Na and K channels: history and structure.

In this perspective, we discuss the physiological roles of Na and K channels, emphasizing the importance of the K channel for cellular homeostasis in animal cells and of Na and K channels for cellular signaling. We consider the structural basis of Na and K channel gating in light of recent structural and electrophysiological findings.

Small Ca releases enable hour-long high-frequency contractions in midshipman swimbladder muscle.

Type I males of the Pacific midshipman fish () vibrate their swimbladder to generate mating calls, or "hums," that attract females to their nests. In contrast to the intermittent calls produced by male Atlantic toadfish (), which occur with a duty cycle (calling time divided by total time) of only 3-8%, midshipman can call continuously for up to an hour. With 100% duty cycles and frequencies of 50-100 Hz (15°C), the superfast muscle fibers that surround the midshipman swimbladder may contract and relax as many as 360,000 times in 1 h.

A perspective on Na and K channel inactivation.

We are wired with conducting cables called axons that rapidly transmit electrical signals (e.g., "Ouch!") from, for example, the toe to the spinal cord.

Structural and functional properties of ryanodine receptor type 3 in zebrafish tail muscle.

The ryanodine receptor (RyR)1 isoform of the sarcoplasmic reticulum (SR) Ca(2+) release channel is an essential component of all skeletal muscle fibers. RyR1s are detectable as "junctional feet" (JF) in the gap between the SR and the plasmalemma or T-tubules, and they are required for excitation-contraction (EC) coupling and differentiation. A second isoform, RyR3, does not sustain EC coupling and differentiation in the absence of RyR1 and is expressed at highly variable levels.

Intracellular calcium movements during relaxation and recovery of superfast muscle fibers of the toadfish swimbladder.

The mating call of the Atlantic toadfish is generated by bursts of high-frequency twitches of the superfast twitch fibers that surround the swimbladder. At 16°C, a calling period can last several hours, with individual 80-100-Hz calls lasting ∼ 500 ms interleaved with silent periods (intercall intervals) lasting ∼ 10 s. To understand the intracellular movements of Ca(2+) during the intercall intervals, superfast fibers were microinjected with fluo-4, a high-affinity fluorescent Ca(2+) indicator, and stimulated by trains of 40 action potentials at 83 Hz, which mimics fiber activity during calling.

Comparison of myoplasmic calcium movements during excitation-contraction coupling in frog twitch and mouse fast-twitch muscle fibers.

Single twitch fibers from frog leg muscles were isolated by dissection and micro-injected with furaptra, a rapidly responding fluorescent Ca(2+) indicator. Indicator resting fluorescence (FR) and the change evoked by an action potential (ΔF) were measured at long sarcomere length (16°C); ΔF/FR was scaled to units of ΔfCaD, the change in fraction of the indicator in the Ca(2+)-bound form. ΔfCaD was simulated with a multicompartment model of the underlying myoplasmic Ca(2+) movements, and the results were compared with previous measurements and analyses in mouse fast-twitch fibers.

Intracellular calcium movements during excitation-contraction coupling in mammalian slow-twitch and fast-twitch muscle fibers.

In skeletal muscle fibers, action potentials elicit contractions by releasing calcium ions (Ca(2+)) from the sarcoplasmic reticulum. Experiments on individual mouse muscle fibers micro-injected with a rapidly responding fluorescent Ca(2+) indicator dye reveal that the amount of Ca(2+) released is three- to fourfold larger in fast-twitch fibers than in slow-twitch fibers, and the proportion of the released Ca(2+) that binds to troponin to activate contraction is substantially smaller.

Measurement and simulation of myoplasmic calcium transients in mouse slow-twitch muscle fibres.

Bundles of intact fibres from soleus muscles of adult mice were isolated by dissection and one fibre within a bundle was micro-injected with either furaptra or mag-fluo-4, two low-affinity rapidly responding Ca(2+) indicators. Fibres were activated by action potentials to elicit changes in indicator fluorescence (ΔF), a monitor of the myoplasmic free Ca(2+) transient ([Ca(2+)]), and changes in fibre tension. All injected fibres appeared to be slow-twitch (type I) fibres as inferred from the time course of their tension responses.

Paying the piper: the cost of Ca2+ pumping during the mating call of toadfish.

