L-type Ca2+ channel charge movement and intracellular Ca2+ in skeletal muscle fibers from aging mice

In this work we tested the hypothesis that skeletal muscle fibers from aging mice exhibit a significant decline in myoplasmic Ca2+ concentration resulting from a reduction in L-type Ca2+ channel (dihydropyridine receptor, DHPR) charge movement. Skeletal muscle fibers from the flexor digitorum brevis (FDB) muscle were obtained from 5-7-, 14-18-, or 21-24-month-old FVB mice and voltage-clamped in the whole-cell configuration of the patch-clamp technique according to described procedures (Wang, Z.-M., M. L. Messi, and O. Delbono. 1999. Biophys. J. 77:2709-2716). Total charge movement or the DHPR charge movement was measured simultaneously with intracellular Ca2+ concentration. The maximum charge movement (Q(max)) recorded (mean +/- SEM, in nC mu F-1) was 53 +/- 3.2 (n = 47), 51 +/- 3.2 (n = 35) (non-significant, ns), and 33 +/- 1.9 (n = 32) (p < 0.01), for the three age groups, respectively. Q(max) corresponding to the DHPR was 43 +/- 3.3, 38 +/- 4.1 (ns), and 25 +/- 3.4 (p < 0.01) for the three age groups, respectively. The peak intracellular [Ca2+] recorded at 40 mV (in mu M) was 15.7 +/- 0.12, 16.7 +/- 0.18 (ns), and 8.2 +/- 0.07 (p < 0.01) for the three age groups, respectively. No significant changes in the Voltage distribution or steepness of the Q-V or [Ca2+]-V relationship were found. These data support the concept that the reduction in the peak intracellular [Ca2+] results from a larger number of ryanodine receptors uncoupled to DHPRs in skeletal muscle fibers from aging mammals.