A1‐and A2‐PURINOCEPTORS IN THE GUINEA‐PIG UTERUS

John M. Haynes, Jocelyn N. Pennefather

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Abstract

1. Radioligand binding and functional studies were undertaken to investigate the P1‐purinoceptors present in the separated myometrial layers and the endometrium of the guinea‐pig uterus. 2. In preparations of endometrium‐denuded circular myometrium, the A2‐selective agonists (2‐p‐(2‐carboxyethyl)phenethylamino‐5'‐N‐ethylcarboxamido‐adenosine (CGS 21680, 100 μmol/L) and N‐ethylcarboxamido adenosine (NECA, 1–10 μmol/L) inhibited contractile responses to phenylephrine. In preparations of endometrium‐intact circular myometrium, NECA (10 μmol/L) enhanced responses to phenylephrine. NECA did not modulate the spontaneous contractions of longitudinal myometrium. 3. Homogenate binding studies with circular myometrium, longitudinal myometrium and endometrium revealed saturable high affinity [3H]‐NECA binding sites. The mean maximal densities of binding sites (Bmax) were 2.08, 14.7 and 15.5 fmol/mg protein, and pKD (neg. log dissociation constant) values were 9.82, 9.19 and 7.44, respectively. 4. (R‐) and (S‐) ‐N6‐(2‐phenylisopropyl)adenosine (R‐ and S‐PIA) both competed for two [3H]‐NECA binding sites in preparations of circular myometrium. CGS 21680 competed for two [3H]‐NECA binding sites in preparations of endometrium and longitudinal myometrium. All other agonist competition was for one site only. The rank orders of potency of high affinity binding were S‐PIA ≥ R‐PIA ≥ CGS 21680 (circular myometrium), R‐PIA ≥ CGS 21680 ≥ S‐PIA (longitudinal myometrium) and CGS 21680 > > S‐PIA ≥ R‐PIA (endometrium). 5. In preparations of circular myometrium, longitudinal myometrium and endometrium the selective A1‐purinoceptor antagonist, 1,3‐dipropyl‐8‐(2‐amino‐4‐chloro)‐phenylxanthine (PACPX), competed for two [3H]‐NECA binding sites, the non‐selective antagonist 3,7‐dimethyl‐1‐propargylxanthine (DMPX), competed for one site only. 6. NECA increased cyclic adenosine monophosphate (CAMP) levels in preparations of both circular myometrium and endometrium. 7. These results indicate that P1‐purinoceptors of the A2‐subtype mediate the inhibitory effects of adenosine analogues on the phenylephrine‐induced contractions of the circular myometrium of the guinea‐pig, this effect is modified by the presence of the endometrium. There is no evidence that the [3H]‐NECA binding sites of the longitudinal myometrium correlate with functional P1‐purinoceptors in this tissue.

Original languageEnglish
Pages (from-to)609-617
Number of pages9
JournalClinical and Experimental Pharmacology and Physiology
Volume20
Issue number10
DOIs
Publication statusPublished - 1 Jan 1993

Keywords

  • A‐purinoceptors
  • CAMP
  • guinea‐pig myometrium
  • N‐ethylcarboxamido adenosine
  • radioligand binding.

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