Table 1.
Effect of t-system Cl− conductance on voltage threshold, twitch size and repriming period
| Treatment | Threshold (V cm−1) | Twitch (%) | RP (ms) |
|---|---|---|---|
| 3 mm Cl− | 33.8 ± 1.2 (12)a,b | 19.4 ± 2.2 (13)a,b | ;4.8 ± 0.1 (13)a,b |
| 0 mm Cl− | 26.8 ± 2.1 (9)a | 33.9 ± 3.1 (17)a | 4.3 ± 0.2 (16)b,c |
| 3 mm Cl−+ phorbol | 26.0 ± 0.7 (5)b | 38.5 ± 2.9 (5)b | 5.8 ± 0.1 (5)a,c |
| Paired data | |||
| 3 mm Cl− | 32.5 ± 1.4 (6)* | 19.6 ± 4.5 (6)* | 5.0 ± 0.0 (6)* |
| 3 mm Cl−+ 9-AC | 25.5 ± 1.6 (6)* | 29.0 ± 5.3 (6)* | 4.3 ± 0.2 (6)* |
| 0 mm Cl− | 30.0 ± 2.3 (4) | 33.4. ± 2.7 (4) | 4.8 ± 0.3 (4) |
| 0 mm Cl−+ 9-AC | 28.1 ± 1.6 (4) | 33.0 ± 3.7 (4) | 4.8 ± 0.3 (4) |
| Phorbol-treated | |||
| 3 mm Cl− | 26.0 ± 0.7 (5) | 38.5 ± 2.9 (5) | 5.8 ± 0.1 (5) |
| 3 mm Cl−+ 9-AC | 24.8 ± 1.3 (5) | 39.8 ± 2.3 (5) | 5.6 ± 0.2 (5) |
Values are the mean (±s.e.m.), with number of fibres shown in parentheses. Twitch responses were elicited in skinned fibres with normal extracellular solution in the t-system and 3 mm Cl− in the intracellular bathing solution (3 mm Cl−) or with no Cl− in either the t-system or intracellular solution (0 mm Cl−) (see Methods). Voltage threshold determined with 1 ms pulses. Twitch force expressed as a percentage of maximum Ca2+-activated force in that fibre. Repriming period (RP) indicates the time between a pair of pulses required for the second pulse to also elicit an AP. Where indicated, measurements were made in same fibre both before and after addition of 100 μm 9-AC. Fibres obtained from muscles pre-exposed to phorbol ester where indicated. Values for the three unpaired treatments (3 mm Cl−, no Cl− and 3 mm Cl− in phorbol-treated muscle) were compared by one-way ANOVA followed by Bonferroni's multiple comparison post hoc test; values significantly different between two treatments are labelled with the same superscript (a, b or c).
Significant difference from indicated paired case (paired t test). Eliminating the chloride conductance by removing Cl−, or by treatment with 9-AC or phorbol ester, caused a decrease in voltage threshold and increase in twitch size, and, except for the phorbol treatment, a decrease in repriming period.