BACKGROUND
extracellular atp plays an important role in catecholamine release from adrenal chromaffin cells, either facilitating cholinergic stimulation via ionotropic  purinoceptors or inhibiting evoked release through a delayed action on metabotropic  purinoceptors .
the latter may provide the basis for an auto-inhibitory feedback loop involving both autocrine and paracrine interactions.
there are at least seven p2x receptor subunits encoded by distinct genes, which may form homo- or heterotrimeric ionotropic purinoceptors; functional p2x receptors are ca2+-permeable and provide an important ca2+ influx pathway, both in neurons and other cell types .
the presence of p2x receptor subtypes in chromaffin cells is species-dependent  <cit>  .
thus, rat chromaffin cells have either been reported to lack p2x receptors  <cit>  or to show a variable expression of p2x receptor subtypes   <cit> ; there is also some evidence that these cells express p2x <dig> receptors in aged animals  <cit> .
guinea-pig chromaffin cells seem to express p2x <dig> receptors  <cit> , although functional studies point strongly to the presence of p2x2-like receptors  <cit> .
there are no studies reporting the expression of specific p2x receptor subtypes in bovine chromaffin cells.
however, the presence of p2x receptors in these cells has been suggested by functional studies involving mostly cytosolic free ca2+ measurements  <cit> .
atp-evoked inward currents have been detected in a limited fraction of the cells  <cit> .
the prevailing p2y receptor subtype in bovine chromaffin cells seems to be an utp-sensitive, gi/o-coupled p2y receptor  <cit> .
atp and other purinergic agonists evoke catecholamine release from either whole glands or isolated chromaffin cells  <cit> .
this action is strictly ca2+-dependent, suggesting that it might be mediated by either ca2+ influx through the receptor-associated pores, opening of voltage-sensitive ca2+ channels or both.
studying p2x receptor-mediated modulation of chromaffin cell function has been made difficult by the lack of specific agonists and antagonists of p2x receptor subtypes.
2-mesatp, for example, has been classically regarded as a p2y receptor agonist; more recently, however, several p2y receptor subtypes have been found to be insensitive to the atp derivative  <cit> .
moreover, 2-mesatp is now known to activate p2x receptor subtypes  <cit> .
there is evidence that 2-mesatp behaves as a p2x agonist in guinea-pig chromaffin cells, as assessed by its ability to evoke inward currents  <cit> .
previous studies provided conflicting evidence regarding the action of 2-mesatp on cytosolic free ca2+ concentration  in bovine chromaffin cells, with one study reporting sizeable responses  <cit>  and another feeble and sporadic responses  <cit> .
whether 2-mesatp might discriminate between p2x and p2y receptors in chromaffin cells remains unknown.
in this work, we aimed at establishing 2-mesatp as a specific p2x receptor agonist in bovine chromaffin cells by single-cell calcium imaging.
we then investigated the effects of 2-mesatp and utp on catecholamine release, aiming at clarifying the relative role of p2x and ca2+-mobilizing p2y receptors.
RESULTS
i rises evoked by purinoceptor agonists
i changes evoked by atp receptor agonists were monitored by digital fluorescence imaging of the f340/f <dig> fura- <dig> fluorescence ratio .
only cells that displayed sizeable i responses to  <dig> μm  <dig> -dimethyl-4-phenylpiperazinium iodide , an established agonist of acetylcholine nicotinic receptors in chromaffin cells, were considered for the study.
to this end, cells were perifused at the tail of the experiments with  <dig> μm dmpp for brief periods of time.
our first basic protocol was to stimulate chromaffin cells with atp, utp and 2-mesatp in order to investigate whether utp-sensitive cells might also be sensitive to 2-mesatp.
cells were typically subjected to ~ <dig> s pulses of solutions containing each purinoceptor agonist and washed extensively for at least  <dig> min to minimize receptor desensitization at the time of the next challenge.
typical pseudocolor fura- <dig> fluorescence images are depicted in fig.
