Excitation contraction coupling in skeletal muscle begins with propagation of a surface action potential into the transverse (T) tubules to give an electrical signal that gates a steeply voltage-dependent release of intracellularly stored Ca²⁺ through the sarcoplasmic reticular (SR) membrane that increases up to e-fold with incremental depolarisations of 2-4 mV. It is thought to take place at localized triadic, T-SR, junctions where the geometrical proximity between nevertheless electrically separate membranes permits their close interaction. This transduction process is likely to involve T-tubular Ca²⁺ channel-dihydropyridine receptors (DHPRs) that act as voltage sensors, acting upon SR ryanodine receptor (RyR)-Ca²⁺ release channels. The conformational changes involved in the voltage-sensing process have been studied electrophysiologically in voltage-clamped intact amphibian muscle fibres. Thus, intramembrane rearrangements in charged functional groups in response to voltage change would displace excess charge from the membrane surface. This would be measurable as a charge movement whose characteristics would offer an electrical signature for the conformational change, provided other membrane ionic currents are minimized.

A specific, q_λ, intramembrane component has been identified with conformational changes in the DHPR. Detubulation experiments specifically localize it to the T-tubular membrane. It is sensitive to the contraction inhibitors tetracaine and dantrolene Na and possesses a steep steady-state voltage-dependence and complex, steeply potential-dependent, ‘humped’ kinetics thereby matching corresponding features of the Ca²⁺ release process. Both processes share a voltage-dependent inhibition by the DHPR inhibitor nifedipine. Such studies emerged with suggestions for the presence of a q_λ system, valence 4 to 6, at a density in the tubular membrane of 200-400 molecules/micron², similar to independently derived estimates of DHPR density. Closer kinetic studies identified the complex and steeply voltage-sensitive kinetics of this intramembrane charge with the direct interactions it may have with RyR-Ca²⁺ channel gating. The kinetics of the q_λ charge movement were separated from remaining components of intramembrane charge through shifts in holding voltage. Isochronic plots of such isolated q_λ charge movement suggested a causally independent intramembrane conformational transition with highly nonlinear kinetics whose embedded rate constants were nevertheless uniquely determined by membrane potential.

Recent pharmacological manoeuvres using DHPR-and RyR-specific reagents have thrown light upon the mechanisms responsible for the complex kinetic features. They suggest the existence of co-operative mechanisms that involve direct interactions between the DHPR-voltage sensor and the RyR-Ca²⁺ release channel. This contrasts with the simple kinetics of corresponding charge movements reflecting Ca²⁺ current gating in cardiac muscle, where intracellularly stored Ca²⁺ is released through indirect, Ca²⁺-induced Ca²⁺ release, mechanisms that would not require such direct functional contact between membrane molecules. Suggestions for a direct interaction between a tubular membrane DHPR and an anatomically close but electrically uncoupled RyR within SR membrane was consistent with steady state findings. These indicated that DHPR modification reduces the total steady-state q_λ charge but RyR modification by daunorubicin or ryanodine preserves the separate identities of steady-state q_λ charge, compatible with actions remote from the DHPR-voltage sensor. A hypothesis that RyR gating is allosterically coupled to configurational changes in DHPRs in skeletal muscle would next predict that such interactions are reciprocal and that RyR modification should influence the transitions shown by intramembrane charge. This prediction of a RyR-DHPR cross talk was confirmed by findings that the RyR antagonists ryanodine and daunorubicin remove, but further addition of the RyR agonist perchlorate restores, the nonlinearity in the q_λ charge movement. Other pharmacological manoeuvres using the RyR-specific agents caffeine and µM-tetracaine produced comparable selective effects on the kinetics of q_λ charge. Similarly, q_λ charge, together with its complex charging kinetics in perchlorate-containing solutions, persisted in experimental preparations whose Ca²⁺ stores were depleted using the Ca²⁺-ATPase inhibitor cyclopiazonic acid (CPA), making it unlikely that such effects result indirectly from Ca²⁺ release rather than such direct allosteric contacts.

A simple analytical model describing the energetics for a possible subunit interaction between DHPRs and RyRs successfully reproduced the major characteristics of this q_λ cooperativity, and could form the basis for further quantitative explorations. This adapted the simplest two-state scheme in which intramembrane charge moves across an energy barrier from a resting energy state to an activated state. It permitted the energy barrier to decline as charge reaches the active state following a depolarising voltage step thereby enhancing subsequent charge transfer, and its return to its previous level with charge recovery at the end of the step.

The author thanks the Wellcome Trust, the Medical Research Council, the British Heart Foundation and the Leverhulme Trust for generous support.