Effects of perchlorate on the molecules of excitation-contraction coupling of skeletal and cardiac muscle.

Abstract

A B S TRACT To understand the nature of the transmission process of excitationcontraction (EC) coupling, the effects of the anion perchlorate were investigated on the voltage sensor (dihydropyridine receptor, DHPR) and the Ca release channel (ryanodine receptor, RyR) of the sarcoplasmic reticulum (SR). The molecules, from rabbit skeletal muscle, were either separated in membrane vesicular fractions or biochemically purified so that the normal EC coupling interaction was prevented. Additionally, the effect of CIO~ was investigated on L-type Ca 2+ channel gating currents of guinea pig ventricular myocytes, as a native DHPR not in the physiological interaction of skeletal muscle. At 20 raM, CIO 4 had minor effects on the activation of ionic currents through Ca channels from skeletal muscle transverse tubular (T) membranes fused with planar bilayers: a +7-mV shift in the midpoint voltage, ~', with no change in kinetics of activation or deactivation. This is in contrast with the larger, negative shift that CIO 4 causes on the distribution of intramembrane charge movement of skeletal muscle. At up to 100 mM it did not affect the binding of the DHP [~H]PN200-110 to triad-enriched membrane fractions (TR). At 8 mM it did not affect the kinetics or the voltage distribution of gating currents of Ca channels in heart myocytes. These negative results were in contrast to the effects of C10 4 on the release channel. At 20 mM it increased several-fold the open probability of channels from purified RyR incorporated in planar bilayers and conducting Ba z+, an effect seen on channels first closed by chelation of Ca 2+ or by the presence of Mg 2+. It significantly increased the initial rate of efflux of 45Ca2+ from TR vesicles (by a factor of 1.75 at 20 mM and 4.5 at 100 mM). I-increased the association rate constant to a similar extent. The binding of ryanodine to purified RyR protein reconstituted into liposomes had a greater affinity than to TR fractions but was similarly enhanced by C10;. The reducing agent dithiothreitol (5 mM) did not reduce the effect of CIO 4, and 5% polyethylene glycol, with an osmolarity equivalent to 20 mM C10;, did not change ryanodine binding. The results are consistent with a primary effect of CIO4 on the release channel and suggest that the effect on charge movement may be secondary, mediated by a mechanical interaction between the voltage sensor and the release channel.

Differential effects of caffeine and perchlorate on excitation—contraction coupling in mammalian skeletal muscle

“…Perchlorate shifted the membrane potential dependence to more negative voltages, similar to the shift described for amphibians (Lüttgau et al 1983; Csernoch et al 1987; Ma et al 1993; Huang, 1998 a ), although the magnitude was less than that reported for frogs. This shift was originally (Lüttgau et al 1983) interpreted as a direct effect on the voltage sensor but recently a more complex view has emerged.…”

“…The former was first proposed to act exclusively on the voltage sensor and shift its membrane potential dependence to more negative voltages (Lüttgau et al 1983). This simple view was questioned following the observation that perchlorate is an effective agonist of the skeletal‐type RyR (Ma et al 1993). Ríos and co‐workers (1993) interpreted the effects of perchlorate using an allosteric model, where the association of adjacent DHPRs and RyRs would be influenced by the drug.…”

Anion-Sensitive Regions of L-Type CaV1.2 Calcium Channels Expressed in HEK293 Cells

“…Cardiac and skeletal muscle contractions, as well as I Ca in skeletal muscle and pancreatic beta cells, are enhanced by low concentrations of perchlorate [13], [14], [16]. Similarly, we found that CaV1.2 currents were enhanced by low perchlorate concentrations.…”

Perchlorate stimulates insulin secretion by shifting the gating of L-type Ca2+ currents in mouse pancreatic B-cells towards negative potentials

“…In this context it may be of relevance that has been reported to enhance excitation-contraction (E-C) coupling in skeletal muscle cells at a concentration as low as 7 mM [19]. The site of ClO 4 -binding relevant to this effect remains to be established but two main hypotheses have been put forward: (1) a direct effect on the voltage sensor in the T-tubules [9,12,17], which contains a DHP-binding site similar to that encountered in the L-type Ca 2+ channel; or (2) an action on the Ca 2+ -release channel of the sarcoplasmic reticulum (SR) [18], which then would affect the voltage sensor in an allosteric way [20,24]. No direct association between ryanodine receptors and L-type Ca 2+ channels is known to exist in the pancreatic B-cell.…”