Poster Presentation 2014 International Biophysics Congress

Retrograde effect of RyR1 on voltage-dependent L-type Ca2+ current inactivation (#476)

Werner Melzer 1 , Zoita Andronache 1
  1. Ulm University, Ulm, Germany

In skeletal muscle cells, dihydropyridine receptors (DHPR) of the transverse tubules (TT) and ryanodine receptors (RyR1) of the sarcoplasmic reticulum (SR) exhibit bidirectional conformational interaction. Orthograde DHPR-RyR1 coupling forms the basis of the voltage-activated Ca2+  release that controls muscle force, whereas retrograde RyR1-DHPR coupling upregulates the DHPR-mediated high voltage-activated L-type Ca2+  inward current. RyR1 appears to also affect DHPR inactivation because muscle cells of malignant hyperthermia-susceptible (MHS) mice expressing mutant RyR1 (Y524S) showed a negative shift in the voltage of half-maximal L-type current availability. Here, we investigated the hypothesis that elevated calcium levels in the TT-SR junctional gap resulting from the hyperactive mutant RyR1 modify DHPR inactivation. We studied voltage-controlled Ca2+  release and Ca2+  inward current in enzymatically isolated adult muscle fibers of WT mice and of heterozygous Y524S+/- mice. Millimolar concentrations of caffeine that favor RyR1 open probability led to a left shift in the voltage-dependent availability curve for L-type current in WT muscle fibers, similar to the effect of the MHS mutation. Thus drug-induced RyR1 hyperactivity mimics the effect of the mutation. However, the difference between WT and Y524S+/- fibers was insensitive to fiber dialysis with an artificial solution containing a high concentration of BAPTA to stabilize the junctional free calcium concentration. Combined, these results suggest that the open RyR1 conformation rather than resulting junctional Ca2+ leads to the observed changes in DHPR inactivation.