‘Quantal’ calcium release operated by membrane voltage in frog skeletal muscle

Abstract

Ca2+ transients and Ca2+ release flux were determined optically in cut skeletal muscle fibres under voltage clamp. ‘Decay’ of release during a depolarizing pulse was defined as the difference between the peak value of release and the much lower steady level reached after about 100 ms of depolarization. Using a double‐pulse protocol, the inactivating effect of release was measured by ‘suppression’, the difference between the peak values of release in the test pulse, in the absence and presence of a conditioning pulse that closely preceded the test pulse. The relationship between decay and suppression was found to follow two simple arithmetic rules. Whenever the conditioning depolarization was less than or equal to the test depolarization, decay in the conditioning release was approximately equal to suppression of the test release. Whenever the conditioning depolarization was greater than that of the test, suppression was complete, i.e. test release was reduced to a function that increased monotonically to a steady level. The steady level was the same with or without conditioning. These arithmetic rules suggest that inactivation of Ca2+ release channels is strictly and fatally linked to their activation. More than a strict linkage, however, is required to explain the arithmetic properties. The arithmetic rules of inactivation result in three other properties that are inexplicable with classical models of channel gating: constant suppression, incremental inactivation and increment detection. These properties were first demonstrated for inositol trisphosphate (IP3)‐sensitive channels and used to define IP3‐induced release as quantal. In this sense, it can now be stated that skeletal muscle Ca2+ release is activated by membrane voltage in a quantal manner. For both classes of intracellular Ca2+ channels, one explanation of the observations is the existence of subsets of channels with different sensitivities (to voltage or agonist dose). In an alternative explanation, channels are identical, but have a complex repertoire of voltage‐ or dose‐dependent responses.

Involvement of multiple intracellular release channels in calcium sparks of skeletal muscle

“…In current skeletal muscle models (5,22,38,41) this directly opens underlying channels, providing Ca 2ϩ to trigger other channels that do not contact the sensor. By not having to directly open all channels, such models increase gain and speed, while displaying the stability and rich modulatory behavior observed experimentally (34,38). The drugs used here appear to exemplify a similar mechanism, where the long event represents the hypothetical voltage-operated opening, whereas the starter reflects the secondary opening of a channel group.…”

“…Arguments for a multichannel origin include the large flux calculated for Ca 2ϩ sparks, which is inconsistent with measurements of unitary Ca 2ϩ currents in bilayers (33), and the complex ''quantal'' properties of the initial peak of release flux elicited by clamp depolarization (34), hardly explicable as the addition of individually determined one-channel contributions. The rapid turn-on and -off of the causative release (21) and the lack of effects of low [Mg 2ϩ ] on spark amplitude (35) lend credence to the one-channel theory.…”

Modal gating transitions in cardiac ryanodine receptors during increases of Ca 2+ concentration produced by photolysis of caged Ca 2+

“…In this regard, it is interesting that the adaptation of RyRs can be accelerated by Mg 2+ and PKA-dependent phosphorylation [23]. Provided it can be sufficiently fast, the shift in gating modes demonstrated here could represent the molecular basis of use-dependent inactivation in which the closure of the channel is caused not by binding of Ca 2+ per se but is rather linked to prior activation [7a, 15,18].…”

A repetitive mode of activation of discrete Ca2+ release events (Ca2+ sparks) in frog skeletal muscle fibres

“…Such inactivation of the other channel(s) must occur without their activation in order for the different group(s) of channel(s) to constitute different release units. The most likely candidate mediating such an inhibitory interaction would be Ca¥-dependent inactivation, but there is evidence that inactivation is strictly linked to activation (Pizarro et al 1997). Modal gating patterns have been described for single SR Ca¥ release channels in bilayers (Percival et al 1994;Armisen et al 1996;Copello et al 1997).…”