A free radical probe was introduced into single barnacle muscle cells, and its freedom of motion inferred from the spin resonance spectra. The probe reported an average local viscosity of 5-10 cp compared with 1 cp for pure water. From a comparison of the temperature dependence of the probe's tumbling rate in model aqueous systems and in the muscle we concluded that in the muscle the probe was undergoing fast exchange between sites of different mobility. Thus 10 cp must be taken as an upper limit for the viscosity of most cell water.

SUMMARY1. The temperature-dependent component of the resting potential in intact, cannulated and dialysed fibres from the muscle of the barnacle Balanus nubilus was studied under a variety of different experimental conditions. A decrease in temperature from 22 to 120 C produced a mean depolarization of 10 mV.2. Neither addition of strophanthidin, nor replacement of external sodium by lithium affect the voltage shift induced by temperature. However, the magnitude of the voltage shift depends on the external chloride and potassium concentration.3. The dialysis technique was applied to measure the potassium, chloride and sodium fluxes as a function of temperature. The QL0 for the passive fluxes of these ions was 1-9, 1P7, and 1P4 respectively.4. The temperature-dependent changes in the passive ionic fluxes combined with the inability of inhibitors of the sodium pump to alter the temperature dependence of the resting potential suggest that the change induced by temperature on the resting potential is primarily caused by a change in the passive permeability ratios, and is not related to active ion transport.

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In contrast, at such negative voltages, Hþ at pH ¼ 6.2 was much less effective in increasing open probability, leading to the estimated C value ¼ ~1.3, placing Hþ between Ca2þ and Mg2þ in the ability to promote opening in the absence of voltage sensor activation. Likewise, at the respective saturation concentrations, Hþ was less effective than Ca2þ at regulating channel kinetics. Our electrophysiological measurements and simulations collectively suggest that Hþ is a partial agonist of the RCK1 high-affinity Ca2þ sensor and that a small change in the relative position of His and Asp residues in the sensor, ~0.1 nm, may underlie the activation of the channel by Hþ. Supported by NIH.