AJP - Regulatory, Integrative and Comparative Physiology, Vol 249, Issue 4 490-R495, Copyright © 1985 by American Physiological Society
Proctolin induces rhythmic contractions and spikes in Limulus heart muscle
W. H. Watson 3rd and T. Hoshi
The Limulus heart is neurogenic. If the cardiac ganglion is removed, all
spontaneous contractions of the heart are abolished. Application of the
pentapeptide proctolin (greater than 1 microM) causes the deganglionated
heart muscle to beat with a frequency and amplitude slightly greater than
those of a normal heart with an intact cardiac ganglion. At a proctolin
concentration of 1 microM, rhythmic beating requires 2-10 min to develop,
and up to 1 h of continuous washing is required to reverse the effect. A
contracture often precedes the rhythmic contractions. Proctolin-induced
rhythmicity occurs in the presence of tetrodotoxin (TTX) and in Na+-free
saline. These effects of proctolin are not mediated by residual portions of
the cardiac ganglion. Contractions are inhibited by Ca2+-free EGTA saline,
CoCl2, MnCl2, and CdCl2. Proctolin causes no significant long-term changes
in the myocardial resting potential or apparent input resistance. However,
proctolin causes rhythmic 10- to 20-mV spikes that precede each contraction
of the myocardium. Production of these spikes appears to be the mechanism
by which proctolin causes rhythmic contractions in normally quiescent
deganglionated myocardium of Limulus.