<dig>  representative time courses of i changes for selected chromaffin cells are depicted in e. resting i was low prior to stimulation .
challenging cells with  <dig> μm atp elicited sizeable, albeit variable peak i responses from a large pool of chromaffin cells .
it is noteworthy that some cells did not respond to atp .
several cells  exhibited pronounced i rises in response to atp but not  <dig> μm utp.
exposure to  <dig> μm 2-mesatp invariably increased the i in these cells.
in contrast, several cells  responded to both atp and utp.
for the most part these cells did not respond at all to 2-mesatp.
a fraction of the atp-sensitive cells responded both to utp and 2-mesatp .
the statistical assessment of the i responses of chromaffin cells to utp and 2-mesatp is presented in fig.
1f in the form of a 3d histogram.
the column at the origin of the histogram  depicts cells that did not respond both to utp and 2-mesatp ; atp evoked i rises in  <dig> of these cells.
cells displaying a variable response to 2-mesatp and no utp response  accounted for approximately 30% of the atp-sensitive pool; the mean size of the 2-mesatp-evoked i responses was not significantly different from the atp responses in these cells .
utp-sensitive cells  were for the most part  insensitive to 2-mesatp, although a minority  displayed a mixed sensitivity.
the mean sizes of the utp and atp responses in utp-sensitive/2-mesatp-insensitive cells were  <dig>  ±  <dig>  and  <dig>  ±  <dig> , respectively .
none of the cells responding to 2-mesatp in presence of ca2+ responded in the virtual absence of extracellular ca2+ , suggesting that the atp derivative is a specific p2x receptor agonist in bovine chromaffin cells.
we have previously reported that suramin blocked the atp-evoked i responses in cells lacking atp responses in the virtual absence of extracellular ca2+; suramin did not affect the utp responses, suggesting that it behaves as a specific p2x receptor antagonist  <cit> .
we have now extended this conclusion to pyridoxalphosphate-6-azophenyl- 2',4'-disulphonic acid .
indeed, the drug blocked the 2-mesatp -evoked i rises in all cells tested .
moreover, in parallel experiments, ppads did not affect the utp-evoked i rises in cells displaying no response to 2-mesatp .
effects of purinergic agonists on catecholamine secretion
both atp and 2-mesatp evoked catecholamine secretion in a dose-dependent way, as assessed amperometrically from cell batches .
in these experiments cells were challenged with brief pulses of either agonist at increasing concentrations, being allowed to rest for ~ <dig> min between successive applications.
the concentration-response curves provided in fig.
3b suggest that 2-mesatp might be a more potent agonist for purinergic receptors .
utp  did not evoke catecholamine secretion.
ppads  abolished 2-mesatp -evoked secretion .
challenging the cells with  <dig> μm 2-mesatp in the virtual absence of extracellular calcium failed to elicit secretion , indicating that the stimulatory action of the purinoceptor agonist is strictly calcium-dependent.
this action may be mediated mostly by activation of voltage-sensitive ca2+ channels, since  <dig>  mm cd2+  reduced 2-mesatp-evoked secretion by  <dig>  ±  <dig> % ; cd2+ inhibited high k+ -evoked catecholamine secretion by ~98% .
replacing extracellular na+ for n-methyl-d-glucamine  abolished 2-mesatp-evoked secretion ; tetrodotoxin  had a modest  inhibitory effect on evoked secretion, suggesting that na+ influx through ttx-sensitive, voltage-dependent na+ channels played a minimal role.
discussion
we found that ~50% of the cells examined by fluorescence imaging exhibited i responses to atp and that, among these, ~40% yielded positive responses to 2-mesatp.
none of the 2-mesatp-sensitive cells responded in the virtual absence of extracellular ca2+.
since cell exposure to egta-containing solutions caused minimal depletion of intracellular ca2+ stores  <cit> , this implies that 2-mesatp elicits i rises by stimulating ca2+ influx.
