Renal proximal tubular cells have been shown to express aromatic l-amino acid decarboxylase (l-AAAD), which converts l-dopa into dopamine and 5-hydroxytryptophan [(OH)Trp] into 5-hydroxytryptamine (5-HT; serotonin). Because 5-HT receptors have been demonstrated in proximal cells, we hypothesized that 5-HT may act as an autocrine/paracrine modulator of proximal transport. We evaluated this possibility in opossum kidney (OK) cells, a renal epithelial cell line with a proximal phenotype expressing 5-HT1B receptors, and in intact anaesthetized rats. 5-HT synthesis by OK cells increased with incubation time and (OH)Trp concentration, and was abolished by benserazide, an l-AAAD inhibitor. 5-HT reversed parathyroid hormone (PTH)-induced cAMP accumulation in a pertussis toxin-sensitive manner and reduced the PTH inhibition of Pi uptake without affecting the NaPi-4 mRNA level. The effects of 5-HT on cAMP generation and Na–Pi co-transport were reproduced by (OH)Trp, except in the presence of benserazide, and by l-propranolol and dihydroergotamine, two 5-HT1B receptor agonists. In rats, (OH)Trp and dihydroergotamine decreased fractional Pi excretion. Benserazide abolished the effect of (OH)Trp but not that of dihydroergotamine. In conclusion: (i) locally generated 5-HT blunts the inhibitory effect of PTH on Na–Pi co-transport in OK cells; (ii) endogenous 5-HT decreases Pi excretion in rats; and (iii) 5-HT is a paracrine modulator involved in the physiological regulation of renal Pi transport.

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