Agonist-induced desensitization of liver alpha 1-receptors; implications for assessment of PI hydrolysis

Alpha 1-adrenergic stimulation of phosphatidyl inositide (PI) hydrolysis was measured in liver slices and in cultured hepatocytes prelabeled with 3H-myoinositol. Incubation of cultured hepatocytes for 15 min with l-epinephrine stimulated the release of 3H-inositol phosphates with an EC50 of 0.28 +/- 0.19 microM and Emax representing 2.88 +/- 0.39% of incorporated radiolabel. Pretreatment of cultures with epinephrine resulted in reductions both in the maximum response and in the sensitivity to epinephrine. In liver slices, epinephrine elicited a similar maximum response (3.38 +/- 0.32%), but the sensitivity to epinephrine (EC50 = 2.27 +/- 1.27 microM) was considerably lower than for cultured cells. Prior treatment of liver slices with 200 microM l-epinephrine had no effect either on the maximum response to l-epinephrine or on the sensitivity to epinephrine. The affinity of alpha 1-adrenergic receptors for agonists was reduced 2.11 fold during incubation of liver slices for 60 min at 37 degrees C in the absence of added agonist (as is required for 3H-myoinositol labeling). Agonist affinity was not further reduced when 200 microM l-epinephrine was included in the incubation. It was concluded that alpha 1-adrenergic stimulation of PI hydrolysis in liver undergoes agonist-induced desensitization. It was also concluded that a limitation exists in the methodology for measuring PI hydrolysis in tissue slices prelabeled with 3H-myoinositol, in that this method assesses the activity of receptors in a desensitized state.