Alterations in sympathetic neuroeffector transmission to mesenteric arteries but not veins in DOCA-salt hypertension

Abstract

We studied hypertension-associated changes in prejunctional alpha(2) adrenergic receptor (alpha(2)-AR) function using amperometry to monitor in vitro norepinephrine (NE) measured as oxidation currents. Vasoconstriction was measured using video imaging. NE release was induced by electrical stimulation of sympathetic nerves associated with mesenteric arteries (MA) and veins (MV) of sham and DOCA-salt hypertensive rats. NE oxidation currents were larger in DOCA-salt compared to sham MA; there were no differences between currents in sham and DOCA-salt MV. Increases in NE oxidation currents followed a multi-exponential time course in sham MA. In DOCA-salt MA and sham and DOCA-salt MV, the time course was mono-exponential. Yohimbine (alpha(2)-AR antagonist, 1 microM), caused a mono-exponential increase in NE oxidation currents in sham and DOCA-salt MA. Yohimbine increased NE oxidation currents and constrictions more in sham compared to DOCA-salt MA and compared to MV. UK 14,304 (alpha(2)-AR agonist, 1.0 microM), reduced currents less in DOCA-salt MA and sham and DOCA-salt MV compared to sham MA. Prazosin (alpha(1)-AR antagonist, 0.1 microM) did not alter NE oxidation currents. Prazosin inhibited constrictions more in DOCA-salt compared to sham MA and almost completely blocked constrictions in sham and DOCA-salt MV. Prazosin-resistant constrictions in MA were blocked by the P2 receptor antagonist, PPADS (10 microM). Prejunctional alpha(2)-ARs modify NE concentrations near neuroeffector junctions in MA and MV. alpha(2)-AR function is most prominent in MA and is impaired in DOCA-salt MA but not MV. Purinergic transmission predominates in sham MA. NE is the dominant vasoconstrictor in DOCA-salt MA and sham and DOCA-salt MV.

Fig. 1

Representative recordings of NE oxidation currents and constrictions of MA (A) and MV (B) from sham rats. Upper traces in each pair are oxidation currents while lower traces are diameter recordings. In both blood vessels responses were evoked by 60 stimuli applied at frequencies of 3 (upper) and 20 Hz (lower) in both blood vessels.

Fig. 2

Frequency–response relationship for oxidation currents evoked by focal electric stimulation in MA (A) and MV (B) from sham and DOCA-salt rats. In “A” * indicates significantly different from responses in sham MA. In “B” # indicates that NE oxidation currents recorded from sham MV are significantly greater than those recorded from sham MA (P< 0.05). Data are mean ±S.E.M. C. Representative recordings of the rising phase of the NE oxidation current in a sham and DOCA-salt MA (left) and sham and DOCA-salt MV (right). In the MA traces “1” and “2” indicate the two components to the time course of the current increase in sham MA.

Fig. 3

Effects of yohimbine on NE oxidation currents and neurogenic constrictions. (A) NE oxidation currents recorded in the presence of yohimbine (1 μM) in sham and DOCA-salt MA. (B) NE oxidation currents recorded in the presence of yohimbine in sham and DOCA-salt MV. Data are expressed as % of the response obtained in each tissue before the application of yohimbine. * Indicates a significant increase over control levels (100%, P< 0.05). # Indicates significantly different from the levels in DOCA-salt MA (P< 0.05). Frequency–response curves for peak neurogenic constrictions in sham and DOCA-salt MA (B) and sham and DOCA-salt MV (D) in the absence and presence of yohimbine. (E) Frequency-dependent increase in the area of neurogenic constrictions in the absence and presence of yohimbine in sham MA. * Indicates significantly different from sham MA in the absence of yohimbine. (F) Frequency-dependent increase in the area of neurogenic constrictions in the absence and presence of yohimbine in DOCA-salt MA. Yohimbine did not change the duration of constrictions in DOCA-salt MA. # Indicates significantly different from constriction area in sham MA at the 20 Hz stimulation. Data are mean ±S.E.M.

Fig. 4

Effect of UK 14,304 on NE oxidation currents and neurogenic constrictions. (A) NE oxidation currents recorded in the presence of UK 14,304 (1 μM) in sham and DOCA-salt MA. (B) NE oxidation currents recorded in the presence of UK 14,304 in sham and DOCA-salt MV. Data are expressed as % of the response obtained in each tissue before UK 14,304 application. * Indicates a significant decrease below control levels (100%, P< 0.05). # Indicates significantly different from the levels in DOCA-salt MA (P< 0.05). Frequency–response curves for neurogenic constrictions in sham and DOCA-salt MA (B) and sham and DOCA-salt MV (D) in the absence and presence of UK 14,304. Data are mean ±S.E.M.

Fig. 5

Effects of prazosin on NE oxidation currents and neurogenic constrictions in sham and DOCA-salt MA and MV. (A) NE oxidation currents recorded in the presence of prazosin (0.1 μM) in sham and DOCA-salt MA. (B) NE oxidation currents recorded in the presence of prazosin in sham and DOCA-salt MV. Data are expressed as % of the response obtained in each tissue before the application of prazosin. Frequency–response curves for neurogenic constrictions in sham and DOCA-salt MA (C) and sham and DOCA-salt MV (D) in the absence and presence of prazosin or prazosin plus PPADS (10 μM). In “C” *indicates significantly different from constriction in the presence of prazosin alone (P< 0.05). Data are mean ±S.E.M.