Charge changes in sarcoplasmic reticulum and Ca2+-ATPase induced by calcium binding and release: a study using lipophilic ions

Comparison of three tetramic acids and their ability to alter membrane function in cultured skeletal muscle cells and sarcoplasmic reticulum vesicles

Cyclopiazonic acid is a potent inhibitor of calcium uptake and Ca(2+)-ATPase activity in sarcoplasmic and endoplasmic reticulum. In L6 muscle myoblasts, cyclopiazonic acid stimulates the uptake of tetraphenylphosphonium, a lipophilic membrane potential probe, and has antioxidant properties. The purpose of the present study was to investigate the structural requirements necessary for causing the surface charge alterations, and the antioxidant activity in L6 skeletal muscle myoblasts, and for inhibition of calcium transport by rat skeletal muscle sarcoplasmic reticulum vesicles. This was accomplished by comparing the effects of two structurally related tetramic acids, cyclopiazonic acid imine and tenuazonic acid, with cyclopiazonic acid. Cyclopiazonic acid imine inhibited oxalate-assisted 45Ca2+ uptake and ATPase activity in sarcoplasmic reticulum vesicles and stimulated tetraphenylphosphonium accumulation by L6 muscle myoblasts. However, these effects required an approximately fourfold higher concentration than that of cyclopiazonic acid. Tenuazonic acid, up to 1 mM, had no effect on oxalate-assisted 45Ca2+ uptake or Ca(2+)-ATPase activity in sarcoplasmic reticulum vesicles and did not stimulate tetraphenylphosphonium accumulation by L6 muscle myoblasts. Cyclopiazonic acid was only slightly more effective than cyclopiazonic acid imine at preventing the patulin-induced increase in thiobarbituric acid positive substance (used to estimate lipid peroxidation); tenuazonic acid was totally ineffective. Previously, it was shown that cyclopiazonic acid was twice as effective as cyclopiazonic acid imine at preventing increases in thiobarbituric acid positive substance in cultured renal cells, LLC-PK1. Thus, the indole nucleus of cyclopiazonic acid is essential for the membrane-associated biological activity; however, modification of the acetyl group reduces the potency of the activity.

Interaction of cyclopiazonic acid with rat skeletal muscle sarcoplasmic reticulum vesicles. Effect on Ca2+ binding and Ca2+ permeability

The interaction of cyclopiazonic acid with rat skeletal muscle sarcoplasmic reticulum (SR) vesicles was investigated in order to study the mechanism of cyclopiazonic acid inhibition of the Ca2+-ATPase (Goeger et al., Biochem Pharmacol 37: 978-981, 1988). Cyclopiazonic acid at 25 microM prevented the binding of Ca2+ to the high affinity binding site of mixed (light and heavy) SR vesicles and inhibited, in a dose-dependent manner, the Ca2+-dependent phosphorylation of SR vesicles by ATP. Binding of Ca2+ to the high affinity site of the CA2+-ATPase is necessary for both Ca2+ transport and for phosphorylation of the Ca2+-ATPase. We conclude that inhibition of Ca2+ binding to the high affinity site may be responsible, at least in part, for the activity of cyclopiazonic acid. The mechanism of inhibition remains unclear. The inhibition was not reduced after dialysis and was only partially reversed by gel filtration of SR vesicles treated with cyclopiazonic acid. Neither 1 mM glutathione nor dithiothreitol pretreatment had any effect on the inhibition of the Ca2+-ATPase. In addition to its inhibition of Ca2+ uptake and the Ca2+-ATPase, cyclopiazonic acid had significant effects on Ca2+ efflux from both passively and actively loaded SR vesicles. Cyclopiazonic acid impeded the efflux of Ca2+ from passively loaded SR vesicles (in the presence of ruthenium red) when compared to either untreated vesicles or those treated with mersalyl acid, a mercurial which also inhibits the Ca2+-ATPase and is known to induce Ca2+ release by both ruthenium red-sensitive and -insensitive pathways. Treatment of actively loaded vesicles with cyclopiazonic acid resulted in a decreased rate of Ca2+ efflux when compared to SR vesicles in which the Ca2+-ATPase activity was inhibited by ATP depletion with hexokinase and glucose. The results are consistent with the hypothesis that, in mixed SR vesicles, cyclopiazonic acid inhibits both the Ca2+ pump and Ca2+ efflux.

Age and growth-related changes in cyclopiazonic acid-potentiated lipophilic cation accumulation by cultured cells and binding to freeze-thaw lysed cells

In a previous study (1) we demonstrated that increased tetraphenylphosphonium (TPP) uptake by renal epithelial cells (LLC-PK1) exposed to the fungal metabolite cyclopiazonic acid (CPA) was not a result of hyperpolarization across the plasma membrane even though CPA-potentiated TPP uptake could be totally inhibited by the depolarizing agent carbonylcyanide-m-chlorophenylhydrazone (CCCP). We now demonstrate that CPA potentiates TPP accumulation by proliferating skeletal muscle (L6) and LLC-PK1 cells but not by nonproliferating primary rat hepatocytes. In LLC-PK1 cells, CPA-potentiated TPP accumulation is observed in cells at all ages. In L6 cells, CPA-potentiated TPP accumulation is maximal soon after subculturing, and as the cells age they become less sensitive to CPA until TPP accumulation by CPA-treated cells approaches that of untreated cells. The temporal change in sensitivity of L6 cells to CPA may be related to biochemical and/or metabolic changes which occur as the cells age in culture. Hepatocytes, LLC-PK1 cells, and L6 cells permeabilized by freeze-thaw lysis, all exhibit CPA-potentiated TPP partitioning, even in the presence of CCCP. This result indicates that both TPP and CPA must have access to the intracellular space in order for potentiated TPP partitioning to be observed. We hypothesize that the site of interaction between CPA and TPP is intracellular and probably associated with the cytoplasmic side of the plasma membrane and possibly the mitochondria.