Dual effect of [D-Pen(2), D-Pen(5)]enkephalin on ion transport in guinea pig colon

Effects of the delta-opioid receptor ligand, [D-Pen(2), D-Pen(5)]enkephalin (DPDPE) on basal and endothelin-1-induced ion secretion in guinea pig colon were investigated. Muscle-stripped segments of guinea pig colon were mounted in Ussing flux chambers and changes in the short-circuit current (I(sc)) were monitored continuously. DPDPE significantly reduced baseline I(sc) at a low dose, 1 nM; however DPDPE increased I(sc) at 10 and 100 microM. Endothelin-1 stimulated ion secretion that was unaltered in tissues pretreated with DPDPE. In guinea pig colon, delta-opioid receptor activation evoked both a proabsorptive and prosecretory response.

Cholinergic and VIP-ergic pathways mediate histamine H2 receptor-induced cyclical secretion in the guinea pig colon

Previous studies demonstrated neurally mediated recurrent increases in short-circuit current (Isc) suggestive of anion secretion in guinea pig distal colon. To determine the neural pathways involved, segments of distal colon from guinea pigs were mounted in flux chambers. In muscle-stripped or whole thickness preparations, serosal addition of the histamine H2 receptor agonist, dimaprit, caused cyclical increases in Isc, which were reduced by the chloride channel blocker, N-phenylanthranilic acid, but not by the sodium channel blocker amiloride. Dimaprit stimulated release of [3H]acetylcholine and vasoactive intestinal polypeptide (VIP) from submucosal/mucosal sheets. Dimaprit caused recurrent increases in Isc, which were significantly decreased by mecamylamine, a nicotinic receptor antagonist, and nearly abolished by the muscarinic antagonist, atropine (M3 > M1 = M2). The muscarinic antagonist, 4-diphenylacetoxy-N-methyl-piperidine methiodide (4-DAMP, M3 > M1), was more potent than pirenzepine (M1 > M3) in reducing recurrent increases in Isc. Dimaprit- and electrically evoked secretion were inhibited by the VIP antagonists [4Cl-D-Phe6, Leu17]VIP and VIP hybrid. The results suggest the involvement of VIP-ergic and cholinergic neurons utilizing nicotinic and muscarinic synapses in mediating secretion.

Vasoactive intestinal polypeptide: a transmitter in submucous neurons mediating secretion in guinea pig distal colon

The aim of the study was to investigate the role of vasoactive intestinal polypeptide (VIP) as a transmitter in regulation of epithelial function. Submucosa/mucosa segments from guinea pig distal colon were mounted in flux chambers and increases in short-circuit current were used as an index of secretion. Serosal addition of VIP, peptide histidine isoleucine, helodermin and pituitary adenylate cyclase-activating peptide caused an increase in short-circuit current. Tissues were less responsive to peptide histidine isoleucine, helodermin and pituitary adenylate cyclase-activating peptide than to VIP. In tissues without neurogenic tone, VIP(10-28), but not tetrodotoxin, reduced the response to exogenous VIP. The secretory response to VIP was attenuated by tetrodotoxin and atropine only in tissues with ongoing neural activity influencing basal rates of transport. In the presence of muscarinic blockade, neural stimulation evoked an increase in short-circuit current which was attenuated by VIP(10-28). High extracellular potassium concentrations or carbachol evoked release of endogenous VIP. VIP and VIP (10-28) displaced the binding of [125I]VIP to epithelial receptors. These results provide evidence that VIP is a neurotransmitter within the submucous plexus involved in regulation of epithelial transport in the guinea pig colon.