QUANTAL CALCIUM-RELEASE OPERATED BY MEMBRANE VOLTAGE IN FROG SKELETAL-MUSCLE

1. Ca2+ transients and Ca2+ release flux were determined optically in cut skeletal muscle fibres under voltage clamp. 'Decay' of release dur ing a depolarizing pulse was defined as the difference between the pea k value of release and the much lower steady level reached after about 100 ms of depolarization. Using a double-pulse protocol, the inactiva ting effect of release was measured by 'suppression', the difference b etween the peak values of release in the test pulse, in the absence an d presence of a conditioning pulse that closely preceded the test puls e. 2. The relationship between decay and suppression was found to foll ow two simple arithmetic rules. Whenever the conditioning: depolarizat ion was less than or equal to the test depolarization, decay in the co nditioning release was approximately equal to suppression of the test release. Whenever the conditioning depolarization was greater than tha t of the test, suppression was complete, i.e. test release was reduced to a function that increased monotonically to a steady level. The ste ady level was the same with or without conditioning. 3. These arithmet ic rules suggest that inactivation of Ca2+ release channels is strictl y and fatally linked to their activation. More than a strict linkage, however, is required to explain the arithmetic properties. 4. The arit hmetic rules of inactivation result in three other properties that are inexplicable with classical models of channel gating: constant suppre ssion, incremental inactivation and increment detection. These propert ies were first demonstrated for inositol trisphosphate (IP3)-sensitive channels and used to define IP3-induced release as quantal. In this s ense, it can now be stated that skeletal muscle Ca2+ release is activa ted by membrane voltage in a quantal manner. 5. For both classes of in tracellular Ca2+ channels, one explanation of the observations is the existence of subsets of channels with different sensitivities (to volt age or agonist dose). In an alternative explanation, channels are iden tical, but have a complex repertoire of voltage- or dose-dependent res ponses.