A calcium‐ and voltage‐dependent chloride current in developing chick skeletal muscle.

Abstract

SUMMARY1. Depolarization of embryonic chick myotubes from negative potentials elicits a rapid spike followed by a long-duration after-potential. The ionic basis of the longduration after-potential was examined by making intracellular recordings from cultured myotubes, and by making whole-cell patch-clamp recordings from myoblasts and myoballs.2. The peak potential of the long-duration after-potential varied with the chloride gradient, suggesting that a conductance increase to chloride is involved in generating the after-potential. However, a calcium current was also implicated, since lowering the extracellular calcium or replacing extracellular calcium with cobalt abolished the after-potential.3. When extracellular calcium was replaced with strontium or barium, shortduration spikes similar to calcium spikes were observed, but only strontium was able to support activation of long-duration after-potentials. Intracellular injection of calcium or strontium into myotubes bathed in calcium-free extracellular solutions restored the ability of depolarization to evoke an after-potential. Intracellular injection of magnesium, barium, nickel or cobalt did not restore this ability. These experiments strongly suggested that the long-duration after-potential was due to a calcium-and voltage-activated chloride current.4. Whole-cell voltage-clamp recordings from myoballs and myoblasts showed that a large chloride conductance could be activated by depolarization when the internal free calcium concentration was buffered at levels greater than 10-7 M. At 2-5 x 10-7 Mcalcium, the voltage dependence of activation was steepest in the range of -30 to -20 mV and the activation kinetics varied with the membrane potential. The time to half-maximal activation ranged from 01 s at positive potentials to greater than 1 s at more negative potentials. The time constant for deactivation was approximately 1 s at -50 mV. No inactivation was observed.5. The selectivity of the chloride current was measured by substituting other anions for chloride. The following permeability series was found: I-> NO3-> Br-> C1-> acetate > F-> S04-= glucuronate. Thus anion permeability decreased as the hydration radius increased.6. Measurements of the resting potential of developing myoblasts and myotubes under 'physiological' conditions (37°C, bicarbonate buffer) suggest that the afterpotential acts to depolarize these cells 10-20 mV above their resting potential (approximately -60 mV) for several seconds. R. 1. HlUMAlE AND S. A. THOMAS 7. We discuss the possibility that the long-duration after-potential may be involved in triggering myoblast fusion and in the generation of bursts of spontaneous contractions in developing myotubes.

Inactivation of calcium-activated chloride channels in smooth muscle by calcium/calmodulin-dependent protein kinase

“…Although the function of these currents in the regulation of cell excitability remains uncertain, evidence suggests that I Cl(Ca) prolongs the action potential and calcium entry (1,5,(8)(9)(10) and mediates fast postsynaptic potentials in smooth muscle (4). Moreover, in smooth muscle, I Cl(Ca) is associated with the sporadic release of calcium from intracellular stores, resulting in spontaneous transient inward currents (STICs) at resting membrane potentials (11), which may act to couple intracellular calcium release to spontaneous phasic electrical activity (12).…”

Control of action potentials and Ca2+ influx by the Ca(2+)‐dependent chloride current in mouse pituitary cells.

“…A Cl-conductance appears to play a role in the termination of the action potentials in mammalian cardiac myocytes (Harvey, Clark & Hume, 1990), but this current is not dependent on [Ca2+]i. In developing chick skeletal muscle however, long duration action potentials seem to be controlled by a Ca2+-activated Cl-conductance similar to that found in AtT-20 cells (Hume & Thomas, 1989 (Fig. 8A).…”

“…In both rat dorsal root ganglion neurons and mouse spinal cord neurons IC1(ca) has been postulated to underlie action potential afterpolarizations (Owen et al 1984;Mayer, 1985). In developing chick skeletal muscle cells (Hume & Thomas, 1989), and in AtT-20 cells (Korn & Weight, 1987), ICl(ca) can be activated rapidly, and therefore may contribute to the shape of the action potential.…”

“…AtT-20 cells express a slowly activating, slowly deactivating chloride current (Icl(ca)) that is gated primarily by intracellular Ca2 , but is also mildly sensitive to membrane voltage (Korn & Weight, 1987). Although ICl(ca) has been observed in a variety of cell types (Bader, Bertrand & Schwartz, 1982;Evans & Marty, 1986;Owen, Segal & Barber, 1984;Mayer, 1985;Matthews, Neher & Penner, 1989;Hume & Thomas, 1989), little is known about its role in the regulation of cell excitability. In both rat dorsal root ganglion neurons and mouse spinal cord neurons IC1(ca) has been postulated to underlie action potential afterpolarizations (Owen et al 1984;Mayer, 1985).…”

Steroid hormone regulation of Clca3 expression in the murine uterus

“…The calcium-activated chloride channels (CLCA) family appears to mediate a calcium-activated chloride conductance in a variety of tissues, including epithelium (Evans & Marty 1986, Huang et al 1993, Arreola et al 1996, smooth muscle (Amedee et al 1990, Clapp et al 1996, skeletal muscle (Hume & Thomas 1989) and neurons (Barnes & Hille 1989, Hallani et al 1998. Six members of this family have been identified, cloned and partially characterized in the mouse: Clca1 (Gandhi et al 1998, Romio et al 1999, Clca2 (Lee et al 1999), Clca3 (Komiya et al 1999), Clca4 (Elble et al 2002), Clca5 and Clca6 (Abdel-Ghany et al 2003, Beckley et al 2004, Evans et al 2004.…”