Frequency response characteristic of sympathetic mediated low-frequency blood pressure fluctuations in conscious rats

Oral Intake of Rosiglitazone Promotes a Central Antihypertensive Effect Via Upregulation of Peroxisome Proliferator-Activated Receptor-γ and Alleviation of Oxidative Stress in Rostral Ventrolateral Medulla of Spontaneously Hypertensive Rats

Rosiglitazone, a synthetic ligand of transcription factor peroxisome proliferator-activated receptor-γ (PPAR-γ), possesses a blood pressure–lowering effect beyond insulin sensitizing and glucose lowering. Oxidative stress in rostral ventrolateral medulla (RVLM), where sympathetic premotor neurons for the maintenance of neurogenic vasomotor tone are located, contributes to neural mechanisms of hypertension. Activation of PPAR-γ protects against oxidative stress in RVLM by upregulation of mitochondrial uncoupling protein 2 (UCP2). We tested the hypothesis that oral intake of rosiglitazone exerts a central antihypertensive effect by ameliorating oxidative stress in RVLM via transcriptional upregulation of UCP2 after PPAR-γ activation. In adult spontaneously hypertensive rats but not normotensive Wistar-Kyoto rats, oral intake of rosiglitazone for 1 week resulted in vasodepression and a reduction in the vasomotor components of the systemic arterial pressure spectrum, our experimental index for sympathetic vasomotor tone. These antihypertensive effects of rosiglitazone in spontaneously hypertensive rats were abrogated by microinjection bilaterally into RVLM of PPAR-γ small interfering RNA. Oral intake of rosiglitazone also upregulated UCP2 and ameliorated the heightened superoxide anion level in RVLM of spontaneously hypertensive rats. Protection against oxidative stress in RVLM by rosiglitazone was abrogated by PPAR-γ small interfering RNA or by antisense oligonucleotide against ucp2 mRNA. Gene knockdown of ucp2 in RVLM also reversed the antihypertensive effect of rosiglitazone. These results suggest that oral intake of rosiglitazone promotes a central antihypertensive effect by decreasing sympathetic vasomotor activity through a PPAR-γ–dependent protection against oxidative stress in RVLM via transcriptional upregulation of the mitochondrial UCP2.

The correlation of mean sympathetic activity with low-frequency blood pressure and sympathetic variability

The low-frequency (0.2-0.8 Hz) component of blood pressure (BP) variability (LF(BP)) is used as an index of the low-frequency variability of sympathetic nerve activity (SNA) (LF(SNA)) in rats. It is unclear whether the LF(BP) can be used as an index of the mean SNA (mSNA). We investigated the correlation of the LF(BP) with different levels of the mSNA in this study to evaluate if it is a feasible tool for detecting differences in mSNA under physiological conditions. Correlation of the LF(SNA) with different mSNA levels was also investigated. The BP and renal SNA of rats were recorded in a nonanesthetized state. Values of the mSNA obtained from 531 recording epochs in six rats were graded into 30 levels with a bin resolution of 0.05 normalized units. A linear regression analysis showed that the correlation between the mSNA and LF(SNA) was higher than that between the mSNA and LF(BP). The mSNA was well correlated with the LF(SNA) over a wider mSNA range, while it was correlated with the LF(BP) only in a restricted range. These results demonstrated a restricted condition under which measuring the LF(BP) can be a definitive index of the mSNA, and further suggest the possibility of using the weighted LF(BP) as an index of the mSNA via intermediation by the LF(SNA) for a wider mSNA range.

Chronic blockade of phosphatidylinositol 3-kinase in the nucleus tractus solitarii is prohypertensive in the spontaneously hypertensive rat

Phosphatidylinositol 3-kinase (PI3K) within brain stem neurons has been implicated in hypertension in the spontaneously hypertensive rat (SHR). Previously, we demonstrated elevated expression of PI3K subunits in rostral ventrolateral medulla and paraventricular nucleus of SHRs compared with Wistar-Kyoto rats. Here, we considered expression levels of PI3K in the nucleus tractus solitarii, a pivotal region in reflex regulation of arterial pressure, and determined its functional role for arterial pressure homeostasis in SHRs and Wistar-Kyoto rats. We found elevated mRNA levels of p110beta and p110delta catalytic PI3K subunits in the nucleus tractus solitarii of adult (12 to 14 weeks old) SHRs relative to the age-matched Wistar-Kyoto rats (fold differences relative to beta-actin: 1.7+/-0.2 versus 1.01+/-0.08 for p110beta, n=6, P<0.05; 1.62+/-0.15 versus 1.02+/-0.1 for p110delta, n=6, P<0.05). After chronic blockade of PI3K signaling in the nucleus tractus solitarii by lentiviral-mediated expression of a mutant form of p85alpha, systolic pressure increased from 175+/-3 mm Hg to 191+/-6 mm Hg (P<0.01) in SHRs but not in Wistar-Kyoto rats. In addition, heart rate increased (from 331+/-6 to 342+/-6 bpm; P<0.05) and spontaneous baroreflex gain decreased (from 0.7+/-0.07 to 0.5+/-0.04 ms/mm Hg; P<0.001) in the SHRs. Thus, PI3K signaling in the nucleus tractus solitarii of SHR restrains arterial pressure in this animal model of neurogenic hypertension.