Ca2+ release from the sarcoplasmic reticulum of barnacle myofibrillar bundles initiated by photolysis of caged Ca2+.

1. Ca2(+)‐induced Ca2+ release (CICR) from the sarcoplasmic reticulum was measured by isometric tension recording from barnacle myofibrillar bundles. Laser‐induced photolysis of the caged calcium molecule, nitr‐5, was used to generate a rapid jump in free Ca2+ (within 1 ms) at the site of the sarcoplasmic reticulum, thus overcoming delays due to Ca2+ diffusion from the bathing solution. 2. The method consisted of equilibrating a myofibrillar bundle (100 micrograms diameter) in a solution containing 0.1 mM‐nitr‐5 (initial pCa 6.8‐6.6) and then exposing it to a UV laser pulse. The resulting phasic contraction had an amplitude of up to 100% maximum tension (P0) in some preparations and a mean half‐time for the rise of tension of 2.3 s at 12 degrees C. Longer half‐times were obtained at low pulse energies. 3. Pre‐treatment of the myofibrillar bundles with ryanodine (10(‐4) M) or the detergent Triton X‐100 abolished a large part of the phasic contraction, confirming its dependence on SR Ca2+ release. The small tonic response which remained had a shorter rise half‐time than the Ca2(+)‐induced Ca2+ release response and was attributed to direct activation of the myofibrils by Ca2+ released from the nitr‐5. 4. The size of the photolytic Ca2+ jump was estimated from the amplitude of the fast tension component. By increasing the laser pulse energy or the initial Ca2+ loading of the nitr‐5, the post‐photolysis pCa was varied from 6.7 to 6.0; the CICR response increased in size over this pCa range. 5. Direct activation of Triton‐treated myofibrils by photolysis of 2.0 mM‐nitr‐5 (initial pCa 6.4) gave contractions of up to 100% P0 and a mean rise half‐time of 164 ms at 12 degrees C (n = 9 for contractions greater than 40% P0). Both the amplitude and the rate of these contractions were dependent on the laser pulse energy. 6. The Ca2(+)‐induced Ca2+ release responses obtained with nitr‐5 photolysis were significantly slower than the fastest rate of tetanus development which has been recorded from intact fibres of barnacle muscle (mean half‐time = 177 ms at 12 degrees C). This could mean that either Ca2(+)‐induced Ca2+ release is less efficient in isolated myofibrillar bundles than in intact fibres or that Ca2(+)‐induced Ca2+ release is not the primary Ca2+ releasing mechanism in excitation‐contraction coupling in barnacle muscle.