This study was undertaken as part of an investigation into the mechanism responsible for the marked increase in the excitability of depolarised muscle upon intracellular acidification1. The preparation used in the study was the mechanically skinned fibre, where the surface membrane is removed by microdissection causing the t-system to seal off and polarize to different levels by placing it in carefully designed solutions. In this preparation all steps in the excitation-contraction coupling process are maintained fully functional2 while allowing direct access to the intracellular environment, thus permitting direct control of intracellular pH and the level of polarization of the t-system.
The skinned fibre preparation was obtained from the extensor digitorum longus muscle of adult rats killed by halothane overdose as previously described2 and the preparation was placed in a solution mimicking the myoplasmic environment. Action potential (AP) induced tetanic force responses were generated with square pulses of 2ms duration and field strength of 70V/cm at 25Hz for 1s 2. Alternatively, the voltage sensors in the t-system were activated independently of APs, by replacing all K+ in the bathing solutions with Na+ 2. Results obtained in carefully balanced solutions at pH 7.1 and pH 6.6. No pH dependence was observed in voltage-sensor activated force responses that were independent of APs in the presence or in the absence of Cl- or in the AP-induced responses in the absence of Cl- in solutions. However, in the presence of Cl-, force responses to APs in the t-system were markedly greater at pH 6.6 than at pH 7.1 when the t-system was depolarized to the same level below about –70mV. These results indicate that Cl- channels mediate the protective effect of intracellular acidosis on AP-induced force responses.
(1) Posterino et al, J Physiol 527, 131, 2000.
(2) Nielsen et al, J Physiol 536, 161, 2001.