L‐Proline as a glutamate antagonist at a crustacean neuromuscular junction

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Abstract
The fast as well as the slow contractions of the adductor muscle in the claw of Procambarus clarkii are inhibited by L‐proline. This inhibition is dose dependent and decreases with increasing frequency of stimulation of the “slow” fiber. Contractions caused by perfusing the adductor muscle with L‐glutamate solutions are also inhibited by L‐proline. The inhibiting potency of L‐proline is small; the effective concentration of this amino acid is 50–100 times that of the L‐glutamate applied. It was postulated that the inhibitory effect of L‐proline is based on competition for excitatory receptor sites of L‐glutamate, which causes depolarization and contraction, and L‐proline, which lacks these actions. Theoretical considerations suggested a linear relationship between the stimulating L‐glutamate and the just‐inhibiting L‐proline concentrations. Experimental evidence supported this model.