1. The effect of caffeine on the initiation of isometric tension in isolated twitch muscle fibres of the frog was recorded with a mechano-electrical transducer.2. In Ringer solution as well as in solutions containing 95 mM-K(2)SO(4), caffeine (6-10 mM) caused reversible contractures. Tension of maximal potassium contractures was reached with a half-time of 2-4 sec.3. Caffeine caused a shift to lower potassium concentrations of the S-shaped curve which relates peak tension to log. [K](o) or membrane potential. In subthreshold concentrations of caffeine (1.5 mM) the potassium concentration at which half of maximal tension was reached shifted from 30 to 16 mM-K (-39 to -53 mV).4. In the ;steady state' the ability of fibres to develop tension is related to log. [K](o) or membrane potential by an S-shaped curve whose half value shifted from 28 to 45 mM-K (-41 to -29 mV) when 1.5 mM caffeine was applied.5. Fibres were most sensitive to caffeine at membrane potentials between -50 and -20 mV.6. The mechanical activity caused by caffeine was ;stabilized' by an increase in [Ca](o) or [Mg](o) resembling the stabilizing action of these ions on potassium contractures or on the sodium permeability of excitable membranes.7. Tetracaine in low concentrations (0.04-0.1 mM) increased the threshold for mechanical activation and shortened the plateau of potassium contractures. Higher concentrations (1-2 mM) suppressed mechanical activity completely.8. Tetracaine, 0.04 mM, was sufficient to suppress tension caused by a 100 times stronger concentration of caffeine. With higher concentrations of caffeine the inhibitory action of tetracaine could be reversed.9. Fibres which were immersed in subthreshold concentrations of caffeine either in Ringer solution or in a solution with 95 mM-K(2)SO(4) developed a strong contracture after a sudden drop in temperature from 20 to 1-3 degrees C.10. The fast activation of the whole cross-section of the muscle fibre caused by caffeine and its dependence on membrane potential, tetracaine and external alkali earth ions favours the idea that the drug acts at some part of the sarcotubular system which is easily accessible for external ions and drugs and in close connextion with the surface membrane.

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1. When a single muscle fibre was externally stimulated to give a propagated action potential, a large decrease in light intensity was measured with the fibre positioned between crossed‐polarizers oriented at +/‐ 45 degrees with respect to the fibre axis. This large optical signal begins just after stimulation and its man phase precedes the development of positive tension. 2. The peak (or plateau) amplitude of the signal, expressed as the peak (plateau) change in light intensity, delta I, divided by the resting light intensity, I, was typically (minus) 1 to 3x10‐3 and the time‐to‐peak (plateau) was 4 to 6 ms (20 degrees C). 3. When mechanical activity was minimized by stretch or Ringer replacement with D2O or hypertonic solution, the peak tension response was reduced in far greater proportion than the peak optical response, suggesting that the early optical signal is not due to changes in tension or gross fibre movement. 4. The magnitude of the optical response was increased by nitrate and double‐shock stimulatin, procedures which potentiate the twitch response. 5. The optical signal was shown to propagate along the fibre length with a conduction velocity appropriate to the surface action potential. 6. The above results suggest that the large, early birefringence signal reflects a step or steps in the sequence of events leading to contractile activation.

Summary. A fluorescence method is described for the measurement of ATP-driven ion fluxes in lipid vesicles containing purified Na.K-ATPase. The membrane voltage of enzyme containing vesicles was measured by using a voltage-sensitive indocyanine dye. By addition of valinomycin the vesicle membrane is made selectively permeable to K-so that the membrane voltage approaches the Nernst potential for K +. With constant external K + concentration. the time course of internal K+ concentration can be continuously measured as change of the fluorescence signal after activation of the pump. The optical method has a higher time resolution than tracer-flux experiments and allows an accurate determination of initial flux rates. From the temperature dependence of active K + transport its activation energy was determined to be ll'i kJ/mol. ATP-stimulated electrogenic pumping can be measured as a fast fluorescence change when the membrane conductance is low (i.e .. at low or zero valinomycin concentration). In accordance with expectation, the amplitude of the fast signal change increases with decreasing passive ion permeability of the vesicle membrane. The resolution of the charge movement is so high that a few pump turnovers can be easily detected.