Carbon dioxide or bicarbonate ions release Ca2+ from internal stores in crustacean myofibrillar bundles

Abstract

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⁺∶20nm) at pH 7.1 with a similar one containing 100% CO₂ and 130mm HCO ₃ ⁻ , 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 20mm caffeine depleted this Ca²⁺ store, (2) procaine (10mm) inhibited the response, and (3) the extracellular space or “clefts” and the mitochondria could be eliminated as possible sources. An effect of the CO₂+HCO ₃ ^t- on the free Ca²⁺/Mg²⁺ ratio in the bathing solution was excluded as a possible mechanism. The diuretic furosemide (1mm) enhanced the response to CO₂+HCO ₃ ^t- . Both furosemide and SITS (1–10mm), 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⁺.