hence, 2-mesatp behaves as a specific p2x receptor agonist in bovine chromaffin cells.
the 2-mesatp-sensitive p2x receptors in these cells are blocked by low concentrations of suramin  <cit>  and ppads .
in agreement with a previous study  <cit> , we show here that 2-mesatp is a stronger secretagogue than atp.
this might suggest that atp is a weaker agonist of the p2x receptor, in apparent disagreement with the information provided by our former i study  <cit>  where the order of potency for purinergic agonists was atp > 2-mesatp >> αβ-meatp, adp , amp.
we note, however, that activation of a p2y receptor coupled to gi/o inhibits exocytotic release of catecholamines from rat chromaffin cells downstream voltage-sensitive ca2+ channels and cytosolic ca2+ elevation  <cit> .
moreover, activation of p2y receptors inhibits voltage-sensitive ca2+ channels via gi/go proteins and, thus, depresses ca2+-dependent exocytosis  <cit> .
subclassification of p2x receptors in bovine chromaffin cells was clearly outside the objectives of the present functional study.
nonetheless, taking into account the sensitivity to inhibitors, as well as the above order of potency for purinergic agonists, p2x receptors expressed in bovine chromaffin cells appear to have the pharmacological profile of p2x <dig> or p2x5receptors  <cit> .
as a note of caution we emphasize that p2x receptors in situ most probably are composed of different unit subtypes with a pharmacology distinct from that of the homomeric receptors cloned and transfected to cells, which difficults any attempt at identifying p2x receptor subtypes on the basis of functional studies.
we have also found that ~50% of the atp-sensitive cells yielded positive responses to utp.
thus, since utp is a specific agonist for p2y receptors in bovine chromaffin cells  <cit> , approximately half of the atp-sensitive cell population expressed functional ca2+-mobilizing p2y receptors.
this agrees with the finding by ennion et al.
that these cells express an as yet unidentified utp-sensitive, gi/o-coupled p2y receptor  <cit> .
it is noteworthy that the authors also provided evidence for the presence of a gi/o-linked, adenine nucleotide-specific p2y <dig> receptor and detected transcripts for p2y <dig> receptors by rt-pcr analysis.
accordingly, our results suggest that bovine chromaffin cells express a ca2+-mobilizing, utp-insensitive p2y receptor.
indeed, ~20% of the atp-sensitive pool did not respond to either 2-mesatp or utp.
taking into account that p2x receptors are fully sensitive to 2-mesatp in bovine chromaffin cells, the most likely explanation for this finding is that, indeed, some cells express a ca2+-mobilizing, utp-insensitive p2y receptor.
chromaffin cells exist in the form of two major phenotypes, epinephrine-secreting  and norepinephrine-secreting  cells  <cit> .
our previous work  <cit>  showed that p2x receptors and utp-sensitive p2y receptors are asymmetrically distributed among two distinct cell pools .
the present data are fully compatible with this finding.
indeed, approximately 30% of the atp-sensitive cells yielded positive responses to 2-mesatp but not to utp, suggesting that they express p2x receptors preferentially.
moreover, approximately 40% of the atp-sensitive cells yielded positive responses to utp but not to 2-mesatp, suggesting that they express ca2+-mobilizing p2y receptors only.
a small fraction of the atp-sensitive cells examined in this study responded both to utp and 2-mesatp, suggesting that they co-express p2x and p2y receptors .
there is evidence that stimulated ca2+ influx is strongly coupled to catecholamine release; in contrast, ca2+ release from intracellular stores appears to be loosely coupled to secretion  <cit> .
the present data support this concept, inasmuch as p2x receptor activation by 2-mesatp stimulated ca2+ influx and evoked catecholamine release, whereas activation of p2y receptors by utp elicited ca2+ release from intracellular stores but did not evoke secretion.
we found that, while 2-mesatp-induced secretion was strictly na+-dependent, the voltage-sensitive na+ channel blocker ttx  <cit>  inhibited evoked release by a modest 20%.
this suggests that voltage-sensitive ca2+ channels were mostly activated by the depolarization brought about by na+ influx across p2x receptor pores.
that voltage-sensitive ca2+ channels are involved in 2-mesatp-induced secretion is suggested by the strong inhibitory effect of cd2+.
indeed, cd2+ blocks ca2+ influx across these channels without affecting p2x receptors  <cit> .
