Evidence for spare alpha 1-adrenoceptors for the accumulation of inositol phosphates in smooth muscle.

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 microM noradrenaline. Clonidine (up to 3 mM) was a partial agonist. Pretreatment of the tissues with the irreversible alpha-adrenoceptor antagonist phenoxybenzamine (0.3-10 microM) 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.