Molecular mechanisms involved in α-adrenergic responses

Noradrenergic fibers were identified in rat cervical lymph nodes by fluorescence histochemistry and confirmed by radioenzymatic determination of norepinephrine. After superior cervical ganglionectomy, lymph node norepinephrine and noradrenergic fluorescence were strikingly decreased. The alpha-adrenergic radioligand [3H] dihydroergocryptine bound to saturable sites on lymph node membranes (0.20 pmol/mg protein; affinity constant, 8 nM) with characteristics expected for alpha-adrenergic receptor binding. The alpha-adrenergic binding sites were present in high concentration in membranes from lymph node stroma but were negligible in membranes of lymphocytes teased from nodes. In lymph nodes the ratio of alpha-adrenergic binding sites to norepinephrine content was higher than ratios of many other sympathetically innervated tissues such as iris or vas deferens. The presence of sympathetic innervation and the high concentration of alpha-adrenergic binding sites relative to that innervation suggest that the sympathetic nervous system modulates an intrinsic lymph node function through alpha-adrenergic mechanisms.

Lymph nodes—A possible site for sympathetic neuronal regulation of immune responses

[3H]Dihydroergocryptine ([3H]DHE) binds to sites in membranes derived from rat myocardium that have the characteristics expected of alpha-adrenergic receptors. The binding is saturable with 41 fmol [3H]DHE bound per mg of protein and of high affinity with KD = 2.9 nM. The binding is rapid and readily reversible. Adrenergic agonists compete with [3H]DHE for binding in the order: epinephrine greater than norepinephrine greater than isoproterenol; and adrenergic antagonists compete for binding in the order: phentolamine greater than propranolol. For comparison, (-)[3H]dihydroalprenolol [(-)[3h]dha] was used to bind to sites in the same membrane preparations having characteristics of beta-receptors. The number and affinity of beta-receptors were quite similar to those of the alpha-receptors with 46 fmol (-)[EH]DHA per mg protein bound at saturation and KD = 2.5 nM. These techniques allowed identification of both beta- and alpha-adrenergic receptors in membranes derived from isolated atria, right ventricular free walls, and left ventricles including interventricular septa. This is the first report documenting direct identification of myocardial alpha-receptors by radioligand-binding techniques and complements the literature previously reporting myocardial inotopic and electrophysiological responses to alpha-adrenergic stimulation.

B S Tsai

Papers

[3H]Dihydroergocryptine binding to alpha adrenergic receptors in canine aortic membranes.