Effect of tetracaine on veratrine-mediated influx of sodium into rat brain synaptosomes

Coupling of excitation to Ca2+ release is modulated by dysferlin

KEY POINTS

Dysferlin, the protein missing in limb girdle muscular dystrophy 2B and Miyoshi myopathy, concentrates in transverse tubules of skeletal muscle, where it stabilizes voltage-induced Ca²⁺ transients against loss after osmotic shock injury (OSI). Local expression of dysferlin in dysferlin-null myofibres increases transient amplitude to control levels and protects them from loss after OSI. Inhibitors of ryanodine receptors (RyR1) and L-type Ca²⁺ channels protect voltage-induced Ca²⁺ transients from loss; thus both proteins play a role in injury in dysferlin's absence. Effects of Ca²⁺ -free medium and S107, which inhibits SR Ca²⁺ leak, suggest the SR as the primary source of Ca²⁺ responsible for the loss of the Ca²⁺ transient upon injury. Ca²⁺ waves were induced by OSI and suppressed by exogenous dysferlin. We conclude that dysferlin prevents injury-induced SR Ca²⁺ leak.

ABSTRACT

Dysferlin concentrates in the transverse tubules of skeletal muscle and stabilizes Ca²⁺ transients when muscle fibres are subjected to osmotic shock injury (OSI). We show here that voltage-induced Ca²⁺ transients elicited in dysferlin-null A/J myofibres were smaller than control A/WySnJ fibres. Regional expression of Venus-dysferlin chimeras in A/J fibres restored the full amplitude of the Ca²⁺ transients and protected against OSI. We also show that drugs that target ryanodine receptors (RyR1: dantrolene, tetracaine, S107) and L-type Ca²⁺ channels (LTCCs: nifedipine, verapamil, diltiazem) prevented the decrease in Ca²⁺ transients in A/J fibres following OSI. Diltiazem specifically increased transients by ∼20% in uninjured A/J fibres, restoring them to control values. The fact that both RyR1s and LTCCs were involved in OSI-induced damage suggests that damage is mediated by increased Ca²⁺ leak from the sarcoplasmic reticulum (SR) through the RyR1. Congruent with this, injured A/J fibres produced Ca²⁺ sparks and Ca²⁺ waves. S107 (a stabilizer of RyR1-FK506 binding protein coupling that reduces Ca²⁺ leak) or local expression of Venus-dysferlin prevented OSI-induced Ca²⁺ waves. Our data suggest that dysferlin modulates SR Ca²⁺ release in skeletal muscle, and that in its absence OSI causes increased RyR1-mediated Ca²⁺ leak from the SR into the cytoplasm.

Quantitation of paralytic shellfish toxins using mouse brain synaptoneurosomes

A membrane potential assay based on synaptoneurosomes prepared from mouse brain was evaluated further for its utility in estimating saxitoxin and related bioactives. Saxitoxin concentrations quantitated in mussel extracts by the synaptoneurosomal technique correlated well with spiked concentrations in these samples (r2 = 0.995; slope=1.048). Other experiments found that the synaptoneurosomal assay can detect saxitoxin-like bioactives in zooplankton samples and the concentrations measured were consistent with preliminary estimations of saxitoxin equivalents using the [3H] saxitoxin receptor binding technique. Veratrine, a mixture of alkaloids that activate sodium channels, had similar potential as a substitute for veratridine in the synaptoneurosomal assay. The results provide additional evidence that the mouse brain synaptoneurosomal membrane potential assay has excellent capability for quantitation of saxitoxin-like activity in shellfish tissues and may also be applied to zooplankton samples.

Osmotic regulation of sodium pump in rat brain synaptosomes: the role of cytoplasmic sodium

The effect of hypoosmolality of incubation medium on the rat of ouabain-sensitive 86Rb+ transport in rat brain synaptosomes was studied. A decreased osmolality from 310 to 250 mOsm increased the rate of 86Rb+ uptake from 3.72 to 6.23 nmol/mg of protein min. To evaluate the involvement of cytoplasmic sodium in sodium pump stimulation inhibitors of ion channels and transport pathways able to increase [Na+]in were used. Tetrodotoxin (1 microM), amiloride (0.5 mM) and verapamil (0.1 mM) had no influence on the osmotic response of the sodium pump. The decrease of sodium concentration in incubation medium to 15 mM, leading to a practical loss of its transmembrane gradient, did not abolish stimulation of pump. No increase in 22Na+ influx or intrasynaptosomal sodium content was registered at hypotonic conditions. It is suggested that osmotic regulation of Na+,K(+)-ATPase is not connected with an increase of internal sodium through opening of sodium channels, or with activation of other membrane sodium-transporting systems.