THE EFFECTS OF GASTRIC AND HOMEOSTATIC AUTONOMIC AFFERENT REFLEXES ON CARDIAC AUTONOMIC EFFERENT ACTIVITY IN HEALTHY HUMAN VOLUNTEERS

Abstract

The effects of gastric autonomic afferent reflexes on cardiovascular autonomic efferent activity are regarded as a direct neural effect of the activation of gastric receptors which send afferent inputs to the central nervous system (CNS) to cause various cardiovascular changes (Van Orshoven et al., 2004; McHugh et al., 2010; Girona et al., 2014). However, the cardiovascular responses to liquid ingestion in humans may be related to gastric distension, volume loading effects, or to its osmotic proprieties. The purpose of this study was to investigate cross autonomic reflex function and to elucidate the effects of the resulting cardiac efferent autonomic activity in resting young healthy subjects. The ingestion of 300 mL of isothermic water increased both the cardiac vagal tone as indicated by increased RMSSD (mean 23.95 ± 20.50 msec increase, p<0.05) and sympathetic activity shown by increased QTc interval (mean 9.86 ± 8.59 msec increase, p< 0.05) during the first 40 minutes post-ingestion. These effects were absent with an identical volume of a physiological (0.9% w/v) saline solution which would increase plasma volume more, indicating that the cardiovascular responses to water drinking are influenced by its hypo-osmotic properties, rather than being related to the volume loading effects. Nevertheless, subjects responded to gastric distension with an ingestion of 300 mL of Fybogel solution with an increase in sympathetic activity during the first 20 minutes post-ingestion, but not in cardiac vagal tone. These results indicate that the mechanisms underlying the cardiovascular responses to water ingestion have additional components to the gastric stretch effect. Contrarily, the cold mediated sympathetic inhibition after drinking the same volume of either cold water or cold Fybogel solution probably happened in the NTS where the two branches of the ANS meet for the first time during their central pathway (Kubin et al., 2006; Thayer and Lane, 2009). In conclusion, the cardiovascular responses to water drinking are influenced by its hypo-osmolality properties and temperature, not by the volume loading effects.