Use of the operational model of agonism and [3H]prazosin binding to assess altered responsiveness of α1-adrenoceptors in the vas deferens of spontaneously hypertensive rat

Abstract

Changes in functional responsiveness to α₁-adrenoceptor activation with noradrenaline and in [³H]prazosin binding in the epididymal portion of vas deferens from normotensive Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) were investigated. The operational model fitting and the nested hyperbolic method were used to analyze the effects of irreversible receptor alkylation by phenoxybenzamine (0.1 μM) on the α₁-adrenoceptor mediated contractile responses to noradrenaline of vasa deferentia from SHR and WKY rats. Saturation isotherms for [³H]prazosin revealed a significant increase (P < 0.05) in the B_max in SHR vas deferens (145 ± 19 fmol/mg protein) compared with vas deferens from normotensive controls (75 ± 12 fmol/mg protein) without changes in the K_D. No differences in the proportion of high and low affinity binding sites for WB-4101 and 5-methylurapidil were observed. The maximum contractile response, α, (P < 0.001) and the pEC₅₀ (P < 0.05) values for noradrenaline were greater for SHR than for WKY rat tissues. The apparent affinity (pK_A) determined by the nested hyperbolic method and by the operational model of agonism was found to be similar in the two strains. In agreement with relative pEC₅₀, the efficacy (τ) value for SHR was greater than for WKY rats. However, the difference in the τ estimates did not reach statistical significance. In summary, in the epididymal portion of SHR vas deferens, the increased maximum contractile response to noradrenaline is due to an increase of E_m. Taken together, the τ values and the results from binding experiments lead to the assumption that the transducer constant K _E must be greater in SHR than in WKY rats, suggesting a deterioration in the transduction of the stimulus provided by the agonist in hypertensive animals.