CONCLUSIONS
2-mesatp is a specific p2x purinoceptor agonist and a potent secretagogue in bovine chromaffin cells.
activation of p2x receptors stimulates ca2+ influx mainly via voltage-sensitive ca2+ channels.
for the most part, these are activated by the depolarization brought about by na+ influx across p2x receptor pores.

 extracellular atp plays an important role in catecholamine release from adrenal chromaffin cells, either facilitating cholinergic stimulation via ionotropic purinoceptors or inhibiting evoked release through a delayed action on metabotropic purinoceptors. the latter may provide the basis for an auto-inhibitory feedback loop involving both autocrine and paracrine interactions. there are at least seven p2x receptor subunits encoded by distinct genes, which may form homo- or heterotrimeric ionotropic purinoceptors; functional p2x receptors are ca2+-permeable and provide an important ca2+ influx pathway, both in neurons and other cell types. the presence of p2x receptor subtypes in chromaffin cells is species-dependent. thus, rat chromaffin cells have either been reported to lack p2x receptors or to show a variable expression of p2x receptor subtypes; there is also some evidence that these cells express p2x receptors in aged animals. guinea-pig chromaffin cells seem to express p2x receptors, although functional studies point strongly to the presence of p2x2-like receptors. there are no studies reporting the expression of specific p2x receptor subtypes in bovine chromaffin cells. however, the presence of p2x receptors in these cells has been suggested by functional studies involving mostly cytosolic free ca2+ measurements. atp-evoked inward currents have been detected in a limited fraction of the cells. the prevailing p2y receptor subtype in bovine chromaffin cells seems to be an utp-sensitive, gi/o-coupled p2y receptor. atp and other purinergic agonists evoke catecholamine release from either whole glands or isolated chromaffin cells. this action is strictly ca2+-dependent, suggesting that it might be mediated by either ca2+ influx through the receptor-associated pores, opening of voltage-sensitive ca2+ channels or both. studying p2x receptor-mediated modulation of chromaffin cell function has been made difficult by the lack of specific agonists and antagonists of p2x receptor subtypes. 2-mesatp, for example, has been classically regarded as a p2y receptor agonist; more recently, however, several p2y receptor subtypes have been found to be insensitive to the atp derivative. moreover, 2-mesatp is now known to activate p2x receptor subtypes. there is evidence that 2-mesatp behaves as a p2x agonist in guinea-pig chromaffin cells, as assessed by its ability to evoke inward currents. previous studies provided conflicting evidence regarding the action of 2-mesatp on cytosolic free ca2+ concentration in bovine chromaffin cells, with one study reporting sizeable responses and another feeble and sporadic responses. whether 2-mesatp might discriminate between p2x and p2y receptors in chromaffin cells remains unknown. in this work, we aimed at establishing 2-mesatp as a specific p2x receptor agonist in bovine chromaffin cells by single-cell calcium imaging. we then investigated the effects of 2-mesatp and utp on catecholamine release, aiming at clarifying the relative role of p2x and ca2+-mobilizing p2y receptors.  i rises evoked by purinoceptor agonists i changes evoked by atp receptor agonists were monitored by digital fluorescence imaging of the f340/f fura- fluorescence ratio. only cells that displayed sizeable i responses to μm -dimethyl-4-phenylpiperazinium iodide, an established agonist of acetylcholine nicotinic receptors in chromaffin cells, were considered for the study. to this end, cells were perifused at the tail of the experiments with μm dmpp for brief periods of time. our first basic protocol was to stimulate chromaffin cells with atp, utp and 2-mesatp in order to investigate whether utp-sensitive cells might also be sensitive to 2-mesatp. cells were typically subjected to ~ s pulses of solutions containing each purinoceptor agonist and washed extensively for at least min to minimize receptor desensitization at the time of the next challenge. typical pseudocolor fura- fluorescence images are depicted in