Caffeine and micromolar Ca2+ concentrations can release Ca2+ from ryanodine-sensitive stores in crab and lobster striated muscle fibres
Ca2+ release mechanisms were studied in striated muscle from the walking legs of crabs using isometric tension recordings from isolated myofibrillar bundles. Caffeine-induced phasic contractions had properties consistent with Ca2+ release from a sarcoplasmic store, which could be optimally loaded in the presence of ATP at pCa 6.4&shy;6.1. Ryanodine (10 &micro;mol l-1) abolished the caffeine-induced contractions and in solutions with low Ca2+ buffering (0.1 mmol l-1 EGTA) itself caused phasic contractions, indicative of Ca2+ release. Ca2+-induced Ca2+ release (CICR) was observed in a pCa 5.8 solution (buffered by 1 mmol l-1 EGTA) as a phasic contraction of variable nature, inhibited by ryanodine (10 &micro;mol l-1), procaine (10 mmol l-1) or benzocaine (5 mmol l-1). Ca2+ release was measured as a function of releasing pCa by using the force&shy;time integral of the caffeine-induced contraction as an estimate of the Ca2+ remaining in the store. After the Ca2+ store had been loaded for 2 min at pCa 6.6, CICR was measured in the presence of 1 mmol l-1 Mg2+, 1 mmol l-1 EGTA and 5 mmol l-1 ATP. The threshold pCa for CICR was 6.0&shy;6.4 under these conditions and more than 90 % of stored Ca2+ was released in 1 min by pCa values in the range 3.5&shy;5.3. Benzocaine totally inhibited the release and promoted extra Ca2+ loading. Preliminary experiments showed a similar caffeine-releasable store in lobster abdominal muscle, which was slightly less sensitive to free [Ca2+]. It is concluded that in crustacean muscle caffeine and micromolar [Ca2+] can release Ca2+ from a ryanodine-sensitive store, which in many respects is similar to the sarcoplasmic reticulum of vertebrate skeletal and cardiac muscle.
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