomycin. In 400 mM symmetrical CsCH3SO3, the average streaming potential was 2.74 +/- 0.2 mV (n = 5, mean +/- SE; 2 osmol/kg) and independent of the osm oticant used (sucrose or diglycine). Identical streaming potential mag nitudes were obtained regardless of which side of the membrane the non electrolyte was placed. This suggests that the narrow part of the pore where single file diffusion occurs is relatively short (i.e., accommo dates a minimum of 3 H2O molecules). This value is comparable to simil ar measurements in a variety of surface membrane channels. Ryanodine-m odified channels had no measurable streaming potential, an increased T ris(+) permeability relative to Cs+, and decreased divalent selectivit y (PCs/PTris 5.1 +/- 1.1 to 1.7 +/- 0.3, n = 3; PBa/PCs 8.2 +/- 0.7 to 1.8 +/- 0.5, n = 4). Cation/anion selectivity was essentially unalter ed in ryanodine-modified channels. These results suggests that the nar row region of the permeation pathway (i.e., the selectivity filter) is relatively short and widens after ryanodine modification.