Involvement of gamma-aminobutyric acid (GABA) B receptors in the hypotensive effect of systemically administered GABA in spontaneously hypertensive rats

Effect of a γ-aminobutyric acid-enriched dairy product on the blood pressure of spontaneously hypertensive and normotensive Wistar–Kyoto rats

We investigated the blood-pressure-lowering effects of γ-aminobutyric acid (GABA) and a GABA-enriched fermented milk product (FMG) by low-dose oral administration to spontaneously hypertensive (SHR/Izm) and normotensive Wistar–Kyoto (WKY/Izm) rats. FMG was a non-fat fermented milk product produced by lactic acid bacteria, and the GABA contained in FMG was made from the protein of the milk during fermentation. A single oral dose of GABA or FMG (5 ml/kg; 0·5 mg GABA/kg) significantly (P>0·05) decreased the blood pressure of SHR/Izm from 4 to 8 h after administration, but did not increase that of WKY/Izm rats. The hypotensive activity of GABA was dose-dependent from 0·05 to 5·00 mg/kg in SHR/Izm. During the chronic administration of experimental diets to SHR/Izm, a significantly slower increase in blood pressure with respect to the control group was observed at 1 or 2 weeks after the start of feeding with the GABA or FMG diet respectively (P>0·05) and this difference was maintained throughout the period of feeding. The time profile of blood-pressure change due to administration of FMG was similar to that of GABA. FMG did not inhibit angiotensin 1-converting enzyme. Furthermore, an FMG peptide-containing fraction from reverse-phase chromatography lacked a hypotensive effect in SHR/Izm rats. The present results suggest that low-dose oral GABA has a hypotensive effect in SHR/Izm and that the hypotensive effect of FMG is due to GABA.

Role of the renal nerves in γ-aminobutyric acid-induced antihypertensive effect in spontaneously hypertensive rats

The aim of this study was to clarify the role of the renal sympathetic nerves in the gamma-aminobutyric acid (GABA)-induced hypotensive effect in spontaneously hypertensive rats. Male spontaneously hypertensive rats (SHR/Izm) aged 7 weeks were divided into four groups on the basis of diet (containing 0.05% GABA, or GABA-free control diet) and operation (renal sympathetic-denervated or sham-operated) (n = 10, each). Water intake, urine volume and urinary sodium were, or tended to be, slightly higher, while plasma renin activity was significantly lower in the GABA group than the GABA-free control group. GABA inhibited the development of hypertension in sham-operated spontaneously hypertensive rats but not in renal-denervated spontaneously hypertensive rats. Plasma renin activity was significantly higher in sham-operated spontaneously hypertensive rats fed the control diet than in the other three groups. These results suggest that a reduction in the effects induced by the renal nerves may play an important role in the hypotensive effect induced in spontaneously hypertensive rats by chronic dietary administration of GABA.

The effects of load on systolic mitral annular velocity by tissue Doppler imaging

Tissue Doppler Imaging (TDI) provides information on systolic function through its systolic mitral annulus velocity wave (Sm), reflecting the peak velocity of shortening of the myocardial fibers oriented in the longitudinal direction. In this study, we evaluated the effect of load changes on Sm. Forty-two cardiac surgical patients with left ventricular ejection fraction >60% were consecutively evaluated. In 24 patients, load was changed with an IV bolus of phenylephrine (50-100 microg) or nitroglycerine (300-500 microg); in 18 patients, preload was changed with a rapid infusion of 500 mL of a gelatin solution. The sample volume of TDI was placed at the lateral side of the mitral annulus in the mid-esophageal 4-chamber view. Changing loading conditions with phenylephrine or nitroglycerine had no effect on Sm; the increase of preload in 18 patients resulted in a statistically significant increase of Sm (baseline, 8.4 +/- 2.6 cm/s; after increase of preload, 9.6 +/- 2.5 cm/s; P = 0.001). We conclude that Sm is dependent on changes in preload obtained by volume loading and cannot be recommended as an index of ventricular contractile performance in critically ill patients where significant changes in ventricular filling occur.