Carbon dioxide or bicarbonate ions release Ca2+ from internal stores in crustacean myofibrillar bundles.
The paper describes an investigation into the increase in intracellular free Ca2+ and resting tension of barnacle muscle fibers when exposed to CO2. Isometric tension was recorded in isolated myofibrillar bundles prepared from barnacles and crabs. On replacement of a low relaxing bathing solution (free Ca2+: 20 nM) at pH 7.1 with a similar one containing 100% CO2 and 13 mM HCO3-, also at pH 7.1, the bundles developed a phasic contraction, which aequorin experiments confirmed was due to a release of Ca2+ from a store within the bundles. The source of this Ca2+ is tentatively identified as the sarcoplasmic reticulum (SR) for the following reasons: (1) prior exposure to 20 mM caffeine depleted this Ca2+ store, (2) procaine (10 mM) inhibited the response, and (3) the extracellular space or "clefts" and the mitochondria could be eliminated as possible sources. An effect of the CO2 + HCO3- on the free Ca2+/Mg2+ ratio in the bathing solution was excluded as a possible mechanism. The diuretic furosemide (1 mM) enhanced the response to CO2 + HCO3-. Both furosemide and SITS (1--10 mM), by themselves, also released Ca2+ in myofibrillar bundles. A scheme is put forward to explain these results: it is suggested that diffusion of dissolved CO2 into the SR produces an acidification of the SR lumen, which modifies either the Ca2+/-ATPase or the Ca2+-induced release process in such a way to release Ca2+.
D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid / Q000494:pharmacology; D000331:Aequorin / Q000378:metabolism; D000818:Animals; D001639:Bicarbonates / Q000494:pharmacology*; D003386:Brachyura / Q000378:metabolism*; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002245:Carbon Dioxide / Q000494:pharmacology*; D005665:Furosemide / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000378:metabolism; D009132:Muscles / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D001468:Thoracica / Q000378:metabolism*
D001639:Bicarbonates; D002245:Carbon Dioxide; D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid; D002110:Caffeine; D000331:Aequorin; D005665:Furosemide; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium