Renal tubular dopamine outward transfer during Na+-H+ exchange activation by α1- and α2-adrenoceptor agonists

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Abstract
1 The present work describes the effects of inhibitors (amiloride and ethylisopropylamiloride) and activators (quinoxaline and phenylephrine) of the Na+-H+ exchanger on the outflow of dopamine in rat kidney slices loaded with l-dihydroxyphenylalanine (l-DOPA). 2 Incubation of kidney slices loaded with 50 μm l-DOPA in the presence of increasing concentrations of amiloride or ethylisopropylamiloride (EIPA) resulted in a concentration-dependent decrease in the outflow of newly-formed dopamine; the IC50 value for EIPA was 5.6 ± 0.3 μm. Phenylephrine and quinoxaline were found to produce a concentration-dependent increase in the outflow of newly-formed dopamine; the EC50 values for the phenylephrine and quinoxaline were, respectively, 0.9 ± 0.1 and 0.08 ± 0.01 μm. 3 The facilitatory effect of phenylephrine on the outflow of dopamine was found not to be affected by yohimbine (100 nm), but was abolished by prazosin (1 μm), whereas that of quinoxaline was found to be selectively antagonized by yohimbine (100 nm), but not by prazosin (1 μm); EIPA (10 μm) was also found to abolish the effect of both phenylephrine and quinoxaline. The facilitatory effect of quinoxaline was also found to be reduced by 42–48% and 56–78% by, dibutyryl adenosine cyclic 3′,5′-monophosphate (dibutyryl cyclic AMP; 250 μm) and forskolin (10 μm), respectively, but not by the protein kinase C (PKC) inhibitor, (+)-sphingosine (10 μm). In contrast, (+)-sphingosine (10 μm) was found to antagonize markedly (43–69% reduction) the facilitatory effect of phenylephrine; dibutyryl cyclic AMP (250 μm) and forskolin (10 μm) were also found to reduce significantly the facilitatory effect of phenylephrine, by 42–53% and 44–59% respectively. 4 A synergistic effect between α1- and α2-adrenoceptors was observed for submaximal concentrations of quinoxaline (50 nm) and phenylephrine or submaximal concentrations of phenylephrine (0.5 μm) and quinoxaline, but not for maximal effective concentrations of either agonist. Dibutyryl cyclic AMP (250 μm) or forskolin (10 μm) produced a marked decrease (35–85% reduction) of the synergistic effect between phenylephrine and quinoxaline. The addition of phorbol 12,13-dibutyrate (PDBu; 500 nm) was found not to alter the outflow of newly-formed dopamine, but did potentiate (18–42% increase) the facilitatory effect of quinoxaline on the amine outflow. This effect was found to occur for submaximal concentrations of quinoxaline (10, 50 and 100 nm) and found to be antagonized by (+)-sphingosine (10 μm). In contrast, PDBu (500 nm) was found not to potentiate the facilitatory effect of phenylephrine on dopamine outflow. 5 In conclusion, inhibition of the Na+-H+ antiport by amiloride and EIPA results in considerable reduction in the outflow of newly-formed dopamine, whereas the activation of this mechanism by both phenylephrine and quinoxaline results in facilitation of the outflow of dopamine; this effect is selectively reversed by α1- and α2-adrenoceptor antagonists and EIPA. The synergistic effect between quinoxaline and phenylephrine may be related to an amplification of a reaction at a given point in the post-receptor transducing pathway.