A comparison of the abilities of CO2/HCO3-., protonophores and changes in solution pH to release Ca2+ from the SR of barnacle myofibrillar bundles.

Increases in solution pH from 6.5 to 7.0 to 7.5 at 0.1 microM free Ca2+ concentration had no effect on the isometric tension of barnacle myofibrillar bundles in relaxing solutions containing 0.1-0.16 mM BAPTA. Decreases in pH in the same range were also without effect. Under the same conditions CO2-induced Ca2+ release from the SR could be readily obtained by replacing the Cl(-)-containing containing relaxing solution with one containing HCO3- and 100% CO2 at the same pH. At a higher free Ca2+ of 2.5 microns, there was a contraction on increasing the pH of the Cl(-)-containing solution from 7.0 to 7.5. This response could be abolished by 1 mM procaine suggesting that it was due to Ca2+ release from the SR. The protonophores monensin, gramicidin, CCCP and FCCP at concentrations of 10-100 microM had no effect on resting tension at either free Ca2+ concentration and did not inhibit the response to 100% CO2. It is concluded that dissipation of a possible pH gradient across the SR membrane by protonophores does not release Ca2+ from the SR of barnacle muscle. Since both CO2 (by possibly lowering SR pH) and an increase in solution pH can release Ca2+ at 2.5 microM free Ca2+, the existence of a Ca2+ release channel which is opened by a change in the trans-SR pH gradient cannot be discounted.(ABSTRACT TRUNCATED AT 250 WORDS)