Inhibition of the intracellular release of calcium by Dantrolene in barnacle giant muscle fibres.

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Abstract
Ca movements in resting and in activated single giant muscle fibers of the barnacle [Balanus nubilus] were analyzed before and after exposure to Dantrolene Na, synthetic hydantoin derivative. In fibers micro-injected with the photoprotein aequorin, the resting rate of light emission (resting glow) reversibly decreased upon exposure to Dantrolene. Similar results were obtained if the fiber had first been equilibrated in a 0 Ca-1 mM-EGTA [ethylene glycol bis (.beta.-amino ethyl ether) tetraacetic acid] medium. The influx of 45Ca into resting muscle fibers was not modified by 35 .mu.M Dantrolene which failed to significantly reduce the influx of 45Ca into muscle fibers depolarized by exposure to external solutions in which K+ had been increased to 60 or 200 mM. In fibers micro-injected with 45Ca, the Ca-efflux was reversibly decreased by Dantrolene. This effect was still observed in 0 Ca medium and in 0 Ca-0 Na medium. A possible effect of Dantrolene on the Na-Ca exchange process at the outer membrane was excluded by showing that when the direction of the Ca2+ movement was inverted in aequorin-loaded fibers by the sudden removal of Na+ from the external medium, a marked increase in the resting glow was recorded which was not affected by exposure to Dantrolene. The reduction of Ca2+ efflux by Dantrolene does not result from any direct inhibitory effect on the metabolically driven Ca pump at the outer membrane, but is rather related to the reduction of the concentration of myoplasmic Ca2+, demonstrated by the reduced resting glow. This in turn may result from a shift in the balance between Ca2+ movements into and out of the intracellular storage sites, and namely the sarcoplasmic reticulum (SR). The Ca2+ transient in aequorin-loaded fibers and the force of the isometric contraction elicited by imposed membrane depolarizations were markedly reduced by Dantrolene. The electrochemical threshold for eliciting intracellular Ca2+ release was not significantly modified. The linear relation between membrane depolarization and Ca2+ transient became less steep. The process of sequestration of myoplasmic Ca2+ back into SR was not significantly affected by Dantrolene which appeared to inhibit rather selectively, Ca2+ release from SR into cytosol.