Hemodynamic characterization of hypertension induced by chronic intrarenal or intravenous infusion of norepinephrine in conscious rats.

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Abstract
The present study was designed to determine the hemodynamic changes underlying the hypertension induced by chronic intrarenal infusion of norepinephrine (NE) in conscious rats. NE was infused for a 5-day period intrarenally with osmotic minipumps via a chronic catheter in the right suprarenal artery at rates of 4 and 36 micrograms . kg-1 . hr-1 or intravenously at a rate of 36 micrograms . kg-1 . hr-1. Control rats received a 1 microliter . hr-1 intrarenal infusion of pyrogen-free 0.9% NaCl. In separate experiments, short-term effects were measured continuously during a 22- to 24-hour intrarenal infusion of 4 and 36 micrograms NE . kg-1 . hr-1 or intravenous infusion of 36 micrograms NE . kg-1 . hr-1. Intrarenal infusion of NE produced a more pronounced long-term hypertensive effect than infusion of the same dose intravenously. This hypertension was characterized by a rapid and sustained increase in total peripheral resistance index (TPRI). Despite of the initial renal vasoconstriction, specifically produced during the first 24 hours of intrarenal NE application, cardiac index (CI) in parallel to stroke volume index (SVI) decreased significantly during intrarenal as well as during intravenous NE infusion. Furthermore, no signs of sodium retention were observed. Both rates of intrarenal NE infusion have been shown previously to produce a significant long-term increase in plasma potassium concentration, and the present study indicates that this is presumably the result of decreased urinary potassium output. It is concluded that chronic hypertension produced by intrarenal or intravenous infusion is not volume-dependent. The relatively greater increase in TPRI during intrarenal NE infusion is attributed to vascular wall receptor sensitization by increased plasma potassium levels resulting from effects of intrarenally present NE on tubular cation exchange mechanisms.