Superfast fibres of toadfish swimbladder muscle generate a series of superfast Ca(2+) transients, a necessity for high-frequency calling. How is this accomplished with a relatively low rate of Ca(2+) pumping by the sarcoplasmic reticulum (SR)? We hypothesized that there may not be complete Ca(2+) saturation and desaturation of the troponin Ca(2+) regulatory sites with each twitch during calling. To test this, we determined the number of regulatory sites by measuring the concentration of troponin C (TNC) molecules, 33.

Calcium indicators and calcium signalling in skeletal muscle fibres during excitation-contraction coupling.

During excitation-contraction coupling in skeletal muscle, calcium ions are released into the myoplasm by the sarcoplasmic reticulum (SR) in response to depolarization of the fibre's exterior membranes. Ca(2+) then diffuses to the thin filaments, where Ca(2+) binds to the Ca(2+) regulatory sites on troponin to activate muscle contraction. Quantitative studies of these events in intact muscle preparations have relied heavily on Ca(2+)-indicator dyes to measure the change in the spatially-averaged myoplasmic free Ca(2+) concentration (Δ[Ca(2+)]) that results from the release of SR Ca(2+).

Low-affinity Ca2+ indicators compared in measurements of skeletal muscle Ca2+ transients.

The low-affinity fluorescent Ca(2+) indicators OGB-5N, Fluo-5N, fura-5N, Rhod-5N, and Mag-fluo-4 were evaluated for their ability to accurately track the kinetics of the spatially averaged free Ca(2+) transient (Delta[Ca(2+)]) in skeletal muscle. Frog single fibers were injected with one of the above indicators and, usually, furaptra (previously shown to rapidly track Delta[Ca(2+)]). In response to an action potential, the full duration at half-maximum of the indicator's fluorescence change (DeltaF) was found to be larger with OGB-5N, Fluo-5N, fura-5N, and Rhod-5N than with furaptra; thus, these indicators do not track Delta[Ca(2+)] with kinetic fidelity.

Comparison of the myoplasmic calcium transient elicited by an action potential in intact fibres of mdx and normal mice.

The myoplasmic free [Ca2+] transient elicited by an action potential (Delta[Ca2+]) was compared in fast-twitch fibres of mdx (dystrophin null) and normal mice. Methods were used that maximized the likelihood that any detected differences apply in vivo. Small bundles of fibres were manually dissected from extensor digitorum longus muscles of 7- to 14-week-old mice.

Simulation of Ca2+ movements within the sarcomere of fast-twitch mouse fibers stimulated by action potentials.

Ca(2+) release from the sarcoplasmic reticulum (SR) of skeletal muscle takes place at the triadic junctions; following release, Ca(2+) spreads within the sarcomere by diffusion. Here, we report multicompartment simulations of changes in sarcomeric Ca(2+) evoked by action potentials (APs) in fast-twitch fibers of adult mice. The simulations include Ca(2+) complexation reactions with ATP, troponin, parvalbumin, and the SR Ca(2+) pump, as well as Ca(2+) transport by the pump.

Effects of tetracaine on voltage-activated calcium sparks in frog intact skeletal muscle fibers.

The properties of Ca(2+) sparks in frog intact skeletal muscle fibers depolarized with 13 mM [K(+)] Ringer's are well described by a computational model with a Ca(2+) source flux of amplitude 2.5 pA (units of current) and duration 4.6 ms (18 degrees C; Model 2 of Baylor et al.

Malaria parasites are rapidly killed by dantrolene derivatives specific for the plasmodial surface anion channel.

Dantrolene was recently identified as a novel inhibitor of the plasmodial surface anion channel (PSAC), an unusual ion channel on Plasmodium falciparum-infected human red blood cells. Because dantrolene is used clinically, has a high therapeutic index, and has desirable chemical synthetic properties, it may be a lead compound for antimalarial development. However, dantrolene derivatives would need to preferentially interact with PSAC over the sarcoplasmic reticulum (SR) Ca2+ release channel to avoid unwanted side effects from antimalarial therapy.

Measurement and Interpretation of Cytoplasmic.

Ca(2+)-indicator dyes are widely used in biology yet difficult to characterize inside cells. Studies in skeletal muscle fibers provide important information about indicator behavior and about Ca(2+) signaling within the cytoplasm.