Calcium transients evoked by action potentials in frog twitch muscle fibres

Changes in threshold for calcium transients in frog skeletal muscle fibres owing to calcium depletion in the T-tubules

Strength-duration curves were measured for voltage-clamp depolarizations required to elicit a just detectable rise in intracellular calcium, as monitored using arsenazo III, in frog twitch muscle fibres. In normal Ringer solution, the threshold for a 5 sec duration depolarization was about 5 mV more negative than for a 200 msec duration pulse. The shift in threshold comparing 200 msec and 5 sec pulses was almost abolished in bathing solutions including magnesium or nickel (4 mM), or where the free calcium concentration was buffered. The shift in threshold was little changed by substitution of barium for calcium. These results can be explained by supposing that the 5 sec depolarization activates an inward calcium flux across the T-tubule membrane, which decreases the calcium concentration in the tubules, and hence alters the threshold for activation of excitation-contraction (e.-c.) coupling because of surface charge effects.

Comparison of isotropic calcium signals from intact frog muscle fibers injected with Arsenazo III or Antipyrylazo III

Intact single skeletal muscle fibers were micro-injected with either of the metallochromic indicator dyes Arsenazo III or Antipyrylazo III, and dye-related Ca2+ signals from each were measured during a twitch. In comparison with the Arsenazo III Ca2+ signal, the signal from Antipyrylazo III had three favorable features: (a) it was temporally faster, (b) its spectral dependence agreed with a cuvette calibration, and (c) its kinetic behavior was consistent with a single Ca2+ -dye stoichiometry. It is therefore suggested that the Antipyrylazo III Ca2+ signal is a more accurate monitor of the time course of the underlying myoplasmic free Ca2+ transient and one that may be more reliably calibrated.

Calcium transients studied under voltage-clamp control in frog twitch muscle fibres

1. Intracellular calcium transients were recorded from frog twitch muscle fibres in response to voltage-clamped depolarizing pulses, using arsenazo III as an intracellular calcium monitor. The object was to investigate the time- and voltage-dependent characteristics of the coupling process between membrane depolarization and calcium release from the sarcoplasmic reticulum (s.r.)2. To examine the extent to which the T-tubule membrane potential was controlled during clamp pulses, the dye NK 2367 was used as an optical probe of tubular potential. This indicated that the tubular time constant is about 0.6 msec.3. Strength-duration curves were obtained for depolarizing pulses required to give both threshold mechanical contraction and calcium signal. Curves measured in these two ways were closely similar.4. Changes in holding potential altered the strength-duration curve for calcium release so that at more positive holding potentials a shorter pulse was needed to obtain a response for any given pulse amplitude.5. A latency of a few milliseconds was observed between the onset of depolarization and the initial rise of the calcium signal. This became shorter with stronger depolarizations, but approached a minimum at potentials above about +25 mV.6. Subthreshold depolarizations applied before a test pulse increased the size and decreased the latency of the calcium signal. Conditioning hyperpolarizations had opposite effects.7. The rate of build-up of potentiation or depression of response size seen with subthreshold de- and hyperpolarizing conditioning pulses was examined using conditioning pulses of different durations. For both pulses this process showed a time constant of about 3 msec (at 10 degrees C).8. The rate of decay of potentiation or depression was similarly measured, using a gap of variable duration between conditioning and test pulses. For both de- and hyperpolarizing pulses this showed a time constant of about 5 msec (10 degrees C).9. The relationship between conditioning pulse potential, and the size of calcium signal elicited by a following test pulse was non-linear.10. Subthreshold pulses immediately following a brief test pulse affected the size of the calcium signal in a similar way to preceding conditioning pulses.11. The relationship between potential and size of the calcium signal was examined using pulses of 3 and 20 msec duration. With the long pulse the relation was roughly sigmoid, but with the short pulse continued to rise even at strongly positive potentials.12. The results are discussed in terms of a model in which the exponential build-up of a hypothetical coupler in the excitation-contraction (e.-c.) coupling process is presumed to lead to calcium release when a threshold level is exceeded.

Calcium transients in frog skeletal muscle fibres following conditioning stimuli

1. Intracellular Ca(2+) transients were recorded from frog twitch muscle fibres, using arsenazo III as a Ca(2+) monitor. When fibres were stimulated by two action potentials, the arsenazo signal to the second stimulus was smaller than the first, for stimulus intervals of up to several seconds.2. The recovery of the amplitude of the second response followed two exponential time courses; a fast one with a time constant of about 70 msec giving recovery to about 90% of the control value, followed by a slow recovery to 100%, with a time constant of about 12 sec (at 10 degrees C).3. The time constant of the fast recovery component was strongly temperature-dependent, with a Q(10) of approximately 2.7, whilst the Q(10) of the slow component was about 1.4.4. Removal of Ca(2+) in the bathing medium lengthened the time constant of the slow recovery component by a factor of three, but had little effect on the fast recovery component. The lengthening of the slow component was not reversed by addition of Mg(2+), but Sr(2+) ions could substitute for Ca(2+).5. The influence of membrane potential on the recovery time-course was investigated after blocking action potentials with tetrodotoxin, using a voltage clamp to control membrane potential. Paired depolarizing stimuli were used, with the potential held to either low (-60 or -80 mV) or high (-110 or -140 mV) potentials between stimuli. No differences were apparent in either the fast or slow recovery components at these holding potentials.6. The arsenazo response elicited by an action potential following a conditioning tetanus was reduced in size even more strongly than following a single action potential. The time course of recovery of the response following a tetanus again comprised two exponential components. After a 20 Hz tetanus for 0.5 sec, the fast component had a time constant of about 400 msec, and gave a recovery to about 60% of the control value. Subsequent recovery to 100% occurred with a time constant of about 12 sec.7. The time constant of the fast recovery component increased markedly with increasing frequency or duration of the conditioning tetanus. The time constant of the slow component was not appreciably altered by conditioning tetani varying between one impulse and sixty impulses. However, the reduction in response size due to the slow component, extrapolated to zero stimulus interval, increased with increasing number of impulses in the tetanus.8. The time constant of the fast recovery component corresponded closely with the decay time constant of the arsenazo response to the conditioning stimulus. This correspondence held over a nearly fifty-fold range of time constants, and for two different conditions which affected the decay time constant (temperature, and frequency of tetanic stimulation).9. The decay time constant of the arsenazo response elicited by an action potential was slowed by a preceding impulse or tetanus. Following a 20 Hz tetanus for 0.5 sec, recovery of the half decay time appeared to follow an exponential time course, with a time constant of about 12 sec.10. These results suggest that the fast recovery component reflects the re-filling of release stores in the sarcoplasmic reticulum by Ca(2+) ions taken up from the cytoplasm. The origin of the slow component is less clear, but it may arise from inactivation of the excitation-contraction (e-c) coupling process between T-tubule depolarization and Ca(2+) release from the sarcoplasmic reticulum.