Dual effects of fluoride on the action potential, contraction, and membrane currents in frog atrial muscle.

The effects of NaF (10(-7) - 10(-2) M) on the membrane potential, current, and contractile tension in bullfrog atrial muscle were investigated under voltage-clamped or unclamped conditions. NaF showed two opposite actions, positive and negative inotropic effects, depending on the concentration of the drug. NaF in lower concentrations produced a marked increase in contractility, a moderate prolongation of action potential, an augmentation of calcium response, and an increase of ICa-dependent tension. Contrarily, NaF in higher concentrations produced an inhibition of contractile tension associated with a shortening of action potential, an increase in potassium conductance, and an inhibition of calcium response. These results indicate that NaF affects the contractile response by dual mechanisms.