L‐type Ca2+ channel and ryanodine receptor cross‐talk in frog skeletal muscle

Abstract

The dihydropyridine receptors (DHPRs)/L-type Ca²⁺ channels of skeletal muscle are coupled with ryanodine receptors/Ca²⁺ release channels (RyRs/CRCs) located in the sarcoplasmic reticulum (SR). The DHPR is the voltage sensor for excitation–contraction (EC) coupling and the charge movement component q_γ has been implicated as the signal linking DHPR voltage sensing to Ca²⁺ release from the coupled RyR. Recently, a new charge component, q_h, has been described and related to L-type Ca²⁺ channel gating. Evidence has also been provided that the coupled RyR/CRC can modulate DHPR functions via a retrograde signal. Our aim was to investigate whether the newly described q_h is also involved in the reciprocal interaction or cross-talk between DHPR/L-type Ca²⁺ channel and RyR/CRC. To this end we interfered with DHPR/L-type Ca²⁺ channel function using nifedipine and 1-alkanols (heptanol and octanol), and with RyR/CRC function using ryanodine and ruthenium red (RR). Intramembrane charge movement (ICM) and L-type Ca²⁺ current (I_Ca) were measured in single cut fibres of the frog using the double-Vaseline-gap technique. Our records showed that nifedipine reduced the amount of q_γ and q_h moved by ∼90% and ∼55%, respectively, whereas 1-alkanols completely abolished them. Ryanodine and RR shifted the transition voltages of q_γ and q_h and of the maximal conductance of I_Ca by ∼4−9 mV towards positive potentials. All these interventions spared q_β. These results support the hypothesis that only q_γ; and q_h arise from the movement of charged particles within the DHPR/L-type Ca²⁺ channel and that these charge components together with I_Ca are affected by a retrograde signal from RyR/CRC.