Properties of a new calcium ion antagonist on cellular uptake and mitochondrial efflux of calcium ions

Effect of alteration of nerve terminal Ca2+ regulation on increased spontaneous quantal release of acetylcholine by methyl mercury

Agents known to disrupt intraterminal Ca2+ buffering, N,N-dimethylamino-8-octyl-3,4,5-trimethoxybenzoate (TMB-8), 25 microM; caffeine, 7.5 mM; N,N-bis(3,4-dimethoxyphenethyl)-N-methylamine (YS035), 180 microM; ouabain, 200 microM; and dantrolene, 50 microM, were tested for the ability to alter effects of methyl mercury (MeHg) on spontaneous quantal release of acetylcholine (ACh) at the rat neuromuscular junction. In particular, we sought to determine whether any of the above agents could prevent the MeHg-induced increase of spontaneous release of ACh, an effect measured electrophysiologically as increased frequency of miniature end-plate potentials (MEPPs). MEPPs were recorded continuously from myofibers of the rat hemidiaphragm using conventional, intracellular recording techniques during pretreatment with an inhibitor of Ca2+ regulation and subsequently with the inhibitor plus MeHg (100 microM). When given alone, caffeine and ouabain, which release Ca2+ from the smooth endoplasmic reticulum and mitochondria, respectively, increased MEPP frequency in a biphasic manner. Following pretreatment, concomitant application of MeHg with caffeine or ouabain increased MEPP frequency after a brief latent period to peak values of 53 and 92 Hz, respectively. TMB-8 and dantrolene, putative inhibitors of Ca2+ release from smooth endoplasmic reticulum, differed in their effects on MEPP frequency; TMB-8 alone decreased MEPP frequency to approximately 10% of drug-free control, whereas dantrolene did not significantly alter control MEPP frequency. Subsequent concomitant application of MeHg with TMB-8 or dantrolene increased MEPP frequency to peak values of 40 and 100 Hz after 17 and 30 min, respectively. YS035, a putative inhibitor of mitochondrial uptake and release of Ca2+, decreased MEPP frequency to less than 10% of control after 15 min when given alone. Application of MeHg following YS035 pretreatment failed to increase MEPP frequency for up to 90 min. YS035 did not mask a MeHg effect by blocking postsynaptic sensitivity to ACh or preventing its release since subsequent treatment with La3+ (2 mM) after YS035 had abolished spontaneous release, increased MEPP frequency within 5 min. Thus, of the five inhibitors of nerve terminal Ca2+ regulation tested, only YS035 prevented the stimulatory action of MeHg on MEPP frequency. Results of the present study suggest that release of Ca2+ from nerve terminal mitochondria contributes to the increased MEPP frequency caused by MeHg while release of Ca2+ from smooth endoplasmic reticulum may not.

Action of the nonsteroidal anti-inflammatory agent, flufenamic acid, on calcium movements in isolated mitochondria

The anti-inflammatory agent flufenamic acid was found to inhibit calcium uptake in isolated mitochondria at low concentrations (IC50 = 7.2 microM). Similar concentrations were required to promote the release of calcium from mitochondria preloaded with the cation (EC50 = 3.5 microM). Identical actions were found with diflunisal, mefenanamic acid and 2,4-dinitrophenol. It was concluded that flufenamic acid was affecting calcium movements across the mitochondrial membrane by virtue of its ability to uncouple oxidative phosphorylation.

Studies on the activation of rat liver pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase by adrenaline and glucagon. Role of increases in intramitochondrial Ca2+ concentration

The administration in vivo of either adrenaline or glucagon alone resulted in increases of about 2-fold in the amounts of active, non-phosphorylated, pyruvate dehydrogenase in the livers of fed male or female rats, whereas when administered together increases of about 4-fold were obtained. Ca2+-dependent increases in the amount of active enzyme of up to about 5-fold could be achieved in isolated rat liver mitochondria by incubating them with increasing extramitochondrial [Ca2+]; from this, two conditions of Ca loading were chosen which caused increases in active enzyme similar to those with the hormone treatments given above. The increases in enzyme activity owing to these Ca loads persisted through the 're-isolation' of mitochondria and their incubation in Na+-free KCl-based media containing EGTA. Differences from values obtained with unloaded controls could be diminished by adding Na+ ions to cause the egress of Ca2+ from the mitochondria, or enough extramitochondrial Ca2+ to saturate the enzyme in its Ca2+-dependent activation; the effects of Na+ could be blocked by diltiazem, an inhibitor of mitochondrial Na+/Ca2+ exchange. The re-isolated, Ca-preloaded, mitochondria also exhibited enhanced activities of 2-oxoglutarate dehydrogenase when assayed at non-saturating [2-oxoglutarate] by two different methods; effects of Na+, Ca2+ or diltiazem on the persistent activations of this enzyme were similar to those for pyruvate dehydrogenase. Na+ caused a marked depletion, which could be blocked by diltiazem, of the 45Ca content of re-isolated mitochondria which had pre-loaded with Ca, containing 45Ca, to the same degrees as above. The activities of pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase in incubated liver mitochondria prepared from rats subjected to the hormone treatments given above were found to behave in a very similar manner to those exhibited in the re-isolated, Ca-preloaded, mitochondria. It is concluded that these hormones each bring about the activations of these rat liver enzymes by causing increases in intramitochondrial [Ca2+], and that their effects, as such, are additive.