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The selectivity of α‐adrenoceptors mediating the pro‐aggregatory response of human and rabbit platelets to adrenaline and the conditions required to permit expression of an aggregatory response to partial agonists at these α‐adrenoceptors have been studied. Yohimbine causes effective blockade of the pro‐aggregatory responses whereas indoramin and prazosin are ineffective. The clonidine analogue, UK‐14304, is nearly as effective as adrenaline in inducing an aggregatory response in human platelets and a pro‐aggregatory response in rabbit platelets. Cross‐tachyphylaxis between adrenaline and UK‐14304 has been demonstrated. Clonidine is a weak agonist for the pro‐aggregatory response of rabbit platelets and in some donors for the aggregatory response of human platelets. Methoxamine induces a pro‐aggregatory response in human platelets which is blocked by indoramin or prazosin but not by yohimbine. No such response to methoxamine is observed in rabbit platelets. The divalent cation ionophore, A‐23187, induces an aggregatory response to clonidine (in platelets from a non‐responsive donor), phenylephrine and methoxamine in human platelets and to adrenaline, UK‐14304 and clonidine in rabbit platelets. A secretory response to clonidine is also induced by A‐23187 in human platelets. The adenylate cyclase inhibitor, SQ‐22536, is ineffective in either inducing a response to the α‐agonists or potentiating the effect of A‐23187. The aggregatory responses to adrenaline and UK‐14304 in rabbit platelets and to clonidine in human and rabbit platelets, which can be induced by A‐23187, are blocked by yohimbine but not by prazosin or indoramin. From these studies we conclude that the pro‐aggregatory responses of human and rabbit platelets to adrenaline are mediated primarily by α2‐adrenoceptors. The presence of α1‐adrenoceptors on human platelets is confirmed but these receptors do not appear to be present on rabbit platelets. The conditions required for expression of an aggregatory response to partial agonists at the human and rabbit platelet α‐adrenoceptors implicate an increase in cytosolic Ca2+ concentration as a key event in stimulus‐response coupling but do not indicate such a role for depression of cyclic adenosine‐3′,5′‐monophosphate concentration.

ADP, adrenaline and vasopressin interact positively as agonists in aggregating human blood platelets in vitro. This interaction is maximal if the addition of two of the agonists is separated by 10--20 s but decreases rapidly at longer intervals especially at low agonist concentrations. The agonist concentrations at which positive interaction gives full aggregation are significantly less than those required for such a response to each agonist alone. The lowest concentrations at which adrenaline and vasopressin interact positively are at least two orders of magnitude greater than the normal blood concentrations of these hormones, and at least an order of magnitude greater than the concentrations achieved in pathological states. Specifically antagonizing the adrenaline and ADP receptors showed that the response was to the second agonist added to the system. An inhibitor of intracellular Ca2+ movement (tetracaine) is equally effective in blocking the responses generated by a single agonist or by interaction of two agonists. Inhibitors which increase cyclic-3',5'-AMP concentration (adenosine, prostaglandin E1, dipyridamole) are more effective against the response to a single agonist than that to agonist interaction. These data suggest that positive agonist interaction results from effects on the concentrations of second messengers within the platelet rather than from a direct interaction on the membrane receptors or the transmembrane coupling mechanisms.

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Previous studies have identified 2', 3'-dialdehyde-ADF (oADP) and -dialcohol-ADP (or ABI as partial agonists at the ADP receptor, and Clonidine and methoxamine as partial agonists at the n-adrenoceptor.oADP and orADP cause shape change but not aggregation (Euxop, J, Biochem, 88, 543): Clonidine and methoxamine fail to cause aggregation but enhance the response to ADP and other agonists (Nature, in press). Preincubation of platelets with non-aggregating concentrations of the ionophore A-23187 induces a primai aggregation response to o ADP, and a primary aggregation + a secretion response to or ADH This response is specific to the partial agonists; is blocked by an ADP antagonist and by agents which raise cyclic AMP levels (e.g. PGE) or block intracellular Ca++ movemenl (e.g. tetracaine); and is not inhibited by specific chelation of extracellular Ca+. Under similar conditions A-23187 provokes a full aggregation response to Clonidine (which is blocked effectively by yohimbine but not by praiosin) and a primary aggrtion response to methoxamine. Preincubation of platelets with an adenylate cyclase inhibit; (SQ-22536) fails to provoke an aggregation response to oADP or orADP. These data suppt the concept that an increase in oytosolic [Ca2+], rather than a decrease in [cAMP], is key step in initiation of the response of human platelets to ADP and adrenaline.