fig. representative time courses of i changes for selected chromaffin cells are depicted in e. resting i was low prior to stimulation. challenging cells with μm atp elicited sizeable, albeit variable peak i responses from a large pool of chromaffin cells. it is noteworthy that some cells did not respond to atp. several cells exhibited pronounced i rises in response to atp but not μm utp. exposure to μm 2-mesatp invariably increased the i in these cells. in contrast, several cells responded to both atp and utp. for the most part these cells did not respond at all to 2-mesatp. a fraction of the atp-sensitive cells responded both to utp and 2-mesatp. the statistical assessment of the i responses of chromaffin cells to utp and 2-mesatp is presented in fig. 1f in the form of a 3d histogram. the column at the origin of the histogram depicts cells that did not respond both to utp and 2-mesatp; atp evoked i rises in of these cells. cells displaying a variable response to 2-mesatp and no utp response accounted for approximately 30% of the atp-sensitive pool; the mean size of the 2-mesatp-evoked i responses was not significantly different from the atp responses in these cells. utp-sensitive cells were for the most part insensitive to 2-mesatp, although a minority displayed a mixed sensitivity. the mean sizes of the utp and atp responses in utp-sensitive/2-mesatp-insensitive cells were ± and ±, respectively. none of the cells responding to 2-mesatp in presence of ca2+ responded in the virtual absence of extracellular ca2+, suggesting that the atp derivative is a specific p2x receptor agonist in bovine chromaffin cells. we have previously reported that suramin blocked the atp-evoked i responses in cells lacking atp responses in the virtual absence of extracellular ca2+; suramin did not affect the utp responses, suggesting that it behaves as a specific p2x receptor antagonist. we have now extended this conclusion to pyridoxalphosphate-6-azophenyl- 2',4'-disulphonic acid. indeed, the drug blocked the 2-mesatp -evoked i rises in all cells tested. moreover, in parallel experiments, ppads did not affect the utp-evoked i rises in cells displaying no response to 2-mesatp. effects of purinergic agonists on catecholamine secretion both atp and 2-mesatp evoked catecholamine secretion in a dose-dependent way, as assessed amperometrically from cell batches. in these experiments cells were challenged with brief pulses of either agonist at increasing concentrations, being allowed to rest for ~ min between successive applications. the concentration-response curves provided in fig. 3b suggest that 2-mesatp might be a more potent agonist for purinergic receptors. utp did not evoke catecholamine secretion. ppads abolished 2-mesatp -evoked secretion. challenging the cells with μm 2-mesatp in the virtual absence of extracellular calcium failed to elicit secretion, indicating that the stimulatory action of the purinoceptor agonist is strictly calcium-dependent. this action may be mediated mostly by activation of voltage-sensitive ca2+ channels, since mm cd2+ reduced 2-mesatp-evoked secretion by ± %; cd2+ inhibited high k+ -evoked catecholamine secretion by ~98%. replacing extracellular na+ for n-methyl-d-glucamine abolished 2-mesatp-evoked secretion; tetrodotoxin had a modest inhibitory effect on evoked secretion, suggesting that na+ influx through ttx-sensitive, voltage-dependent na+ channels played a minimal role. discussion we found that ~50% of the cells examined by fluorescence imaging exhibited i responses to atp and that, among these, ~40% yielded positive responses to 2-mesatp. none of the 2-mesatp-sensitive cells responded in the virtual absence of extracellular ca2+. since cell exposure to egta-containing solutions caused minimal depletion of intracellular ca2+ stores, this implies that 2-mesatp elicits i rises by stimulating ca2+ influx. hence, 2-mesatp behaves as a specific p2x receptor agonist in bovine chromaffin cells. the 2-mesatp-sensitive p2x receptors in these cells are blocked by low concentrations of suramin and ppads. in agreement with a previous study, we show here that 2-mesatp is a stronger secretagogue than atp. this might suggest that atp is a weaker agonist of the p2x receptor, in apparent disagreement with the information provided by our former