Dihydropyridine-sensitive ion currents and charge movement in vesicles derived from frog skeletal muscle plasma membranes

Ciliary neurotrophic factor promotes inactivation of muscle Ca2+ channels via PKC

The actions of the ciliary neurotrophic factor (CNTF) were assessed on adult mouse skeletal muscle L-type Ca2+ currents and on Ca2+ release from sarcoplasmic reticulum. Currents were measured with the whole cell patch clamp technique. Ca2+ signals in response to single action potentials were recorded with Fluo3-AM. CNTF (20 ng/ml) reversibly reduced the amplitude of Ca2+ channel currents by 50% within 15 min. In addition, CNTF greatly increased the rate of inactivation during depolarizing pulses and shifted the steady state inactivation curve by -12 mV. The effects of CNTF were mimicked by the PKC activator PMA and prevented by the PKC-inhibitor chelerythrine. In contrast to the effects on the Ca2+ conductance, charge movement and Ca2+ signals remained unaffected by CNTF. These results suggest that CNTF can rapidly decrease muscle Ca2+ channel currents by promoting inactivation, probably through an intracellular PKC-dependent mechanism.

AKAP79 increases the functional expression of skeletal muscle Ca2+ channels in Xenopus oocytes

The actions of the kinase A anchoring protein, AKAP79, a key element in the regulation of the cardiac L-type Ca2+ channel, were assessed on skeletal muscle Ca2+ channels expressed in Xenopus oocytes. The channels were reconstituted by expressing the pore forming alpha1s subunit and its accessory subunits, alpha2-delta, beta, and gamma. We report, for the first time, that peak Ca2+ channel currents are greatly increased (3.5-fold) by AKAP79 when co-expressed with the truncated form of the alpha1s subunit. Immunoblots revealed that the increase in current amplitude is not accompanied by a corresponding increase in the membrane levels of the alpha1s subunit. This suggests that AKAP79 does not increase the trafficking of the channel. In addition, we show that the transcript of AKAP150, the rat ortholog of the human AKAP79, is expressed in rat skeletal muscle and propose that AKAP79/150 modulates Ca2+ channel function.

Ca2+ current and charge movements in skeletal myotubes promoted by the beta-subunit of the dihydropyridine receptor in the absence of ryanodine receptor type 1

The beta-subunit of the dihydropyridine receptor (DHPR) enhances the Ca(2+) channel and voltage-sensing functions of the DHPR. In skeletal myotubes, there is additional modulation of DHPR functions imposed by the presence of ryanodine receptor type-1 (RyR1). Here, we examined the participation of the beta-subunit in the expression of L-type Ca(2+) current and charge movements in RyR1 knock-out (KO), beta1 KO, and double beta1/RyR1 KO myotubes generated by mating heterozygous beta1 KO and RyR1 KO mice. Primary myotube cultures of each genotype were transfected with various beta-isoforms and then whole-cell voltage-clamped for measurements of Ca(2+) and gating currents. Overexpression of the endogenous skeletal beta1a isoform resulted in a low-density Ca(2+) current either in RyR1 KO (36 +/- 9 pS/pF) or in beta1/RyR1 KO (34 +/- 7 pS/pF) myotubes. However, the heterologous beta2a variant with a double cysteine motif in the N-terminus (C3, C4), recovered a Ca(2+) current that was entirely wild-type in density in RyR1 KO (195 +/- 16 pS/pF) and was significantly enhanced in double beta1/RyR1 KO (115 +/- 18 pS/pF) myotubes. Other variants tested from the four beta gene families (beta1a, beta1b, beta1c, beta3, and beta4) were unable to enhance Ca(2+) current expression in RyR1 KO myotubes. In contrast, intramembrane charge movements in beta2a-expressing beta1a/RyR1 KO myotubes were significantly lower than in beta1a-expressing beta1a/RyR1 KO myotubes, and the same tendency was observed in the RyR1 KO myotube. Thus, beta2a had a preferential ability to recover Ca(2+) current, whereas beta1a had a preferential ability to rescue charge movements. Elimination of the double cysteine motif (beta2a C3,4S) eliminated the RyR1-independent Ca(2+) current expression. Furthermore, Ca(2+) current enhancement was observed with a beta2a variant lacking the double cysteine motif and fused to the surface membrane glycoprotein CD8. Thus, tethering the beta2a variant to the myotube surface activated the DHPR Ca(2+) current and bypassed the requirement for RyR1. The data suggest that the Ca(2+) current expressed by the native skeletal DHPR complex has an inherently low density due to inhibitory interactions within the DHPR and that the beta1a-subunit is critically involved in process.

The beta1a subunit regulates the functional properties of adult frog and mouse L-type Ca2+ channels of skeletal muscle

The beta(1a) subunit, one of the auxiliary subunits of Ca(V)1.1 channels, was expressed in COS-1 cells, purified by electroelution and electrodialysis techniques and identified by Western blot using monoclonal antibodies. The purified beta(1a) subunit strongly interacted in vitro with the alpha interaction domain (AID) of Ca(V)1.1 channels. The actions of the purified beta(1a) subunit on Ca(V)1.1 channel currents were assessed in whole cell voltage clamp experiments performed in vesicles derived from frog and mouse adult skeletal muscle plasma membranes. L-type inward currents were recorded in solutions containing Ba(2+) (I(Ba)). Values of peak I(Ba) were doubled by the beta(1a) subunit in frog and mouse muscle vesicles and the amplitude of the slow component of tail currents was greatly increased. The actions of the beta(1a) subunit on Ca(V)1.1 channel currents reached a steady state within 20 min. The beta(1a) subunit had no effect on the time courses of activation or inactivation of I(Ba) or shifted the current-voltage relation. Non-linear capacitive currents were recorded in solutions that contained mostly impermeant ions. Charge movement depended on voltage with average Boltzmann parameters: Q(max) = 28.0 +/- 6.6 nC microF(-1), V = -58.0 +/- 2.0 mV and k = 15.3 +/- 1.1 mV (n = 24). In the presence of the beta(1a) subunit, these parameters remained unchanged: Q(max) = 29.8 +/- 3.5 nC microF(-1), V = -54.5 +/- 2.2 mV and k = 16.4 +/- 1.3 mV (n = 21). Overall, the work describes a novel preparation to explore in situ the role of the beta(1a) subunit on the function of adult Ca(V)1.1 channels.

Inactivation of I(A) channels of frog skeletal muscle is modulated by ATP

Whole cell, voltage clamp experiments were performed in vesicles derived from frog skeletal muscle plasma membranes to characterize the influence of ATP on the kinetic properties of fast inactivating K(+) currents (I(A)). I(A) was recorded in ATP-free solutions. Peak I(A) decayed with a time constant of 27 ms at large depolarizations. Steady state inactivation reached half maximal values at -66 mV. In the presence of ATP, these values were 196 ms and -41 mV, respectively, indicating a major effect of ATP on inactivation. In contrast, activation of I(A) was unaffected by ATP. The protein kinase C (PKC) inhibitors, H7 and staurosporine, greatly prevented the effects of ATP on inactivation. Inactivation remained unchanged by the protein kinase A inhibitor HA1004 or by the catalytic subunit of cAMP protein kinase. We conclude that ATP decreases inactivation of skeletal muscle I(A) and that this effect may be mediated by protein kinase C.