Changes in extracellular pH mediate the chronotropic responses to L-arginine.

UNLABELLED: We have recently shown that exogenous nitric oxide (NO) elicits a positive chronotropic response by stimulating the hyperpolarization activated current, I(f). OBJECTIVE: To examine whether L-arginine (L-Arg) can mimic the chronotropic effect of NO by enhancing its endogenous production. METHODS: In spontaneously beating guinea pig atria we evaluated the heart rate (HR) response to increasing concentrations of L-Arg (1 mumol/l to 10 mmol/l), and compared it with that for D-Arg or L-lysine (L-Lys) (all in free base (FB) or hydrochloride (HCl) formulation). RESULTS: L-ArgFB > 100 mumol/l caused a reversible dose-dependent increase in HR (peak effect +64 +/- 7 bpm at 10 mmol/l, P < 0.05, n = 8). However, a similar HR response occurred with D-ArgFB (n = 7) or L-LysFB (n = 6). All FB formulations increased the perfusate pH (peak [pH]o = 8.61 +/- 0.03). Although alkalinization can stimulate NO release from the endothelium, this is unlikely to have contributed to HR changes in our preparation, since neither NG-methyl-L-arginine, (100-500 mumol/l, which per se reduced HR by 8 +/- 1%, P < 0.05, n = 9) nor NO scavenging (fresh 5% red blood cells, n = 9) caused a rightward shift of the concentration-response curve to L-ArgFB. Furthermore, as opposed to FB formulations, L-ArgHCl, D-ArgHCl or L-LysHCl > 1 mmol/l significantly decreased HR and [pH]o (n = 17). The chronotropic effects of L-ArFB or L-ArgHCl were reproduced by changing [pH]o with NaOH (n = 8) or HCl (n = 7), whereas the HR increase with L-ArgFB was prevented by clamping [pH]o at 7.42 +/- 0.07 (n = 10). CONCLUSIONS: In vitro, L-Arg can markedly affect HR through a pH-mediated, NO-independent mechanism. Our data show that the opposing changes in [pH]o induced by different formulations of L-Arg can importantly confound the assessment of the biological effects of this amino acid.