Caffeine-evoked contractures in single slow (tonic) muscle fibres of the frog (Rana temporaria and R. esculenta).

Single slow (tonic) muscle fibres were dissected from cruralis muscles of Rana temporaria and R. esculenta. Increasing concentrations of caffeine were applied in Ringer solution, and contractures were measured isometrically. Sigmoid caffeine concentration-response curves were obtained, the threshold value being near 1.2 mmol/l, and maximum contractures being obtained with 10 to 20 mmol/l concentrations of caffeine. Contracture solutions were modified by varying the Ca2+ concentration or by replacing Ca2+ with 1.8 mmol/l Mg2+, Ni2+, Co2+ or with 0.1-5.0 mmol/l La3+. The effects of low pH (5.3), K+ (6,10 and 95 mmol/l), adenosine (10 mmol/l) and gallopamil (D600; 30 micromol/l) were examined too. The caffeine threshold was lowered by Mg2+, K+, 0 .1 mmol/l La3+ and D600, while all other substances including 0.5-5.0 mmol/l La3+ increased it. The amplitude of contractures evoked by high caffeine concentrations was unaffected. Caffeine (1-40 mmol/l) was also pressure injected into slow fibres. The composition of the solution was modified in a number of ways, but a contractile response was not observed or measured. Extracellular application of caffeine from the same pipettes evoked local contractures. Similar injection experiments in twitch fibres revealed the same results. These observations suggest that an extracellular binding site seems to be involved in the initiation of caffeine-evoked contractures in intact frog muscle fibres. Possible reasons for the ineffectiveness of intracellular caffeine are discussed.