The α1-adrenergic receptor in human erythrocyte membranes mediates interaction in vitro of epinephrine and thyroid hormone at the membrane Ca 2+-ATPase

Abstract

Membrane Ca 2+-ATPase activity was stimulated in vitro separately by T 4 (10 −10 M) and by epinephrine (10 −6 M). In the presence of a fixed concentration of T 4, additions of 10 −8 and 10 −6 M epinephrine reduced the T 4 effect on the enzyme. β-Adrenergic blockade with propranolol (10 −6 M) prevented stimulation by epinephrine of Ca 2+-ATPase activity, but did not prevent the suppressive action of epinephrine on T 4-stimulable Ca 2+-ATPase. In contrast α 1-adrenergic blockade with unlabelled prazosin restored the effect of T 4 on Ca 2+-ATPase activity in the presence of epinephrine. Like propranolol, prazosin prevented enhancement of enzyme activity by epinephrine in the absence of thyroid hormone. Neither prazosin nor propranolol had any effect on the stimulations by T 4 of red cell Ca 2+-ATPase in the absence of epinephrine. Analysis of radiolabelled prazosin binding to human red cell membranes revealed the presence of a single class of high-affinity binding sites ( K d, 1.2 × 10 −8 M; B max, 847 fmol/mg membrane protein). Thus, the human erythrocyte membrane contains α 1-radrenergic receptor sites that are capable of regulating Ca 2+-ATPase activity.