μ opioid receptors modulate net chloride secretion in the guinea pig colonic mucosa in a dual fashion

The present voltage-clamp data from guinea pig colonic mucosa indicate that μ opioid receptors both enhance and inhibit net chloride secretion stimulated by PGE 1 plus theophylline. Actually, enhancement by the μ opioid agonist DAGO (0.2 nmol/l) declines and finally turns into inhibition with increasing net chloride secretion observed in control preparations taken from the same animals. Moreover, inhibition by the μ opioid antagonist CTOP-NH 2 (100 nmol/l) increases with increasing inhibition (or decreasing enhancement) by the μ opioid agonist DAGO. Thus, the μ opioid agonist DAGO and the μ opioid antagonist CTOP-NH 2 display opposite effects on net chloride secretion only to a limited degree but otherwise show parallel, either stimulatory or inhibitory, effects. To explain this complex function of μ opioid receptors in the control of net chloride secretion, a working hypothesis is being proposed.