Actions of beta-endorphin peptides on the contractions of mouse diaphragm muscle

The effect of derivatives of beta-endorphin on the contractile response to indirect stimulation in mouse diaphragm muscle was studied to determine whether the action of the peptide to increase muscle tension is an opioid effect. beta-Endorphin (1-27), beta-endorphin (30-31), and a beta-endorphin (28-31) analogue all increased the amplitude of the contractions. The C-terminal peptides were more potent than beta-endorphin or beta-endorphin (1-27). The beta-endorphin (28-31) analogue, like beta-endorphin, decreased the time to peak but beta-endorphin (1-27) did not. The effect of beta-endorphin (1-27), but not that of the beta-endorphin (28-31) analogue, was blocked by naloxone. Thus, beta-endorphin acts on muscle via both opioid and nonopioid receptors.