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

Abstract

The dihydropyridine receptors (DHPRs)/L-type Ca 2+ channels of skeletal muscle are coupled with ryanodine receptors/Ca 2+ 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 Ca 2+ release from the coupled RyR. Recently, a new charge component, q h , has been described and related to L-type Ca 2+ 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 Ca 2+ channel and RyR/CRC. To this end we interfered with DHPR/L-type Ca 2+ 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 Ca 2+ 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 Ca 2+ channel and that these charge components together with I Ca are affected by a retrograde signal from RyR/CRC.

Cytoskeleton, L-type Ca2+ and stretch activated channels in injured skeletal muscle

“…These treatments in fact, acting on RyRs, increased the number of L-type Ca 2+ /DHPR receptors uncoupled to ryanodine receptors. Accordingly, blocking the interaction between DHPRs and RyRs by heptanol or octanol depressed in size L-type Ca 2+ current, whereas the block of RYR shifted the Q and Qh charge and ICa voltage-activation towards more positive potential [19,20]. In agreement to these findings, the shift towards more positive potentials of L-type Ca 2+ current that we observed in injured skeletal muscle fibres could indicate an increased number of uncoupled DHPRs with RyR1.…”

“…RyR1 activation can, in turn, act as a signal for the DHPR/L-CaC, with a sort of cross talk between RyR1/CRC and DHPR/LCaC [19,20]. Precisely, there is a retrograde action from RyR1/CRC that, once activated, acts as a signal for DHPR/L-CaC, causing an increase of Q and Qh charge and L-CaC currents (ICa) as demonstrated by the reduced size of these charges and ICa when RyR1/CRC are blocked by ryanodine or rhutenium red [19,20]. Other retrograde signals come from the cytoskeleton expression and activity.…”

Excitation-contraction coupling and mechano-sensitivity in denervated skeletal muscles

“…The effect of ryanodine together with those of nifedipine and 1-alkanols, are a further demonstration that qγ and qh both reside in the DHPR/L-CaC and represent separate independent processes from qβ. Moreover, these results demonstrate that, in the absence of RyR/CRC blockers, the opening of the RyR/CRC facilitates qγ and qh movement as well as the opening of L-CaC by a retrograde signal and, in turn, the ECC process [60,61]. Conversely, it may be supposed that any reduction of RyR1/CRC functionality or expression could affect the ECC.…”

“…So, the total T-tubule Ca 2+ content could be reduced and this could be a factor that enhances the L-CaC inactivation due to T-tubule Ca 2+ depletion. Moreover, the increased number of RyR1 uncoupled with L-CaC may be another factor of the altered ECC since the reduced potentiating action of the retrograde action of RyR1 on L-CaC [4,60,61]. However, the presence of residual functional voltagedependent Na + and L-type Ca 2+ channels may allow FES to activate Na + and L-CaC and in turn the affected EC coupling process.…”

Effects of heptanol and carbenoxolone on noradrenaline induced contractions in guinea pig vas deferens

“…One of the principle non-selective action reported for carbenoxolone and heptanol is their modulatory action on VGCCs (in retina: Vessey et al, 2004; in skeletal muscle: Squecco et al, 2004). To see if either of these compound was affecting VGCCs in GPVD we have compared their effect with those effects of nifedipine, a specific blocker of VGCC involved in the generation of noradrenaline-induced contraction i.e., L-type Ca 2+ channels.…”