Activation of a slow outward current by the calcium released during contraction of cultured rat skeletal muscle cells

Na+-Ca2+ exchange activity in rat skeletal myotubes: effect of lithium ions

The whole-cell patch-clamp technique coupled with intracellular [Ca2+] measurements was used to investigate the sodium-calcium exchange mechanism in rat skeletal muscle cells in primary culture. Replacing external Na+ ions with Li+ or N-methyl-D-glucamine (NMDG+) ions generated outward currents which were correlated with significant increases of free cytosolic-calcium concentration. These results strongly argue for a functional Na+-Ca2+ exchange mechanism working in its reverse mode. Moreover, the outward currents were sensitive to the new compound KB-R7943 (10 microM), which has been shown to be a potent inhibitor of the sodium-calcium exchanger. Outward Na+-Ca2+ exchange current densities were reduced in the presence of external Li+ as compared to those measured in the presence of NMDG+. After replacing internal sodium by lithium ions, rapid changes of external lithium concentrations generated sarcolemmal currents which were accompanied by subsequent variations of intracellular calcium activity. The currents were dependent on extracellular Li+ with a half-maximal activation at 67 mM and a Hill coefficient of 2.9. This work shows that the Na+-Ca2+ exchanger is able to significantly influence the myoplasmic calcium concentration of cultured rat myotubes. On the other hand, our results suggest that Li+ ions may substitute Na+ ions to catalyse an electrogenic Li+/Ca2+ counter transport.

Measurement of membrane potential and myoplasmic [Ca2+] in developing rat myotubes at rest and in response to stimulation

In this study, the membrane potential and cytosolic [Ca2+] were measured in rat myotubes developing in culture from days 6-14. It was found that as the myotubes developed in culture, the resting membrane potential (RMP) became more negative during days 6-8, and then did not significantly change until after day 13, when it started to become less negative. The mean RMP measured at days 8-13 was -59 +/- 1 mV (n = 70). The amplitude of action potentials elicited in the myotubes by anode break stimulation increased in size during development (range: 47.5-119 mV) and this closely correlated with the development of a more negative RMP. Cytosolic [Ca2+] was measured in the rat myotubes using the Ca2+ indicator Fura-2, and no significant change in the resting [Ca2+] was observed during development (days 6-14). Ca2+ responses triggered by action potentials varied from small slow increases in [Ca2+] that failed to return to the baseline to rapid [Ca2+] transients. The size of the [Ca2+] transients positively correlated with both the observed increase in the RMP during development and the size of the action potential. Larger [Ca2+] transients also had more rapid rates of [Ca2+] decay, indicating a tandem increase in the ability of the sarcoplasmic reticulum to release and resequester Ca2+ during development of rat myotubes. Repetitive stimulation (10 Hz) of the myotubes exhibiting small [Ca2+] transients produced a step-like rise in [Ca2+]. Many myotubes exhibiting larger [Ca2+]transients could not be stimulated at 10 Hz by anode break stimulation due to the presence of action potentials with large hyperpolarisations. However, when these myotubes were depolarised at 10 Hz, they produced a tetanic Ca2+ response similar to that seen in adult skeletal muscle.

Properties of calcium currents and contraction in cultured rat diaphragm muscle

The characterization of calcium currents and contraction simultaneously measured in cultured rat diaphragm muscle cells was carried out in the present study. Whole-cell patch-clamp experiments were designed to further elucidate the mechanism of excitation-contraction (E-C) coupling in diaphragm which, though generally considered a skeletal-type muscle, has been reported to exhibit properties indicative of a cardiac-like E-C coupling mechanism. Normalized current/voltage (I/V) curves for two concentrations of external calcium (2.5 and 5 mM) were obtained from diaphragm myoballs. Both curves showed peaks corresponding to the activation of a T-type calcium current and a dihydropyridine-sensitive L-type calcium current. The normalized curve for the voltage dependence of the activation of contraction in diaphragm myoballs followed a typical Boltzmann-type relationship to the peak of contraction. Thereafter, the curve declined in a manner that was more pronounced in diaphragm compared to that measured in additional experiments using cultured rat limb muscle myoballs. This effect could be interpreted in terms of a more pronounced participation of the L-type current in E-C coupling in cultured diaphragm muscle. An increased likelihood of cultured diaphragm muscle to undergo depletion of sarcoplasmic reticular calcium stores during repetitive stimulation, or a heightened propensity for the voltage sensor for E-C coupling in diaphragm to enter the inactive state could also explain this effect. Maximal contractile activity was only slightly affected when the L-type current was blocked by externally applied cadmium (2 mM) or cobalt (3 mM), suggesting that a pronounced calcium-current-dependent component of contraction is unlikely in cultured diaphragm muscle. These results show that T- and L-type calcium channels are expressed in cultured rat diaphragm muscle cells and that, in contrast to cardiac muscle, the entry of calcium ions via L-type voltage-dependent calcium channels is not a prerequisite for contraction. Differences in the voltage sensitivity of contraction, observed at depolarized membrane potentials in cultured rat diaphragm and limb muscle cells, suggest that the voltage sensor for E-C coupling in diaphragm might more readily enter an inactivated configuration - possibly by a mechanism which is dependent on the concentration of external calcium.