Guanine nucleotides and alpha 1 adrenergic receptors in the heart

Abstract

Guanine nucleotides such as GTP and Gpp(NH)p are known to regulate the affinity of beta and alpha 2 adrenergic receptors for agonists as assessed by radioligand binding techniques. Recent studies in the rat heart using the radioligand [ 3H]WB4101, which reportedly labels alpha 1 adrenergic receptors, have suggested that the affinity of alpha 1 adrenergic receptors for epinephrine is altered by guanine nucleotides. To assess the possible role of guanine nucleotides in alpha 1 adrenergic function, we have constructed (−)epinephrine competition curves in the absence and presence of guanine nucleotides using the highly alpha 1 subtype selective agents [ 3H]prazosin and [ 125I]BE2254 in rat cardiac membranes. Epinephrine competition curves in the absence of guanine nucleotides were steep and uniphasic in character, and the addition of Gpp(NH)p (10 −4 M) had no effect on the ability of epinephrine to compete for either [ 3H]prazosin or [ 125I]BE2254 binding sites. In this same membrane preparation, the ability of (−)isoproterenol to compete with [ 3H]dihydroalprenolol for beta adrenergic receptors binding sites was decreased significantly by Gpp(NH)p. These findings demonstrate that, under conditions where guanine nucleotide regulation of agonist-receptor binding in these membranes can be observed, no nucleotide regulation of agonist-alpha 1 receptor interactions was evidenced using subtype selective radioligands. These results suggest that previous reports of agonist-alpha 1 receptor regulation by guanine nucleotides may represent a manifestation of the anomalous binding characteristics of [ 3H]WB4101 as compared with [ 3H]prazosin and [ 125I]BE2254.