Purine and pyrimidine nucleotide receptors in the apical membranes of equine cultured epithelia.

1. The short circuit current (ISC) technique was used to quantify electrolyte transport by equine cultured sweat gland epithelia. Adenosine 5'-triphosphate (ATP) and certain related compounds, caused transient increases in ISC when added to the apical solution. The order of potency was uridine triphosphate (UTP) > ATP > ADP > > AMP = adenosine. 2. The responses to apical nucleotides were due to chloride and bicarbonate secretion and were reduced in pertussis toxin-treated cells. P2-receptors sensitive to uridine 5'-triphosphate (UTP), that interact with inhibitory G proteins, therefore appear to be present in the apical membrane. 3. Responses to ATP and UTP were reduced in cells loaded with BAPTA, a calcium chelator. BAPTA attenuated the response to ATP more than the response to UTP suggesting that these nucleotides may not act via a common pathway. 4. Cross-desensitization experiments indicated that two populations of UTP-sensitive receptor were present. One was sensitive to UTP and ATP, whereas the second was sensitive only to UTP. Uridine diphosphate appeared to activate the ATP-insensitive receptor population selectively. 5. These data suggest that apical pyrimidinoceptors may be expressed by these cells. The physiological role of these receptors is unknown but they may allow the autocrine regulation of epithelial function.