Effects of dopamine on ion transport across rat colon
Dopamine (5.10-6 5.10-4 mol.l-1) when added serosally induced a concentrationdependent decrease in short-circuit current (Isc) across rat distal and proximal colon. This response was preceded by a transient and inconsistent increase in Isc. A part of the catecholamine action is mediated by subepithelial actions sites as it was indicated by the ... inhibiton of dopamine effects by the cyclooxygenase inhibitor, indomethacin (10-6 mol.l-1), and the neuronal blocker, tetrodotoxin (10-6 mol.l-1). The positive Isc evoked by dopamine was due to chloride secretion as indicated when both Cl- and HCO3- were substituted (using HEPES as buffer) and when of the basolateral membrane was depolarizing by high potassium concentration.The negative Isc evoked by dopamine was due to potassium secretion. This was demonstrated by unidirectional flux experiments. 86Rb+ efflux experiments revealed a redistribution of cellular K+ efflux in favour of the apical K+ conductance in the presence of dopamine. The negative Isc evoked by dopamine was inhibited by the blocker of apical K+ channels, quinine (10-3 mol.l-1), indicating that a stimulation of K+ secretion underlies the measured current. Both the alpha-adrenoceptor blocker phentolamine (10-4 mol.l-1) and as well as inhibitors of D2-like receptors such as L-741,626 (10-5 mol.l-1) and L-745,870 (10-5 mol.l-1) inhibited the dopamine response. All these observations indicate similarities between dopamine and the other catecholamines derivatives in their effect on ions transport in rat colon.