NEONATAL RAT PINEALOCYTES - TYPICAL AND ATYPICAL CHARACTERISTICS OF [I-125]IODOHYDROXYBENZYLPINDOLOL BINDING AND ADENOSINE-3',5'-MONOPHOSPHATE ACCUMULATION

Techniques are described which make it possible to study .beta.-adrenergic receptors on intact neuroendocrine cells. Receptors were characterized on neonatal pinealocytes using the radioligand [125I]iodohydroxybenzylpindolol ([125I]HYP). Specific binding of [125I]IHYP, which is 4-fold greater than nonspecific binding, is concentration and temperature dependent, reversible and saturable. [125I]IHYP binds noncooperatively (Kd = 35 pM), and Scatchard analysis indicates that only a single class of receptor sites for [125I]IHYP is present. Under the conditions used, it appears that there are about 12,000 .+-. 1100 sites/cell. Inhibition of specific [125I]IHYP binding by .beta.-adrenergic agonist and antagonists [practolol, alprenol, butoxamine, isoproterenol, phenylephrine, norepinephrine, octopamine, dopamine, tryptamine, epinephrine, taurine, propranolol, oxprenolol, phentolamine and tyramine] is stereospecific, and the relative potency of agonists is characteristic of binding to .beta.-adrenergic receptors. Analysis of adrenergic stimulation of intracellular cAMP accumulation indicates thdt similar half-maximal concentrations of antagonists inhibit [125I]IHYP binding and adrenergically stimulated cAMP accumulation. .beta.-adrenergic agonists are considerably more potent in stimulating cAMP than in inhibiting [125I]IHYP binding. Unexpected differences, not previously reported, were found in the shapes of the cAMP accumulation dose-response curves of norepinephrine and isoproterenol. The relative potencies of these 2 agonists appear to be partially concentration dependent. This raises the possibility that there may be distinct differences in the intrinsic effects of these compounds on the regulation of intracellular cAMP accumulation in pinealocytes.