CAFFEINE EVOKED CONTRACTURES IN SINGLE SLOW (TONIC) MUSCLE-FIBERS 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 caf feine were applied in Ringer solution, and contractures were measured isometrically. Sigmoid caffeine concentration-response curves were obt ained, the threshold value being near 1.2 mmol/l, and maximum contract ures being obtained with 10 to 20 mmol/l concentrations of caffeine. C ontracture solutions were modified by varying the Ca2+ concentration o r by replacing Ca2+ with 1.8 mmol/l Mg2+, Ni2+, Co2+ or with 0.1-5.0 m mol/l La3+. The effects of low pH (5.3), K+ (6,10 and 95 mmol/l), aden osine (10 mmol/l) and gallopamil (D600; 30 mu mol/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+ increas ed it. The amplitude of contractures evoked by high caffeine concentra tions was unaffected. Caffeine (1-40 mmol/l) was also pressure injecte d into slow fibres. The composition of the solution was modified in a number of ways, but a contractile response was not observed or measure d. Extracellular application of caffeine from the same pipettes evoked local contractures. Similar injection experiments in twitch fibres re vealed the same results. These observations suggest that an extracellu lar binding site seems to be involved in the initiation of caffeine-ev oked contractures in intact frog muscle fibres. Possible reasons for t he ineffectiveness of intracellular caffeine are discussed.