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E. Monjok, K.L. Shivers, K.H. Kulkarni, C.A. Opere, S.E. Ohia; ATYPICAL H1–RECEPTORS MEDIATE CONTRACTIONS OF PORCINE ISOLATED IRIDES TO HISTAMINE . Invest. Ophthalmol. Vis. Sci. 2004;45(13):411.
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In a previous study, we showed evidence that histamine induced contractions of the bovine isolated irides were antagonized in a non–competitive manner by mepyramine and triprolidine suggesting that atypical H1–receptors mediated the observed responses in this tissue (Kulkarni et al. The Pharmacologist 44: abs. no. 25.11, 2002). It was, therefore, of interest to establish the pharmacological profile of histamine receptors mediating contractions in the iris of another species. Purpose. The aim of the present study was to classify the pharmacological subtype of histamine receptor that mediates contraction of the porcine isolated irides. Methods. Isolated porcine iris muscle strips were set up in organ baths containing oxygenated Krebs buffer solution at 37oC. Longitudinal isometric tension was recorded via a Grass FT03 force–displacement transducer and analyzed using the Polyview computer software. An initial load of 150 mg was placed on each tissue after which they were allowed to equilibrate for one hour. Cumulative contractile concentration–response curves were constructed to the histamine in the absence or presence of antagonists. Results. Histamine (10 nM – 100 µM) caused a concentration–dependent contraction of the isolated iris muscle yielding an EC50 of 700 nM. In concentrations up to 10 nM, both mepyramine and triprolidine (selective H1–receptor antagonists) had no significant effect (P > 0.05) on histamine contractions. As with bovine irides, concentrations of triprolidine higher than 100 nM elicited a non–competitive antagonism of the histamine contractions. Conclusions. In conclusion, atypical H1–receptors mediate histamine contractions in the porcine iris. These atypical H1–receptors appear to be similar to the ones previously identified in bovine isolated irides.
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