LOCAL CALCIUM-RELEASE IN MAMMALIAN SKELETAL-MUSCLE

1. Fluo-5 fluorescence associated with Ca2+ release was recorded with confocal microscopy in single muscle fibres mechanically dissected fro m fast twitch muscle of rats or frogs, voltage clamped in a two Vaseli ne-gap chamber. 2. Interventions that elicited Ca2+ sparks in frog ske letal muscle (low voltage depolarizations, application of caffeine) ge nerated in rat fibres images consistent with substantial release from triadic regions, but devoid of resolvable discrete events. Ca2+ sparks were never observed in adult rat fibres. In contrast, sparks of stand ard morphology were abundant in myotubes from embryonic mice. 3. Depol arization-induced gradients of fluorescence between triadic and surrou nding regions (which are proportional to Ca2+ release flux) peaked at about 20 ms and then decayed to a steady level. Gradients were greater in frog fibres than in rat fibres. The ratio of peak over steady grad ient (R) was steeply voltage dependent in frogs, reaching a maximum of 4.8 at -50 mV (n = 7). In rats, R had an essentially voltage-independ ent value of 2.3 (n = 5). 4. Ca2+-induced Ca2+ release, resulting in c oncerted opening of several release channels, is thought to underlie C a2+ sparks and the peak phase of release in frog skeletal muscle. A di ffuse 'small event' release, similar to that observed in these rats, i s also present in frogs and believed to be directly activated by volta ge. The present results suggest that in these rat fibres there is litt le contribution by CICR to Ca2+ release triggered by depolarization, a nd a lack of concerted channel opening.