The voltage-dependent blocking effect of phalloidin on the delayed potassium current of voltage-clamped frog skeletal muscle fibres

Toxins in the characterization of potassium channels

Several recently characterized toxins (apamin, charybdotoxin, dendrotoxin and noxiustoxin) are proving invaluable for establishing what kinds of potassium channel are expressed in neurones, and what the roles of the channels might be.

[50 years of phalloidine: its discovery, characterization and current and future applications in cell research]

Phalloidin, like the later-detected phallotoxins, consists of a cyclic heptapeptide backbone, the ring being crosslinked by a 2'-indolylthioether moiety (tryptathionine). After intraperitoneal administration--not per os--it will, after a short time, damage the liver specifically, presumably in consequence of its very tight binding to F-actin preventing its dissociation. This affinity can be utilized for a sensitive visual identification of F-actin by using fluorescent derivatives.

Antamanide antagonizes the phalloidin-induced negative inotropic effect and blocks voltage dependently the fast outward K+ current in voltage-clamped frog muscle fibres

The effects of antamanide (10(-14)-10(-5) M) and N-acetyl-secophalloidin (10(-7)-5 X 10(-3) M) a neutral non-toxic derivative of phalloidin, were tested on voltage-clamped single frog muscle fibres. Antamanide protected muscle fibres against the negative inotropic effect of phalloidin but blocked the fast potassium permeability in the same concentration range and the same voltage-dependent manner as did phalloidin. N-Acetyl-secophalloidin exhibited a strongly attenuated blocking effect on K+ permeability in a 1,000-fold higher concentration range than phalloidin. Neither antamanide nor N-acetyl-secophalloidin affected the contractile properties. These results suggest the existence in the frog muscle membrane of a receptor with two sites for phalloidin and antamanide which acts on potassium conductance.

What we have learned from phalloidin

In contrast to popular opinion phallotoxins do not play a role in poisoning with Amanita phalloides when the fungi are ingested orally. All toxic properties of this mushroom are due to amatoxins which, in contrast to the phallotoxins, are absorbed upon ingestion. Nearly all experiments on intact animals were performed by parenteral injection of phalloidin and therefore, most of these are unsuitable for practical consideration. In the present survey, however, a series of important findings are discussed, which provide insight into various functions of liver cells. When present in the blood, phalloidin and other phallotoxins are selectively taken up by hepatocytes. No other types of cells are sensitive to the toxin. No extrahepatic tissue is primarily impaired by phalloidin. Phalloidin cannot be degraded by peptidases or by proteases occurring in animals. Phalloidin is therefore a useful model substance for studies on the uptake of cyclopeptides by liver cells. The carrier system responsible for the active uptake of phalloidin can also translocate antamanide and several cyclic modifications of somatostatin. Phallotoxins bind with high affinity to microfilamentous structures, in particular to F-actin [Govindan et al., Naturwissenschaften, 59 (1972) 521-522] whereas phallotoxins are not bound to the monomer (G-actin). With respect to the strong organotropism of phallotoxins, intravenously injected phalloidin binds preferentially to microfilamentous F-actin of hepatocytes. Phalloidin is therefore a tool for inactivation of microfilamentous functions specifically in liver cells, and is suitable as a prototype of a cholestatic agent. In perfused livers arrest of bile flow is the earliest effect seen after addition of the toxin. In cells from other tissues phalloidin is only toxic when applied by intracellular microinjection. Phalloidin poisoning has been often used as a model for liver damage in the testing of hepatoprotective drugs. This substance is, however, not useful for such studies since the mechanism of phalloidin poisoning is too specific for interpretation in the sense of general liver protection.

The apamin-sensitive potassium current in frog skeletal muscle: its dependence on the extracellular calcium and sensitivity to calcium channel blockers

Slow outward potassium currents were recorded in isolated frog skeletal muscle fibres using the double mannitol-gap voltage-clamp technique. Detubulated fibres failed to generate a slow outward current, and apamin had no effect on the remaining current. The maximum blocking effect of organic and inorganic Ca2+-channel blockers on the slow outward channels of intact fibres was larger than that of apamin. Apamin failed to induce an additional block when applied after Ca2+-channel blockers. In a low-Ca2+ solution (OCa, EGTA 1 mM) the slow outward current was slightly increased and the blocking effect of apamin was enhanced. A Ca2+-rich solution (Ca2+ X 10) increased the slow outward current and the blocking effect of apamin was drastically reduced. It is concluded that the apamin-sensitive current which is a component of the slow outward K+ current is located in the tubular membrane. Its activation seems barely dependent on the Ca2+ influx via the slow inward Ca2+ current. Apamin-receptor binding appears to be dependent on the extracellular Ca2+ concentration. Blockade of slow outward current by Ca2+-channel blockers is likely to be the result of a direct action on the slow K+ permeability rather than a consequence of Ca2+ channel inhibition.