Levamisole and ryanodine receptors. I: A contraction study in Ascaris suum

Cholinergic anthelmintics (like levamisole) are important drugs but resistance with reduced responses by the parasite to these compounds is a concern. There is a need to study and understand mechanisms that affect the amplitude of the responses of parasites to these drugs. In this paper, we study interactions of levamisole and ryanodine receptors on contractions of Ascaris suum body muscle flaps. In our second paper, we extend these observations to examine electrophysiological interactions of levamisole, ryanodine receptors (RyRs) and AF2. We report that the maximum force of contraction, g(max), was dependent on the extracellular concentration of calcium but the levamisole EC(50) (0.8 microM) was not. The relationship between maximum force of contraction and extracellular calcium was described by the Michaelis-Menten equation with a K(m) of 1.8mM. Ryanodine inhibited g(max) without effect on EC(50); ryanodine inhibited only 44% of the maximum contraction (K(i) of 40 nM), revealing a ryanodine-insensitive component in the levamisole excitation-contraction pathway. Dantrolene had the same effect as ryanodine but was less potent. The neuropeptide AF2 (1 microM) decreased the levamisole EC(50) to 0.2 microM without effect on g(max); 0.1 microM ryanodine and 100 microM dantrolene, inhibited the g(max) of the AF2-potentiated levamisole response. High concentrations of caffeine, 30 mM, produced weak contraction of the body-flap preparation. Caffeine behaved like ryanodine in that it inhibited the maximum force of contraction, g(max), without effects on the levamisole EC(50). Thus, RyRs play a modulatory role in the levamisole excitation-contraction pathway by affecting the maximum force of contraction without an effect on levamisole EC(50). The levamisole excitation-contraction coupling is graded and has at least two pathways: one sensitive to ryanodine and one not.