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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-K₂SO₄, 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-K₂SO₄ developed a strong contracture after a sudden drop in temperature from 20 to 1-3° 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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