A characterization of dynorphin-(1-13) on the guinea pig ileal longitudinal muscle

Using naloxone as the antagonist, a comparison of pA2 values obtained from the guinea pig ileal longitudinal muscle preparation revealed that the pA2 value for dynorphin-(1-13) was significantly different from that of the pure narcotic agonists such as morphine, beta h-endorphin, Leu- and Met-enkephalin and that of the mixed agonist-antagonists such as nalorphine. In addition, no cross-tolerance to dynorphin-(1-13) could be demonstrated whereas a pronounced cross-tolerance existed for other opioid peptides on a ileal strip made tolerant to morphine by implantation of morphine pellets to the guinea pig for 72 h. Thus, dynorphin-(1-13) would appear to have a unique pharmacology on this peripheral opioid receptor preparation quite distinct from that of other known opioid peptides.