i study where the order of potency for purinergic agonists was atp > 2-mesatp >> αβ-meatp, adp, amp. we note, however, that activation of a p2y receptor coupled to gi/o inhibits exocytotic release of catecholamines from rat chromaffin cells downstream voltage-sensitive ca2+ channels and cytosolic ca2+ elevation. moreover, activation of p2y receptors inhibits voltage-sensitive ca2+ channels via gi/go proteins and, thus, depresses ca2+-dependent exocytosis. subclassification of p2x receptors in bovine chromaffin cells was clearly outside the objectives of the present functional study. nonetheless, taking into account the sensitivity to inhibitors, as well as the above order of potency for purinergic agonists, p2x receptors expressed in bovine chromaffin cells appear to have the pharmacological profile of p2x or p2x5receptors. as a note of caution we emphasize that p2x receptors in situ most probably are composed of different unit subtypes with a pharmacology distinct from that of the homomeric receptors cloned and transfected to cells, which difficults any attempt at identifying p2x receptor subtypes on the basis of functional studies. we have also found that ~50% of the atp-sensitive cells yielded positive responses to utp. thus, since utp is a specific agonist for p2y receptors in bovine chromaffin cells, approximately half of the atp-sensitive cell population expressed functional ca2+-mobilizing p2y receptors. this agrees with the finding by ennion et al. that these cells express an as yet unidentified utp-sensitive, gi/o-coupled p2y receptor. it is noteworthy that the authors also provided evidence for the presence of a gi/o-linked, adenine nucleotide-specific p2y receptor and detected transcripts for p2y receptors by rt-pcr analysis. accordingly, our results suggest that bovine chromaffin cells express a ca2+-mobilizing, utp-insensitive p2y receptor. indeed, ~20% of the atp-sensitive pool did not respond to either 2-mesatp or utp. taking into account that p2x receptors are fully sensitive to 2-mesatp in bovine chromaffin cells, the most likely explanation for this finding is that, indeed, some cells express a ca2+-mobilizing, utp-insensitive p2y receptor. chromaffin cells exist in the form of two major phenotypes, epinephrine-secreting and norepinephrine-secreting cells. our previous work showed that p2x receptors and utp-sensitive p2y receptors are asymmetrically distributed among two distinct cell pools. the present data are fully compatible with this finding. indeed, approximately 30% of the atp-sensitive cells yielded positive responses to 2-mesatp but not to utp, suggesting that they express p2x receptors preferentially. moreover, approximately 40% of the atp-sensitive cells yielded positive responses to utp but not to 2-mesatp, suggesting that they express ca2+-mobilizing p2y receptors only. a small fraction of the atp-sensitive cells examined in this study responded both to utp and 2-mesatp, suggesting that they co-express p2x and p2y receptors. there is evidence that stimulated ca2+ influx is strongly coupled to catecholamine release; in contrast, ca2+ release from intracellular stores appears to be loosely coupled to secretion. the present data support this concept, inasmuch as p2x receptor activation by 2-mesatp stimulated ca2+ influx and evoked catecholamine release, whereas activation of p2y receptors by utp elicited ca2+ release from intracellular stores but did not evoke secretion. we found that, while 2-mesatp-induced secretion was strictly na+-dependent, the voltage-sensitive na+ channel blocker ttx inhibited evoked release by a modest 20%. this suggests that voltage-sensitive ca2+ channels were mostly activated by the depolarization brought about by na+ influx across p2x receptor pores. that voltage-sensitive ca2+ channels are involved in 2-mesatp-induced secretion is suggested by the strong inhibitory effect of cd2+. indeed, cd2+ blocks ca2+ influx across these channels without affecting p2x receptors.  2-mesatp is a specific p2x purinoceptor agonist and a potent secretagogue in bovine chromaffin cells. activation of p2x receptors stimulates ca2+ influx mainly via voltage-sensitive ca2+ channels. for the most part, these are activated by the depolarization brought about by na+ influx across p2x receptor pores. 
