Osmolality, NaCl dietary intake, and regulation of ventilation by CO2

The ventilatory response to CO2 (VR) was examined in six awake dogs during the 2nd week of sequential dietary periods of low NaCl (less than 5 meq NaCl/day), high NaCl (approximately 120 meq NaCl/day), and then repeated low NaCl. Water intake was maintained constant at 77 ml.kg-1.day-1. PCO2 of arterial blood (PaCO2) and the PaCO2 threshold of the VR increased between the high-NaCl and the second low-NaCl period; the PaCO2 threshold did not change between the first low-NaCl and the high-NaCl period. In contrast, arterial [H+] ([H+]a) threshold of the VR increased between the first low-NaCl period and the high-NaCl period, but did not change further in the second low-NaCl period. Variations in [H+]a threshold of the VR between dogs on high- and low-NaCl diets were correlated with plasma osmolality, and this accounted for a positive linear relation between [H+]a and plasma osmolality. During the high-NaCl period, the arterial strong ion difference [[SID]a = ([Na+] + [K+]) - ([lactate-] + [Cl-])] decreased, forcing [H+]a to increase. However, during high-NaCl diet, PaCO2 decreased relative to plasma osmolality, counterbalancing the decrease in [SID]a and maintaining the relation between [H+]a and plasma osmolality. The compensatory mechanism for the decrease in PaCO2 during high NaCl was a shift in the relation between PaCO2 threshold of the VR and plasma osmolality to a lower PaCO2 threshold. Chemoreceptor, osmoreceptor, and/or humoral mechanisms are potentially involved in these respiratory adaptations to alterations in electrolyte and water balance.