Rabbit femoral arteries retain a memory of previous maximum receptor activation for up to 3-4 h after complete cessation of the stimulus, as reflected by a reduction in the steady-state contraction produced by a subsequent exposure to KCl. The present study examined the hypothesis that this modulatory effect involves alterations in postreceptor signal transduction. To quantify the degree of cellular downregulation induced by an episode of alpha 1-adrenoceptor stimulation, tissues were pretreated for 30 min with 10(-5) M phenylephrine (PE), washed for 10 min to cause complete relaxation, and activated with increasing concentrations of KCl. Pretreatment of tissues with PE resulted in a large reduction compared with control tissues in the ability of 20-60 mM KCl to increase stress and myosin light-chain phosphorylation. However, only at low (20 and 26 mM), not high (> 26 mM), KCl concentrations did PE pretreatment reduce the ability of KCl to increase intracellular free Ca2+ concentration ([Ca2+]i). These data support the hypothesis that memory of receptor activation involves reductions in both Ca2+ mobilization and the sensitivity of contractile proteins to [Ca2+]i.