the average sizes of the atp- and 2-mesatp-evoked i responses were identical in utp-insensitive cells. using single cell calcium imaging techniques, we have determined whether 2-mesatp might be a specific p2x receptor agonist in bovine chromaffin cells and assessed the relative role of p2x and p2y receptors on catecholamine secretion from these cells. this indicates that 2-mesatp behaves as a specific ionotropic purinoceptor agonist in bovine chromaffin cells. 2-mesatp is a specific p2x purinoceptor agonist and a potent secretagogue in bovine chromaffin cells. removing extracellular ca2+ suppressed the i-raising ability of 2-mesatp, observed in ~40% of the atp-sensitive cells. utp raised the i in ~40% of the atp-sensitive cells, indicating that these expressed ca2+-mobilizing p2y receptors. utp-sensitive receptors may not be the only p2y receptors present, as suggested by the observation that ~20% of the atp-sensitive pool did not respond to either 2-mesatp or utp. 2-mesatp-induced secretion was strongly inhibited by cd2+ and suppressed by extracellular ca2+ or na+ removal. 2-methylthioadenosine 5'-triphosphate, formerly regarded as a specific p2y purinergic receptor agonist, stimulates ca2+ influx and evokes catecholamine release from adrenal chromaffin cells. these cells express p2y and p2x purinoceptors, with the latter providing an important ca2+ influx pathway. 2-mesatp stimulated ca2+ influx and evoked catecholamine release, whereas utp elicited ca2+ release from intracellular stores but did not evoke secretion. activation of 2-mesatp-sensitive receptors stimulates ca2+ influx mainly via voltage-sensitive ca2+ channels. for the most part, these are activated by the depolarization brought about by na+ influx across p2x receptor pores.
 2-methylthioadenosine 5'-triphosphate, formerly regarded as a specific p2y purinergic receptor agonist, stimulates ca2+ influx and evokes catecholamine release from adrenal chromaffin cells. these cells express p2y and p2x purinoceptors, with the latter providing an important ca2+ influx pathway. using single cell calcium imaging techniques, we have determined whether 2-mesatp might be a specific p2x receptor agonist in bovine chromaffin cells and assessed the relative role of p2x and p2y receptors on catecholamine secretion from these cells.  atp raised the i in ~50% of the cells. removing extracellular ca2+ suppressed the i-raising ability of 2-mesatp, observed in ~40% of the atp-sensitive cells. this indicates that 2-mesatp behaves as a specific ionotropic purinoceptor agonist in bovine chromaffin cells. the 2-mesatp-induced i-rises were suppressed by ppads. utp raised the i in ~40% of the atp-sensitive cells, indicating that these expressed ca2+-mobilizing p2y receptors. utp-sensitive receptors may not be the only p2y receptors present, as suggested by the observation that ~20% of the atp-sensitive pool did not respond to either 2-mesatp or utp. the average sizes of the atp- and 2-mesatp-evoked i responses were identical in utp-insensitive cells. 2-mesatp stimulated ca2+ influx and evoked catecholamine release, whereas utp elicited ca2+ release from intracellular stores but did not evoke secretion. 2-mesatp-induced secretion was strongly inhibited by cd2+ and suppressed by extracellular ca2+ or na+ removal. ttx inhibited 2-mesatp-evoked secretion by ~20%.  2-mesatp is a specific p2x purinoceptor agonist and a potent secretagogue in bovine chromaffin cells. activation of 2-mesatp-sensitive receptors stimulates ca2+ influx mainly via voltage-sensitive ca2+ channels. for the most part, these are activated by the depolarization brought about by na+ influx across p2x receptor pores. 