Objective: To determine the acute hemodynamic effects of the nonessential amino acid, glycine, and its precursor, L-serine, in normotensive and hypertensive rats.
Methods: Changes in mean arterial pressure and heart rate evoked by comparable intravenously administered doses (0.3-3.0 mmol/kg) of L-serine, D-serine and glycine were examined in anaesthetized normotensive 14-week-old male Sprague-Dawley, Wistar-Kyoto (WKY) rats, spontaneously hypertensive rats and WKY rats subjected to chronic nitric oxide synthase inhibition by treatment with NG nitro L-arginine methyl ester (0.7 mg/ml in drinking water for 5 days).
Results: L-Serine evoked a greater maximal fall in mean arterial pressure [L-serine vs. D-serine in Sprague-Dawley rats, mean +/- standard error of the mean values (mmHg): 30 +/- 3 vs. 20 +/- 5, P < 0.05; in control WKY rats: 46 +/- 3 vs. 30 +/- 4, P < 0.05; in NG nitro L-arginine methyl ester-treated WKY rats: 93 +/- 6 vs. 41 +/- 5, P < 0.01; in spontaneously hypertensive rats: 81 +/- 7 vs. 39 +/- 5 P < 0.01]. The effects of L-serine were significantly reduced in rats pretreated with a combination of apamin and charybdotoxin, inhibitors of the small conductance and intermediate conductance calcium-activated potassium (KCa) channels. Glycine elicited a dose-dependent fall in mean arterial pressure in normotensive WKY rats (25 +/- 4; P < 0.01) and evoked pressor responses in both spontaneously hypertensive rats (29 +/- 3; P < 0.01) and NG nitro L-arginine methyl ester-pretreated hypertensive WKY (39 +/- 5; P < 0.01) rats. Both the depressor and pressor responses to glycine were abolished by pretreatment with the N-methyl D-aspartate receptor antagonist, MK-801.
Conclusion: The profound stereo-selective antihypertensive effect of L-serine is neither mediated nor mimicked by glycine. It does not require N-methyl D-aspartate receptor activation by glycine but likely involves activation of endothelial KCa channels. L-Serine is a potential antihypertensive agent.