The neuropeptide proctolin acts directly on Limulus cardiac muscle to increase the amplitude of contraction.

The pentapeptide proctolin increases the amplitude of contraction but not heart beat frequency of the isolated heart of Limulus polyphemus. It acts directly on the heart muscle and has no effects on the neurones of the cardiac ganglion or on the cardiac neuromuscular EJPs. A peptide with molecular weight, enzymatic susceptibilities and physiological effects similar to those of proctolin occurs in the Limulus cardiac ganglion. It is suggested that proctolin, or a family of proctolin-like peptides, may modulate muscle contraction in more than one subphylum of the Arthropoda.