Sarcoplasmic reticulum Ca2+ release in rat slow- and fast-twitch muscles

The same isoform of ryanodine receptor (RYR1) is expressed in both fast and slow mammalian skeletal muscles. However, differences in contractile activation and calcium release kinetics in intact and skinned fibers have been reported. In this work, intracellular Ca2+ transients were measured in soleus and extensor digitorum longus (EDL) single muscle fibers using mag-fura-2 (KD for Ca2+ = 49 microM) as Ca2+ fluorescent indicator. Fibers were voltage-clamped at Vh = -90 mV and sarcoplasmic reticulum calcium release was measured at the peak (a) and at the end (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, respectively. Ratios (b/a-b) in soleus and EDL fibers were 0.41 +/- 0.05 and 1.01 +/- 0.13 (n = 12), respectively. This result suggested that the proportion of dihydropyridine receptor (DHPR)-linked and unlinked RYRs is different in soleus and EDL muscle. The number of DHPR and RYR were determined by measuring high-affinity [3H]PN200-110 and [3H]ryanodine binding in soleus and EDL rat muscle homogenates. The Bmax values corresponded 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. These data suggest that soleus muscle has a larger calcium-gated calcium release component and a larger proportion of DHPR-unlinked RYRs.