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)
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D000818:Animals; D001639:Bicarbonates / Q000494:pharmacology*; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000032:analysis / Q000378:metabolism* / Q000494:pharmacology; D002245:Carbon Dioxide / Q000494:pharmacology*; D002712:Chlorides / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007476:Ionophores / Q000494:pharmacology*; D008274:Magnesium / Q000032:analysis; D009119:Muscle Contraction / Q000187:drug effects; D009210:Myofibrils / Q000378:metabolism*; D011343:Procaine / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D001468:Thoracica
D001639:Bicarbonates; D002712:Chlorides; D007476:Ionophores; D002245:Carbon Dioxide; D002110:Caffeine; D011343:Procaine; D008274:Magnesium; D002118:Calcium