Adrenergic modulation of the K+ contractures in tonic skeletal muscle fibers of the frog

Convergent regulation of skeletal muscle Ca2+ channels by dystrophin, the actin cytoskeleton, and cAMP-dependent protein kinase

The skeletal muscle L-type Ca2+ channel (Ca(V)1.1), which is responsible for initiating muscle contraction, is regulated by phosphorylation by cAMP-dependent protein kinase (PKA) in a voltage-dependent manner that requires direct physical association between the channel and the kinase mediated through A-kinase anchoring proteins (AKAPs). The role of the actin cytoskeleton in channel regulation was investigated in skeletal myocytes cultured from wild-type mice, mdx mice that lack the cytoskeletal linkage protein dystrophin, and a skeletal muscle cell line, 129 CB3. Voltage dependence of channel activation was shifted positively, and potentiation was greatly diminished in mdx myocytes and in 129 CB3 cells treated with the microfilament stabilizer phalloidin. Voltage-dependent potentiation by strong depolarizing prepulses was reduced in mdx myocytes but could be restored by positively shifting the stimulus potentials to compensate for the positive shift in the voltage dependence of gating. Inclusion of PKA in the pipette caused a negative shift in the voltage dependence of activation and restored voltage-dependent potentiation in mdx myocytes. These results show that skeletal muscle Ca2+ channel activity and voltage-dependent potentiation are controlled by PKA and microfilaments in a convergent manner. Regulation of Ca2+ channel activity by hormones and neurotransmitters that use the PKA signal transduction pathway may interact in a critical way with the cytoskeleton and may be impaired by deletion of dystrophin, contributing to abnormal regulation of intracellular calcium concentrations in dystrophic muscle.

Adrenaline diminishes K+ contractures and Ba2+-current in chicken slow skeletal muscle fibres

The effects of adrenaline and the beta-adrenergic agonist isoprenaline on K+-evoked tension (K+-contracture) and Ba2+ current were investigated in chicken slow (anterior latissimus dorsi (ald)) muscle using isometric-tension measurements and current recording. Addition of adrenaline (10(-7) - 10(-5) M) or isoprenaline (10(-6) - 10(-5) M) to the bath reduced the amplitude of the K+-contractures. These effects were blocked by the beta-antagonist propranolol (5 x 10(-6) M). External application of a cAMP analogue (8-bromo cyclic AMP; 1 x 10(-4) M) also decreased the amplitude of the K+-contractures. To analyze the possible relationship between the induced tension reduction and effects on sarcolemmal Ca2+ channels, a slow action potential and a slow inward membrane current were studied in intact ald chicken muscle fibres. When the ald muscle was immersed in a Na+- and Cl--free solution containing Ba2+ and depolarizing pulses were delivered from a -80 mV holding potential, the muscle fibres exhibited a small, slow Ba2+-dependent potential (observed at about -26 mV, peak amplitude, around -10 mV). The response was blocked by the addition of Co2+ (5 mM) or Cd2+ (2 mM). Using the three-microelectrode voltage-clamp technique, a slow inward membrane current underlying the Ba2+ potential could be discerned. The current had a mean threshold of -60 mV, reached maximum at about -5 mV and ranged from ca. 9 to 19 pA/cm2 (depending on the external Ba2+ concentration). It had a mean reversal potential of +45 mV. The Ba2+ inward current was diminished when adrenaline or isoprenaline was added to the bath (1 x 10(-5) M); however, this decrease did not occur when propranolol was present (5 x 10-6 M). These results suggest that the decreases in the tension of K+-contractures induced by adrenaline and isoprenaline may occur through beta-adrenergic effects on sarcolemmal Ca2+ channels in ald chicken slow muscle fibres.

Regulation of mouse skeletal muscle L-type Ca2+ channel by activation of the insulin-like growth factor-1 receptor

We investigated the modulation of the skeletal muscle L-type Ca2+ channel/dihydropyridine receptor in response to insulin-like growth factor-1 receptor (IGF-1R) activation in single extensor digitorum longus muscle fibers from adult C57BL/6 mice. The L-type Ca2+ channel activity in its dual role as a voltage sensor and a selective Ca2+-conducting pore was recorded in voltage-clamp conditions. Peak Ca2+ current amplitude consistently increased after exposure to 20 ng/ml IGF-1 (EC50 = 5.6 +/- 1.8 nM). Peak IGF-1 effect on current amplitude at -20 mV was 210 +/- 18% of the control. Ca2+ current potentiation resulted from a shift in 13 mV of the Ca2+ current-voltage relationship toward more negative potentials. The IGF-1-induced facilitation of the Ca2+ current was not associated with an effect on charge movement amplitude and/or voltage distribution. These phenomena suggest that the L-type Ca2+ channel structures involved in voltage sensing are not involved in the response to the growth factor. The modulatory effect of IGF-1 on L-type Ca2+ channel was blocked by tyrosine kinase and PKC inhibitors, but not by a cAMP-dependent protein kinase inhibitor. IGF-1-dependent phosphorylation of the L-type Ca2+ channel alpha1 subunit was demonstrated by incorporation of [gamma-32P]ATP to monolayers of adult fast-twitch skeletal muscles. IGF-1 induced phosphorylation of a protein at the 165 kDa band, corresponding to the L-type Ca2+ channel alpha1 subunit. These results show that the activation of the IGF-1R facilitates skeletal muscle L-type Ca2+ channel activity via a PKC-dependent phosphorylation mechanism.

Modulation of the cloned skeletal muscle L-type Ca2+ channel by anchored cAMP-dependent protein kinase

Ca2+ influx through skeletal muscle Ca2+ channels and the force of contraction are increased in response to beta-adrenergic stimulation and high-frequency electrical stimulation. These effects are thought to be mediated by cAMP-dependent phosphorylation of the skeletal muscle Ca2+ channel. Modulation of the cloned skeletal muscle Ca2+ channel by cAMP-dependent phosphorylation and by depolarizing prepulses was reconstituted by transient expression in tsA-201 cells and compared to modulation of the native skeletal muscle Ca2+ channel as expressed in mouse 129CB3 skeletal muscle cells. The heterologously expressed Ca2+ channel consisting of alpha1, alpha2delta, and beta subunits gave currents that were similar in time course, current density, and dihydropyridine sensitivity to the native Ca2+ channel. cAMP-dependent protein kinase (PKA) stimulation by Sp-5,6-DCl-cBIMPS (cBIMPS) increased currents through both native and expressed channels two- to fourfold. Tail currents after depolarizations to potentials between -20 and +80 mV increased in amplitude and decayed more slowly as either the duration or potential of the depolarization was increased. The time- and voltage-dependent slowing of channel deactivation required the activity of PKA, because it was enhanced by cBIMPS and reduced or eliminated by the peptide PKA inhibitor PKI (5-24) amide. This voltage-dependent modulation of the cloned skeletal muscle Ca2+ channel by PKA also required anchoring of PKA by A-Kinase Anchoring Proteins because it was blocked by peptide Ht 31, which disrupts such anchoring. The results show that the skeletal muscle Ca2+ channel expressed in heterologous cells is modulated by PKA at rest and during depolarization and that this modulation requires anchored protein kinase, as it does in native skeletal muscle cells.