Co-existence of P(2Y)- and PPADS-insensitive P(2U)-purinoceptors in endothelial cells from adrenal medulla

Abstract

1 We have studied the effects of purinoceptor stimulation on Ca2+ signals in bovine adrenomedullary endothelial cells. [Ca2+]i was determined with the fluorescent probe fura‐2 both in population samples and in single, isolated, endothelial cells in primary culture and after subculturing.

2 In endothelial cells, maintained in culture for more than one passage, several purinoceptor agonists elicited clear [Ca2+]i transient peaks that remained in the absence of extracellular Ca2+. Adenosine 5′‐triphosphate (ATP) and uridine 5′‐triphosphate (UTP) were equipotently active, with EC50 values of 8.5 ± 0.9 μm and 6.9 ± 1.5 μm, respectively, whereas 2‐methylthioadenosine 5′‐triphosphate (2MeSATP), adenosine 5′‐(α, β‐methylene)triphosphate (α, β‐MeATP) and adenosine(5′)tetraphospho(5′)adenosine (AP4A) were basically inactive. Adenosine 5′‐O‐(2‐thiodiphosphate) (ADPβS) was a weak agonist. The apparent potency order was UTP = ATP > ADPβS > > 2MeSATP > α, β‐MeATP.

3 Cross‐desensitization experiments revealed that UTP or ATP, added sequentially at concentrations of maximal effect, could completely abolish the [Ca2+]i response to the second agonist. ADPβS exerted only a partial desensitization of the response to maximal ATP, in accordance with its lower potency in raising [Ca2+]i.

4 The effect on [Ca2+]i of 100 μm ATP in subcultured cells was reduced by only 25% with 100 μm suramin pretreatment and was negligibly affected by exposure to 10 μm pyridoxalphosphate‐6‐azophenyl‐2′,4′‐disulphonic acid (PPADS). The concentration‐effect curve for ATP was not significantly affected by PPADS, but was displaced to the right by a factor of 6.5 by 100 μm suramin.

5 In primary cultures, clear [Ca2+]i responses were elicited by 2MeSATP. Suramin totally and selectively blocked 2MeSATP responses, whereas UTP‐evoked [Ca2+]i transients were mainly unaffected by suramin or PPADS. Over 80% of cells tested showed responses to both 2MeSATP and UTP. The [Ca2+]i response to UTP was not desensitized in the presence of 2MeSATP.

6 ATP and UTP stimulated the release of preloaded [3H]‐arachidonic acid ([3H]‐AA), both in the presence and in the absence of extracellular Ca2+, by approximately 135% with respect to basal levels. Suramin and PPADS enhanced, rather than inhibited, the [3H]‐AA releasing effect of ATP by 2.5 times. Suramin also potentiated the effect of the calcium ionophore A23187.

7 These results indicate that endothelial cells from adrenomedullary capillaries co‐express both P2Y‐and P2U‐purinoceptors. P2Y‐purinoceptors are lost in culture with the first passage of the cells. The P2U‐purinoceptor subtype present in these cells is insensitive to PPADS and thus similar to that found in aortic endothelial cells.