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‘Quantal’ calcium release operated by membrane voltage in frog skeletal muscle
Author(s) -
Pizarro Gonzalo,
Shirokova Natalia,
Tsugorka Alexander,
Ríos Eduardo
Publication year - 1997
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1997.289bn.x
Subject(s) - depolarization , chemistry , conditioning , pulse (music) , biophysics , membrane potential , time constant , mathematics , voltage , physics , biochemistry , biology , statistics , electrical engineering , engineering , quantum mechanics
1 Ca 2+ transients and Ca 2+ 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. 2 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. 3 These arithmetic rules suggest that inactivation of Ca 2+ release channels is strictly and fatally linked to their activation. More than a strict linkage, however, is required to explain the arithmetic properties. 4 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 (IP 3 )‐sensitive channels and used to define IP 3 ‐induced release as quantal. In this sense, it can now be stated that skeletal muscle Ca 2+ release is activated by membrane voltage in a quantal manner. 5 For both classes of intracellular Ca 2+ 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.