Evidence that alpha-dihydrograyanotoxin II does not bind to the sodium gate

Biochemical interaction of six OP delayed neurotoxicants with several neurotargets

Five organophosphorous insecticides: Leptophos, EPN, Cyanofenphos, trichloronate and salithion proved to cause irreversible ataxia not only to chicken but also to mice and sheep. TOCP was included as a reference. Cyanofenphos blocked the catecholamine B-receptor binding activity with 3H-norepinephrine at a level similar to that of the specific inhibitor propranolol in the mouse heart preparation. In the lamb heart preparation, the B-receptor was more sensitive to Leptophos, salithion and TOCP than to propranolol. The six compounds and their oxons were screened for their in-vitro inhibition to monamine oxidase (MAO), acetyl cholinesterase (AChE) and neurotoxic esterase (NTE) in the brain of either mouse, lamb or chicken. It is believed that their AChE inhibition stands for their acute toxicity, while NTE inhibition is responsible for their paralytic ataxia.

Sensitivity of excitable and inexcitable membranes to alpha-dihydrograyanotoxin II

A variety of excitable and inexcitable preparations have been examined for their sensitivity to the depolarizing action of alpha-dihydrograyanotoxin II. alpha-Dihydrograyanotoxin II was effective in producing membrane depolarization in giant axons from the crayfish (Procambarus clarki) and sartorius muscle fibers from the frog (Rana pipiens) and the salamander (Taricha torosa). The toxin-induced depolarization was reversed by lowering the external sodium concentration. High concentrations of tetrodotoxin and saxitoxin were also effective in reversing the depolarization in the various grayanotoxin-sensitive preparations studied except in the salamander sartorius muscle, in which sodium-dependent action potentials were highly resistant to tetrodotoxin and saxitoxin. In contrast, alpha-dihydrograyanotoxin II was ineffective in changing the resting membrane potential in the muscle fibers of the crayfish (Procambarus clarki and Orconectes virilis) and in the salivary gland giant cells from Drosophila virilis. Of the preparations that were examined, only the membranes that produce a sodium-dependent action potential are sensitive to the depolarizing action of alpha-dihydrograyanotoxin II.

Pharmacologic characterization of the Na+ ionophores in L6 myotubes

We present a pharmacologic characterization of the Na+ ionophores present in L6 myotubes in vitro. Action potentials are abolished by replacement of the external Na+ by Tris. The amplitude of the action potential is generally resistant to high concentrations of tetrodotoxin (10(-5) M) and saxitoxin (10(-6 M), but the effect of these agents is highly variable. Veratridine (10(-4 M) consistently induces, as a short-term effect, a marked prolongation of the falling phase of the action potential. As a long-term effect, veratridine consistently induces a Na+-dependent reduction in the resting potential of the cell. The effects of veratridine on the action potential are not antagonized by tetrodotoxin or saxitoxin. However, the effects of veratridine on the resting potential are strongly antagonized by tetrodotoxin (10(-5) M) and fully inhibited by saxitoxin (10(-6) M). Significantly, under conditions where saxitoxin has fully inhibited the effects of veratridine on the resting potential, the myotubes are capable of generating overshooting action potentials. In contrast to their sensitivity to veratridine, L6 myotubes are insensitive to 10(-5) M alpha-dihydro-grayanotoxin-II. These results are discussed in the contexts of developmental significance and current views about Na+ ionophores.