Interactions of ionophores (A23187 and X-537A) with sarcolemma and sarcoplasmic reticulum

Clotrimazole, an antimycotic drug, inhibits the sarcoplasmic reticulum calcium pump and contractile function in heart muscle

Clotrimazole (CLT), an antimycotic drug, has been shown to inhibit proliferation of normal and cancer cell lines and its systemic use as a new tool in the treatment of proliferative disorders is presently under scrutiny (Benzaquen, L. R., Brugnara, C., Byers, H. R., Gattoni-Celli, S., and Halperin, J. A. (1995) Nature Med. 1, 534-540). The action of CLT is thought to involve depletion of intracellular Ca2+ stores but the underlying mechanism has not been defined. The present study utilized membrane vesicles of rabbit cardiac sarcoplasmic reticulum (SR) to determine the mechanism by which CLT depletes intracellular Ca2+ stores. The results revealed a strong, concentration-dependent inhibitory action of CLT on the ATP-energized Ca2+ uptake activity of SR (50% inhibition with approximately 35 microM CLT). The inhibition was of rapid onset (manifested in <15 s), and was accompanied by a 7-fold decrease in the apparent affinity of the SR Ca2+-ATPase for Ca2+ and a minor decrement in the enzyme's apparent affinity toward ATP. Exposure of SR to CLT in the absence or presence of Ca2+ resulted in irreversible inhibition of Ca2+ uptake demonstrating that the Ca2+-bound and Ca2+-free conformations of the Ca2+-ATPase are CLT-sensitive. Introduction of CLT to the reaction medium subsequent to induction of enzyme turnover with Ca2+ and ATP resulted in instantaneous cessation of Ca2+ transport indicating that an intermediate enzyme species generated during turnover undergoes rapid inactivation by CLT. The inhibition of Ca2+ uptake by CLT was accompanied by inhibition of Ca2+-stimulated ATP hydrolysis and Ca2+-induced phosphoenzyme intermediate formation from ATP in the ATPase catalytic cycle. Phosphorylation of the Ca2+-deprived enzyme with Pi in the reverse direction of catalytic cycle and Ca2+ release from Ca2+-preloaded SR vesicles were unaffected by CLT. It is concluded that CLT depletes intracellular Ca2+ stores by inhibiting Ca2+ sequestration by the Ca2+-ATPase. The mechanism of ATPase inhibition involves a drug-induced alteration in the Ca2+-binding site(s) resulting in paralysis of the enzyme's catalytic and ion transport cycle. CLT (50 microM) caused marked depression of contractile function in isolated perfused, electrically paced rabbit heart preparations. The contractile function recovered gradually following withdrawal of CLT from the perfusate indicating the existence of mechanisms in the intact cell to inactivate, metabolize, or clear CLT from its target site.

Identification of two populations of cardiac microsomes with nitrendipine receptors: correlation of the distribution of dihydropyridine receptors with organelle specific markers

Conventional sarcolemma and microsome preparations from rabbit and cat ventricular muscle were fractionated on continuous linear sucrose gradients. The distribution of nitrendipine receptors was compared with the distribution of organelle specific markers. For the conventional sarcolemma preparation, the dihydropyridine receptor distribution matched the pattern for external membrane markers in position and shape. The number of nitrendipine receptors was three times the number of muscarine binding sites (approximately 1.0 pmol/mg protein) at the isopycnic point of the vesicles. In contrast, two populations of vesicles with nitrendipine receptors were found in the microsome preparations. One population banded with the external membrane vesicles at a mean buoyant density of 24% (w/w) sucrose. The specific content of dihydropyridine receptors (0.2 pmol/mg) was 1/5 that for the muscarine receptors. The second and major population followed the distribution of an Mr 300K polypeptide, a marker for the junctional cisternae of the sarcoplasmic reticulum (SR). Muscarine receptors, however, were also present throughout that band, albeit at a reduced specific content (approximately 0.1 pmol/mg) compared to the light vesicles. The nitrendipine specific content increased over threefold from that of the light vesicles such that the relative content (nitrendipine/muscarine) was twice that determined for the conventional sarcolemma preparation. Nitrendipine receptors were not associated with nonjunctional SR or mitochondria. The light and heavy microsome populations were incubated with 0.2 mg digitonin/mg protein, a treatment which preferentially perturbs the isopycnic point of external membrane vesicles. For the light vesicles, the membranes with muscarine and nitrendipine receptors became heavier than the bulk of the SR. In contrast, after digitonin treatment of the heavy vesicle population, the nitrendipine and muscarine receptors and the SR marker appeared to comigrate into a sharpened band at 39% sucrose. The possibility that the dihydropyridine binding sites in the heavy microsome population are on external membrane vesicles physically linked to the junctional SR is discussed.

Neurogenic origin of an experimental myopathy induced by the ionophore A23187

An experimental myopathy was induced in frog skeletal muscle incubated in vitro with the calcium ionophore A23187. This myopathy could be prevented if Ca2+ ions in the incubating medium were replaced by either Mg2+ or Co2+ ions. Similarly, preincubation of the preparation with acetylcholine (ACh) receptor blockers such as d-tubocurarine (d-TC) or alpha-bungarotoxin (alpha-BuTX) prevented the development of muscle damage. In muscles that had been previously denervated or treated with botulinum toxin (BTX) the ionophore failed to induce morphological alterations. These results suggest that spontaneous transmitter release, greatly increased by the ionophore A23187, triggers Ca2+ influx into the muscle fibres at the endplate region. Elevated levels of intracellular Ca2+ presumably activate proteolytic systems leading to myofilament disassembly.

