SARCOPLASMIC-RETICULUM CA2-TWITCH AND FAST-TWITCH MUSCLES( RELEASE INRAT SLOW)

The same isoform of ryanodine receptor (RYR1) is expressed in both fas t and slow mammalian skeletal muscles. However, differences in contrac tile activation and calcium release kinetics in intact and skinned fib ers have been reported. In this work, intracellular Ca2+ transients we re measured in soleus and extensor digitorum longus (EDL) single muscl e fibers using mag-fura-2 (K-D for Ca2+ = 49 mu M) as Ca2+ fluorescent indicator. Fibers were voltage-clamped at V-h = -90 mV and sarcoplasm ic reticulum calcium release was measured at the peak (a) and at the e nd (b) of 200 msec pulses at +10 mV. Values of a-b and b were assumed to correspond to Ca2+-gated and voltage-gated Ca2+ release, respective ly. Ratios (b/a-b) in soleus and EDL fibers were 0.41 +/- 0.05 and 1.0 1 +/- 0.13 (n = 12), respectively. This result suggested that the prop ortion of dihydropyridine receptor (DHPR)-linked and unlinked RYRs is different in soleus and EDL muscle. The number of DHPR and RYR were de termined by measuring high-affinity [H-3]PN200-110 and [H-3]ryanodine binding in soleus and EDL rat muscle homogenates. The B-max values cor responded to a PN200-110/ryanodine binding ratio of 0.34 +/- 0.05 and 0.92 +/- 0.11 for soleus and EDL muscles (n = 4-8), respectively. Thes e data suggest that soleus muscle has a larger calcium-gated calcium r elease component and a larger proportion of DHPR-unlinked RYRs.