Effects of procaine and caffeine on calcium release from the sarcoplasmic reticulum in frog skeletal muscle

Klein, M.G.; Simon, B.J.; Schneider, M.F.

Journal of Physiology 453: 341-366

1992


ISSN/ISBN: 0022-3751
PMID: 1464833
DOI: 10.1113/jphysiol.1992.sp019232
Accession: 007282724

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Abstract
1. Resting myoplasmic free [Ca2+] and [Ca2+] transients (.DELTA.[Ca2+]) were measured in single voltage-clamped frog skeletal muscle fibres in the presence and absence of procaine, caffeine or procaine plus caffeine using Fura-2 fluorescence and antipyrylazo III (Ap III) absorbance signals. The rate of release (Rrel) of calcium from the sarcoplasmic reticulum (SR) was calculated from the calcium transients and corrected for the relatively small decline due to depletion of calcium from the SR. 2. Procaine (1 mM) reversibly suppressed .DELTA.[Ca2+] and the corresponding Rrel by about 40% for 60-100 ms depolarizing steps to -40 to +20 mV. Procaine had little effect on either the waveform or voltage dependence of the Rrel records. 3. [Ca2+] Transients calculated from Fura-2 fluorescence changes in the presence or absence of procaine had similar time courses and amplitudes as those calculated from the Ap III absorbance changes suggesting that 1 mM-procaine did not interfere with the ability of Ap III or Fura-2 to monitor .DELTA.[Ca2+]. 4. Although 1 mM-procaine depressed Rrel it had no effect on intramembrane charge movements (IQ) calculated from membrane currents recorded simultaneously with .DELTA.[Ca2+]. 5. Procaine (1 mM) reversibly inhibited the potentiating effect of 0.5 mM-caffeine on .DELTA.[Ca2+]. The amplitude and waveform of the Rrel records were similar in control fibres and in the presence of 1 mM-procaine plus 0.5 mM-caffeine. 6. In the presence of 0.5 mM-caffeine .DELTA.[Ca2+] after 10-20 ms voltage steps exhibited an increase in the time to peak and a slower decay time course compared with caffeine-free controls, suggestive of significant calcium-induced calcium release in the presence of caffeine. These effects of caffeine were completely and reversibly blocked by 1 mM-procaine. 7. In the absence of caffeine, 1 mM-procaine caused a small decrease in time to peak of .delta.[Ca2+] after 10-30 ms duration voltage steps compared to the bracketing controls and wash runs without procaine. Rrel turned off faster after 10 ms pulses in procaine than in the absence of procaine, but the turn-off of release was about equally fast with or without procaine after pulses of 20 ms or longer. The effect of procaine after 10 ms pulses in the absence of caffeine may indicate suppression of a component of calcium-induced calcium release in control that inactivates during the pulse. 8. The absence of a significant effect of procaine on the Rrel waveform and voltage dependence for long pulses suggest that the major component of suppression of release by procaine may be a non-specific block of SR calcium-release channels.