A range of 2-(,5-dihydroimidazolyl)-benzene, -quinoline, and -quinoxaline derivatives and 2-morpholino-4-catechol have been characterized as agonists or

Decreased responsiveness to adrenaline has been observed in five apparently normal unrelated human donors. In four of the donors this trait is inherited. Three of the donors, as well as their affected relatives, also exhibited depressed responsiveness to collagen and vasopressin but normal responsiveness to ADP and thrombin. The other two affected donors exhibit normal responsiveness to most other agonists. Normal responsiveness can be restored in all instances either by incubating the platelet-rich plasma at 20 degrees C or by addition of a low concentration of the divalent cation ionophore, A-23187. All affected platelets which have been examined have ATP and ADP contents, cholesterol to phospholipid ratios, and phospholipid class compositions within the normal range. Both the resting level of cyclic-3'5'-AMP and the ability of adrenaline to prevent elevation of cyclic-3',5'-AMP levels by prostaglandin E1 are normal. Mixing experiments demonstrate the absence of a circulating inhibitor of platelet function and suggest that the defect resides in the platelets. We conclude that the depressed responsiveness of human platelets to adrenaline may result from a defect in Ca2+ mobilization to the cytosol.

1. Thrombin induces Ca2+ uptake into both stirred and unstirred human platelets in the presence or absence of acetylsalicylate. This Ca2 + uptake is closely correlated with adenine nucleotide secretion in accord with previous observations [Massini, P. and Luscher, E. F. (1974) Biochim. Biophys. Actu 372, 109-1211 but a low level of secretion is observed in the absence of significant CaZ + uptake.2. l-O-Alkyl-2-acetyl-sn-glycero-3-phosphocholine (1-0-alkylAcGEPC) also induces Ca2+ uptake into both stirred and unstirred human platelets in the presence and absence of acetylsalicylate. Aggregation and adenine nucleotide secretion induced by 1-0-alkylAcGEPC can be observed in the absence of added fibrinogen, but addition of fibrinogen causes a very marked shift to the left in the dose/response curves for aggregation and Ca2+ uptake induced by I-0-alkylAcGEPC. In the absence of added fibrinogen a close correlation is observed between Caz+ uptake and adenine nucleotide secretion induced by 1 -0-alkylAcGEPC. In the presence of added fibrinogen significant aggregation can be observed in the absence of detectable Ca2+ uptake.3. Adrenaline induces CaZ+ uptake only into stirred human platelets in the presence of added fibrinogen. Blockade of secretion, e.g. by addition of acetylsalicylate, also prevents Ca2 + uptake. 4. Addition of adrenaline fails to cause breakdown of phosphatidylinositol or phosphatidylcholine in acetylsalicylate-treated platelets under conditions where such a response is observed on addition of thrombin. We conclude that Ca2+ uptake into human platelets induced by thrombin, I-0-alkylAcGEPC and adrenaline is closely associated with the secretory response and in some circumstances, e.g. stimulation by thrombin, is clearly a consequence of this latter response. Previous reports of CaZ+ uptake as an initiating event in the response of human platelets to adrenaline [Owen, N. E., Feinberg, H. and Le Breton, G. C. (1981) Am J. Physiol. 239, H483-4881 have not been confirmed in this study.It is widely accepted that an increase in cytosolic [Ca"] is one mechanism by which excitatory agonists induce platelet aggregation and secretion (cf. [I]). Divalent cation ionophotes which show selectivity for Ca2+ e.g. A23187, ionomycin, induce these responses (21 and direct evidence for an increase in cytosolic [Ca"] on stimulation by thrombin and plateletactivating factor has been obtained in studies where this parameter was monitored directly using Quin I1 [3, 41. However, the source from which Ca2+ is mobilised to permit an increase in the cytosolic concentration of this cation is much less clear. Uptake of 45Ca2+ into human platelets is stimulated by addition of thrombin but such stimulation is closely correlated in time with 5-hydroxytryptamine secretion induced by this agonist [5, 61. A similar correlation of 45Ca2+ uptake with secretion has been suggested for stimulation by collagen, ADP and adrenaline [6]. Furthermore Brass and Shattil [7] were unable to find an enhancement of net 4sCa2+ uptake by ADP or adrena...

Fig. I . Sephucyl S -3 0 0 H R gel permeation of ummoriium-sulphate-fructioriuted crirde extruct of H. carbonum 10.5 ml (approx. 20 nig) injected, 2.5 ml fractions collected] (-) Absorbance at 280 pm;