A nonlinear electrostatic potential change in the T-system of skeletal muscle detected under passive recording conditions using potentiometric dyes.

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Abstract
Voltage-sensing dyes were used to examined the electrical behavior of the T-system under passive recording conditions similar to those commonly used to detect charge movement. These conditions are designed to eliminate all ionic current and render the T-system potential linear with respect to the command potential applied at the surface membrane. However, we found an unexpected nonlinearity in the relationship between the dye signal for the T-system and the applied clamp potential. An additional voltage- and time-dependent optical signal appears over the same depolarizing range of potentials where charge movement and mechanical activation occur. This nonlinearity is not assiciated with unblocked ionic currents and cannot be attributed to lack of voltage clamp control of the T-system, which appears to be good under these conditions. We propose that a local electrostatic potential change occurs in the T-system upon depolarization. An electrostatic potential would not be expected to extend beyond molecular distances of the membrane and therefore would be sensed by a charged dye in the membrane but not by the voltage clamp, which responds solely to the potential of the bulk solution. Results obtained with different dyes suggest that the location of the phenomena giving rise to the extra absorbance change is either intramembrane or at the inner surface of the T-system membrane.