Endothelial modulation of α1-adrenoceptor contractile responses in the tail artery of spontaneously hypertensive rats

1. Vascular contraction induced by phenylephrine was studied in tail artery rings from spontaneously hypertensive (SHR) and Wistar Kyoto rats (WKY) with particular focus on the role of endothelium. The influence of receptor reserve and the density of alpha 1-adrenoceptors on the possible differences observed were also analysed. 2. Phenylephrine (0.01-100 microM) induced concentration-dependent vasoconstrictions. The maximum response (alpha, P < 0.001) was greater but the pEC50 (P < 0.05) smaller in rings from SHR than from WKY rats irrespective of the presence or absence of endothelium. 3. Removal of endothelial cells resulted in a decrease of the maximum contraction with no modification in the pEC50 in arteries from both WKY and SHR. 4. The density of alpha 1-adrenoceptors (Bmax) and the dissociation constant (KD) were found to be the same for preparations from SHR and WKY rats in [3H]-prazosin binding experiments. 5. The apparent affinity (pKA) determined by the nested hyperbolic method and the operational model was similar in tail arteries from the two rat strains, irrespective of the presence or absence of endothelium. However, in endothelium-denuded rings, the pKA value was enhanced when compared with intact rings, in both SHR and WKY rats. 6. In rings from hypertensive rats, the operational parameter maximum possible effect (Em) was greater and the agonist efficacy (tau) was smaller than in rings from normotensive rats. When the endothelium was removed log tau and Em diminished in preparations from both rat strains. 7. In summary, the increased maximum responsiveness to phenylephrine in rings from SHR could be due to enhancement in Em. The log tau values indicate a deterioration in the transduction of the stimulus provided by the agonist in tail arteries from hypertensive animals. This study also suggests that the absence of endothelium modifies the alpha 1-adrenoceptor-mediated vasoconstriction probably by altering the transduction signalling mechanisms. The importance of analysing the degree of endothelium functionality when comparing results from different groups of rats is stated.