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1. The post-receptor mechanisms of alpha 1 and alpha 2-adrenoceptor subtypes in guinea-pig aorta and human digital arteries have been explored. 2. Nifedipine antagonized contractile responses of human digital arteries to TL99 and methoxamine to a similar degree, thus suggesting that neither the alpha 1 nor the alpha 2 receptor is preferentially linked to calcium entry through voltage-operated channels of the cell membrane. 3. In the guinea-pig aorta, which contains only alpha 1-adrenoceptors, methoxamine-stimulated inositol phosphate (IP) production at similar concentrations was required to produce contractile responses. 4. In the human digital artery, noradrenaline also produced a significant increase in IP formation, but preliminary experiments have suggested that both TL99 and methoxamine stimulate IP production. 5. Thus, the present authors have been unable as yet to confirm, in a tissue which contains both alpha 1- and alpha 2-adrenoceptors, that the post-receptor mechanisms of the alpha subtypes are different.

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