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Reduced Ca2+ current, charge movement, and absence of Ca2+ transients in skeletal muscle deficient in dihydropyridine
C Strube1, M Beurg, P A Powers
1Department of Physiology, University of Wisconsin School of Medicine, Madison 53706, USA.
Abstract:
The Ca2+ currents, charge movements, and intracellular Ca2+ transients in mouse skeletal muscle cells homozygous for a null mutation in the cchb1 gene encoding the beta 1 subunit of the dihydropyridine receptor have been characterized. I beta null, the L-type Ca2+ current of mutant cells, had a approximately 13-fold lower density than the L-type current of normal cells (0.41 +/- 0.042 pA/pF at + 20 mV, compared with 5.2 +/- 0.38 pA/pF in normal cells). I beta null was sensitive to dihydropyridines and had faster kinetics of activation and slower kinetics of inactivation than the L-type current of normal cells. Charge movement was reduced approximately 2.8-fold, with Qmax = 6.9 +/- 0.61 and Qmax = 2.5 +/- 0.2 nC/microF in normal and mutant cells, respectively. Approximately 40% of Qmax was nifedipine sensitive in both groups. In contrast to normal cells, Ca2+ transients could not be detected in mutant cells at any test potential; however, caffeine induced a robust Ca2+ transient. In homogenates of mutant muscle, the maximum density of [3H]PN200-110 binding sites (Bmax) was reduced approximately 3.9-fold. The results suggest that the excitation-contraction uncoupling of beta 1-null skeletal muscle involves a failure of the transduction mechanism that is due to either a reduced amount of alpha 1S subunits in the membrane or the specific absence of beta 1 from the voltage-sensor complex.
Insights
Mice lacking the beta 1 subunit of the dihydropyridine receptor show significantly reduced L-type calcium currents and impaired excitation-contraction coupling in skeletal muscle. This suggests the beta 1 subunit is crucial for normal muscle function.
Area of Science:
- Muscle physiology
- Molecular biology
- Ion channel function
Background:
- Dihydropyridine receptors are critical for skeletal muscle excitation-contraction coupling.
- The beta 1 subunit's role in these processes is not fully understood.
Purpose of the Study:
- To investigate the function of the beta 1 subunit in skeletal muscle.
- To characterize calcium currents, charge movement, and calcium transients in cells lacking the beta 1 subunit.
Main Methods:
- Electrophysiological recordings of calcium currents (I beta null).
- Measurement of charge movement (Qmax).
- Analysis of intracellular calcium transients.
- Radioligand binding assays ([3H]PN200-110).
Main Results:
- Mutant cells exhibited a 13-fold lower L-type calcium current density.
- Charge movement was reduced 2.8-fold, with 40% nifedipine sensitivity.
- No detectable calcium transients were observed in mutant cells, but caffeine induced them.
- A 3.9-fold reduction in [3H]PN200-110 binding sites was noted.
Conclusions:
- The absence of the beta 1 subunit leads to excitation-contraction uncoupling in skeletal muscle.
- This uncoupling results from a failure in the transduction mechanism.
- This failure may be due to reduced alpha 1S subunits or the absence of beta 1 from the voltage-sensor complex.