Evidence for spare α1-adrenoceptors for the accumulation of inositol phosphates in smooth muscle

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Abstract
The accumulation of inositol phosphates (IP) in smooth muscle from rat vas deferens and caudal artery was maximally increased 3- to 4-fold in response to exposure of the tissues to 100 μM noradrenaline. Clonidine (up to 3 mM) was a partial agonist. Pretreatment of the tissues with the irreversible α-adrenoceptor antagonist phenoxybenzamine (0.3–10 μM) shifted the noradrenaline concentration-response curve to the right before depressing the maximum. The maximum of the clonidine concentration-response curve was depressed without significant change in the EC50 by the same treatment. These data, which are most easily interpreted as demonstrating the presence of a receptor reserve for IP accumulation, are discussed.