Neurohormonal regulation of ion transport in the porcine distal jejunum. Enhancement of sodium and chloride absorption by submucosal opiate receptors

The actions of opiates in modulating ion transport across the porcine distal jejunal mucosa were examined in vitro. Opiate receptors were functionally characterized using [D-Pen²,D-Pen⁵] -enkephalin (DPDPE), [D-Ala²,N-Me-Phe⁴,Gly⁵-ol]-enkephalin (DAMGO) and U-50,488 {trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl] -benzeneacetamide}, agonists selective for delta, mu and kappa receptor types, respectively. Serosal administration of opiate agonists produced concentration-dependent decreases in basal short-circuit current (I(sc)). DPDPE and DAMGO also increased tissue conductance (G(t)). DPDPE (EC₅₀ = 4 nM) was 7- and 86-fold more potent in decreasing basal I(sc) than DAMGO and U-50,488, respectively. U-50,488 displayed the greatest efficacy in decreasing I(sc). Serosal naloxone decreased DPDPE and DAMGO potencies under basal conditions with K(e) values of 11 and 6 nM, respectively. DPDPE- and DAMGO-induced decreases in basal I(sc) were associated with increases in net Cl absorption; in addition, DAMGO produced an increase in net Na absorption. 8-Bromocyclic AMP (0.3 mM) increased I(sc), decreased G(t) and inhibited net Na and Cl absorptive fluxes. Selective opiate agonists decreased cyclic AMP-induced elevations in I(sc) with a rank order of potency (DPDPE, EC₅₀ = 3 nM) of DPDPE > DAMGO > U-50,488. DPDPE reversed the action of cyclic AMP on I(sc) and Cl absorption but had no effect on net Na transport. Cyclic AMP-induced decreases in G(t) were not altered by DPDPE. The neuronal conduction blocker tetrodotoxin (0.1 μM) or the eicosanoid synthesis blocker 5,8,11,14-eicosatetraynoic acid (0.1 mM) inhibited the actions of opiates on basal ion transport, a result indicating that enteric neurons and eicosanoids are involved in opiate action. Phentolamine, hexamethonium and atropine did not alter peak I(sc) responses to DPDPE (0.1 μM) and DAMGO (0.1 μM), suggesting that opiate proabsorptive activity is not mediated by adrenergic or cholinergic neurons. Opiate agonists decreased mucosal I(sc) elevations in response to electrical stimulation of submucosal neurons (300 pulses at 10 Hz, 0.5 msec pulse width, 2.8 mA cm^-2). DPDPE (EC₅₀ = 10 nM) was 3- and 26-fold more potent in decreasing mucosal responses to electrical stimulation than DAMGO and U-50,488, respectively. The selective delta-opiate antagonist ICI 174,864 inhibited DPDPE and DAMGO actions with respective K(e) values of 36 and 96 nM. These results suggest that opiate receptors associated with submucosal neurons inhibit the release of nonadrenergic, noncholinergic neurotransmitters. These undefined transmitters may tonically reduce Cl absorption.