THE INFLUENCE OF CAFFEINE ON INTRAMEMBRANE CHARGE MOVEMENTS IN INTACTFROG STRIATED-MUSCLE

1. The influence of caffeine, applied over a 25-fold range of concentr ations, on intramembrane charge movements was examined in intact volta ge-clamped amphibian muscle fibres studied in the hypertonic gluconate -containing solutions that were hitherto reported to emphasize the fea tures of q(gamma) at the expense of those of q(beta) charge. 2. The to tal charge, Q(max), the transition voltage, V, and the steepness fact or, k, of the steady-state charge-voltage relationships, Q(V), were al l conserved to values expected with significant contributions from the steeply voltage-dependent q(gamma) species (Q(max) approximate to 20 nC mu F-1, V approximate to -50 mV, k approximate to 8 mV) through al l the applications of caffeine concentrations between 0.2 and 5.0 mM. This differs from recent reports from studies in cut as opposed to int act fibres. 3. The delayed transients that have been attributed to tra nsitions within the q(gamma) charge persisted at low (0.2 mM) and inte rmediate (1.0 mM) caffeine concentrations. 4. In contrast, the time co urses of such q(gamma) currents became more rapid and their waveforms consequently merged with the earlier q(beta) decays at higher (5.0 mM) reagent concentrations. The charging records became single monotonic decays from which individual contributions could not be distinguished. This suggests that caffeine modified the kinetic properties of the q( gamma) system but preserved its steady-state properties. These finding s thus differ from earlier reports that high caffeine concentrations e nhanced the prominence of delayed transient components in cut fibres. 5. Caffeine (5.0 mM) and ryanodine (0.1 mM) exerted antagonistic actio ns upon q(gamma) charge movements. The addition of caffeine restored t he delayed time courses that were lost in ryanodine-containing solutio ns, reversed the shift these produced in the steady-state charge-volta ge relationship but preserved both the maximum charge, Q(max), and the steepness, k, of the steady-state Q(V) relationships. 6. Caffeine als o antagonized the actions of tetracaine on the total available q(gamma ) charge, but did so only at the low and not at the high applied conce ntrations. Thus, 0.2 mM caffeine restored the steady-state q(gamma) ch arge, the steepness of the overall Q(V) function and the appearance of delayed q(gamma) charge movements that had been previously abolished by the addition of 2.0 mM tetracaine. 7. In contrast, the higher appli ed (1.0 and 5.0 mM) caffeine concentrations paradoxically did not modi fy these actions of tetracaine. The total charge and voltage dependenc e of the Q(V) curves, and the amplitude and time course of charge move ments remained at the reduced values expected for the tetracaine-resis tant q(beta) charge. 8. These results permit a scheme in which caffein e acts directly upon ryanodine receptor (RyR)-Ca2+ release channels wh ose consequent activation then dissociates them from the tubular dihyd ropyridine receptor (DHPR) voltage sensors that produce q(gamma) charg e movement, with which they normally are coupled in reciprocal alloste ric contact.