Dorflinger L J, Schonbrunn A
Endocrinology. 1985 Dec;117(6):2330-8. doi: 10.1210/endo-117-6-2330.
Although purine nucleosides have been shown to regulate the secretion of several peptide and steroid hormones, effects on pituitary hormone release have not been reported. We show here that in the clonal GH4C1 pituitary cell line maximal concentrations of adenosine (greater than or equal to 50 microM) inhibited PRL and GH secretion by 40%. Adenosine deaminase abolished the inhibitory effect of adenosine but not that of SRIF or (-)N6(R-2-phenylisopropyl)adenosine (PIA), a nonhydrolyzable adenosine analog. Furthermore, this enzyme increased basal secretion by 50%, and analysis of the incubation medium by HPLC demonstrated that the cells secreted biologically effective concentrations of adenosine. These results indicate that adenosine produced in culture tonically inhibits hormone release. In other target cells, adenosine inhibition is mediated by two types of binding sites: an extracellular Ri-site requiring an intact ribose moiety or an intracellular P-site requiring an intact purine ring. Four lines of evidence indicate that in GH4C1 cells, adenosine acts at an Ri-site. PIA, an Ri-site-specific agonist, was a potent inhibitor of hormone release (ED50 = 30 nM). Theophylline, an Ri-site antagonist, competitively inhibited the action of PIA (Ki = 2.4 microM). 3) 2'5'-Dideoxyadenosine, a P-site-specific agonist, did not inhibit PRL release even at a concentration of 1 mM. 4) Dipyridamole, an adenosine uptake inhibitor, did not reduce adenosine inhibition. In addition to its effect on basal secretion, PIA inhibited stimulation of hormone release by vasoactive intestinal peptide and TRH. PIA also reduced vasoactive intestinal peptide-stimulated cAMP accumulation by 75%, consistent with its action to inhibit adenylate cyclase via Ri receptors in other targets. Since PIA inhibition of PRL release and cAMP accumulation was not additive with the effects of SRIF and carbamyl choline, these inhibitors may act via a common rate-limiting step. Our results demonstrate that adenosine activates an Ri-type of adenosine receptor in GH4C1 cells and that the production of adenosine under normal culture conditions causes autocrine inhibition of secretion.
尽管嘌呤核苷已被证明可调节多种肽类和类固醇激素的分泌，但尚未见其对垂体激素释放有影响的报道。我们在此表明，在克隆的GH4C1垂体细胞系中，腺苷的最大浓度（大于或等于50 microM）可使PRL和GH分泌减少40%。腺苷脱氨酶消除了腺苷的抑制作用，但对SRIF或（-）N6（R-2-苯异丙基）腺苷（PIA，一种不可水解的腺苷类似物）的抑制作用无影响。此外，该酶使基础分泌增加50%，通过HPLC对孵育培养基进行分析表明，细胞分泌的腺苷具有生物学活性浓度。这些结果表明，培养过程中产生的腺苷持续抑制激素释放。在其他靶细胞中，腺苷的抑制作用由两种类型的结合位点介导：一种是需要完整核糖部分的细胞外Ri位点，另一种是需要完整嘌呤环的细胞内P位点。有四条证据表明，在GH4C1细胞中，腺苷作用于Ri位点。PIA是一种Ri位点特异性激动剂，是激素释放的有效抑制剂（ED50 = 30 nM）茶碱是一种Ri位点拮抗剂，竞争性抑制PIA的作用（Ki = 2.4 microM）。3）2'5'-二脱氧腺苷是一种P位点特异性激动剂，即使在浓度为1 mM时也不抑制PRL释放。4）双嘧达莫是一种腺苷摄取抑制剂，并不降低腺苷的抑制作用。除了对基础分泌的影响外，PIA还抑制血管活性肠肽和TRH对激素释放的刺激。PIA还使血管活性肠肽刺激的cAMP积累减少75%，这与其在其他靶细胞中通过Ri受体抑制腺苷酸环化酶的作用一致。由于PIA对PRL释放和cAMP积累的抑制作用与SRIF和氨甲酰胆碱的作用无相加性，这些抑制剂可能通过共同的限速步骤起作用。我们的结果表明，腺苷在GH4C1细胞中激活了一种Ri型腺苷受体，并且在正常培养条件下腺苷的产生导致自分泌抑制分泌。