Opposing interactions of ionophores (valinomycin and monensin) on calcium ion uptake in rat retinal preparations

Valinomycin is a potent inhibitor of taurine-stimulated ATP-dependent calcium ion uptake in rat retinal membrane preparations but had no effect on ATP-dependent calcium ion uptake in the absence of taurine and no effect on ATP-independent calcium ion uptake. The presence of potassium ions in the buffer systems were required for valinomycin to be inhibitory. On the contrary, monensin stimulated calcium ion uptake in the ATP-dependent system but had no effect on ATP-independent calcium ion uptake. The crude retinal homogenate was also fractionated into various subcellular components. The fraction which contained photoreceptor cell synaptosomes (P1) had a higher specific activity for taurine-stimulated ATP-dependent calcium ion uptake than the crude homogenate or either the fractions which contained synaptosomes derived from the plexiform layer (P2) or rod outer segments (ROS). No differences in calcium ion uptake were observed in the various subcellular fractions compared to the homogenate when assayed for ATP-dependent calcium ion uptake. Valinomycin inhibited both ATP-dependent and taurine-stimulated ATP-dependent calcium ion uptake in the P1, P2, and ROS fractions while monensin stimulated the ATP-dependent calcium ion uptake in the subcellular fractions.

Inhibition of rapid axonal transport in vitro by the ionophores X-537 A and A 23187

The effects of the ionophores X-537 A and A 23187 on axonal transport (AXT)' high energy phosphates, levels of Ca2+ and ultrastructure were investigated in frog sciatic nerves in vitro. X-537 A at 0.09 muM blocked AXT of [3H]labelled proteins by 50% as judged by ligature experiments while the levels of ATP and CrP remained unchanged. Pulse-label experiments showed that the amount of transported material decreased whereas the rate of AXT was only slightly retarded. Elevated extracellular levels of Ca2+ (5mM) potentiated the inhibitory effect of the ionophore. In contrast, ruthenium red counteracted the ionophore induced inhibition of AXT. A 23187 at 10 muM but not at 2 muM inhibition AXT but also reduced the ATP and CrP levels. In pulse-label experiments A 23187 displayed similar but less pronounced effects than X-537 A. Both ionophores increased the total calcium content of the nerve and appeared to decrease the amount of axonal mictotubles (MT). As X-537 is a very potent inhibitor of AXT it may prove a valuable tool in exploring the relations between AXT, Ca2+ and MT.

Lysozyme production by a granuloma in vivo: output in blood and lymph in relation to ultrastructure and immunochemistry

The output of lysozyme into the venous and lymphatic drainage of a B.C.G. experimental granuloma has been studied. There is a massive output of lysozyme by both routes maximal in the lymph at 28 days, and in the serum at 35 days after induction of the granuloma. There is no similar excretion of acid phosphatase. The lactic dehydrogenase level both in blood draining the granuloma and blood draining the normal limb is elevated but not in precise synchrony with lysozyme output. Macrophages in the granuloma show ultrastructural features suggestive of secretion of dense granules, by appearance of electron dense material in the Golgi zone, and condensation therefrom. Granules are discharged on to the surface of lipid droplets, presumably representing attempted phagocytosis, but are not discharged elsewhere on the cell surface. Granules are released by cell disintegration both of macrophages and polymorphs. Lysozyme was demonstrated immunocytochemically in macrophage granules. The macrophages in the granuloma are biosynthetically active as shown by uptake of tritiated tyrosine. The findings support the view that granulomas export lysozyme and that this is due partly to exocytosis of material by macrophages on to the surface of lipid globules and partly to cell disintegration.

Effects of the ionophores X537A and A23187 on GABA, glycine, and taurine release from chick retinal subcellular fractions

The ionophore X537A at concentrations of 5--20 microM stimulated the release of [3H]GABA and [35S]taurine from retinal subcellular crude nuclear (P1) and crude synaptosomal (P2) fractions. The release of [3H]GABA increased 114% and 136% over control values in P1 and P2 fractions, respectively. The efflux of [35S]taurine from P1 was increased by 45% and that from P2 by 21%. X537A increased 45Ca2+ uptake in the P2 fraction but not in the P1 fraction. The effect of X537A on the amino acid release was not dependent on the presence of exogenous calcium. X537A did not affect [3H]GABA or [35S]taurine uptake by the retinal fractions. A23187 enhanced [3H]GABA release from P1 and P2 by 52% and 105%, respectively. The ionophore also increased [14C]glycine liberation in both P1 (35%) and P2 (50%) but failed to stimulate [35S]taurine release. A23187 produced a transient increase of 45Ca2+ uptake of 38% in P1 and 30% in P2. The effects of A23187 on the release of amino acids were calcium dependent. The amino acid uptake was not affected by the ionophore. These results are consistent with the suggested neurotransmitter role for GABA at the outer synaptic layer and for GABA and glycine at the inner synaptic layer of the retina. A neurotransmitter role for taurine is not supported by the present results.