Effects of Renin-Angiotensin Blockade on Sympathetic Reactivity and β-Adrenergic Pathway in the Spontaneously Hypertensive Rat

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Abstract
Abstract As interactions between the renin-angiotensin and sympathetic nervous systems have been suggested in the pathogenesis of hypertension, we wanted to investigate the effect of chronic renin-angiotensin blockade with losartan and enalaprilat on the sympathetic reactivity to hypotension and on the cardiac β-adrenergic–coupled adenylyl cyclase pathway in 12-week-old Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Both treatments, exerting equipotent shifts of angiotensin-pressure responses, lowered blood pressure and attenuated cardiac hypertrophy similarly in SHR. The nitroprusside-induced hypotension was similar in both strains, but the associated increases in plasma catecholamines and heart rate were higher in SHR. In SHR treated with losartan and enalaprilat, the nitroprusside-induced hypotension was greater and associated with markedly attenuated increases in norepinephrine and heart rate. The binding affinity of cardiac β-adrenoceptors was significantly lower, and β 2 -adrenoceptor subtype was dominant in untreated SHR in contrast to WKY, in which β 1 -adrenoceptor subtype was dominant. Enalaprilat treatment increased total β-adrenoceptor density, whereas both treatments restored the binding affinity and β 1 - and β 2 -adrenoceptor proportions to normal in SHR. Isoproterenol-, guanylylimidodiphosphate [Gpp(NH)p]–, and forskolin-stimulated adenylyl cyclase reactivity was increased in SHR. Enalaprilat restored adenylyl cyclase reactivity to normal in SHR and reduced the sensitivity (EC 50 ) of Gpp(NH)p-induced cAMP formation in both strains. The present study supports the possibility that functional alterations of the renin-angiotensin and sympathetic systems are involved in hypertension in SHR. The antihypertensive action of losartan and enalaprilat in SHR may be partly mediated through the normalization of sympathetic hyperreactivity and the restoration of β-adrenergic signaling pathway sensitivity.