Acid-base disturbance and ventilatory response to changes in plasma osmolality in Pekin ducks

Vasopressin and Breathing: Review of Evidence for Respiratory Effects of the Antidiuretic Hormone

Vasopressin (AVP) is a key neurohormone involved in the regulation of body functions. Due to its urine-concentrating effect in the kidneys, it is often referred to as antidiuretic hormone. Besides its antidiuretic renal effects, AVP is a potent neurohormone involved in the regulation of arterial blood pressure, sympathetic activity, baroreflex sensitivity, glucose homeostasis, release of glucocorticoids and catecholamines, stress response, anxiety, memory, and behavior. Vasopressin is synthesized in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus and released into the circulation from the posterior lobe of the pituitary gland together with a C-terminal fragment of pro-vasopressin, known as copeptin. Additionally, vasopressinergic neurons project from the hypothalamus to the brainstem nuclei. Increased release of AVP into the circulation and elevated levels of its surrogate marker copeptin are found in pulmonary diseases, arterial hypertension, heart failure, obstructive sleep apnoea, severe infections, COVID-19 due to SARS-CoV-2 infection, and brain injuries. All these conditions are usually accompanied by respiratory disturbances. The main stimuli that trigger AVP release include hyperosmolality, hypovolemia, hypotension, hypoxia, hypoglycemia, strenuous exercise, and angiotensin II (Ang II) and the same stimuli are known to affect pulmonary ventilation. In this light, we hypothesize that increased AVP release and changes in ventilation are not coincidental, but that the neurohormone contributes to the regulation of the respiratory system by fine-tuning of breathing in order to restore homeostasis. We discuss evidence in support of this presumption. Specifically, vasopressinergic neurons innervate the brainstem nuclei involved in the control of respiration. Moreover, vasopressin V1a receptors (V1aRs) are expressed on neurons in the respiratory centers of the brainstem, in the circumventricular organs (CVOs) that lack a blood-brain barrier, and on the chemosensitive type I cells in the carotid bodies. Finally, peripheral and central administrations of AVP or antagonists of V1aRs increase/decrease phrenic nerve activity and pulmonary ventilation in a site-specific manner. Altogether, the findings discussed in this review strongly argue for the hypothesis that vasopressin affects ventilation both as a blood-borne neurohormone and as a neurotransmitter within the central nervous system.

Responses of gull kidneys and salt glands to NaCl loading

Glomerular filtration rate was measured in Glaucous-winged Gulls, Larus glaucescens, acclimated to freshwater or saline and was also measured (along with salt gland secretion) following intravenous and intragastric infusion of 700 mM sodium chloride (NaCl). Freshwater- and saline-acclimated gulls had the same mean glomerular filtration rate, 5.6 +/- 0.8 and 6.1 +/- 1.0 mL.min-1.kg-1; urine flow rate, 44 +/- 20 and 46 +/- 19 microL.min-1.kg-1; urine/plasma osmolality ratio, 1.2 +/- 0.2 and 1.7 +/- 0.1; urine/plasma [14C]polyethylene glycol ratio, 210 +/- 69 and 211 +/- 64; and Na+ reabsorption, 99.6 +/- 0.2 and 99.6 +/- 0.3%, respectively. Renal function during 143 mM NaCl infusion (glomerular filtration rate, 3.4 +/- 0.3 mL.min-1.kg-1; urine flow rate, 19.7 +/- 2.8 microL.min-1.kg-1; urine/plasma osmolality ratio, 1.7 +/- 0.1; urine/plasma [14C]polyethylene glycol ratio, 223 +/- 26, and Na+ reabsorption 99.9 +/- 0.0%) was not affected by subsequent infusion of 700 mM NaCl, although plasma osmolality was significantly increased. The urine/plasma ratio for [14C]polyethylene glycol, but not osmolality, was higher in gulls than in other avian species. Intravenously infused NaCl elicited less concentrated salt gland secretion at a significantly lower threshold plasma osmolality than intragastric NaCl infusion. Salt gland secretion and glomerular filtration rates were inversely related.

Hyperosmolality alters the ventilatory response to acute hypercapnia and hypoxia

Acute hyperosmolality in the Pekin duck results in an extracellular acidosis and hypercarbia without any stimulation of ventilation. The development of the extracellular acidosis is accompanied by the concurrent development of an intracellular alkalosis systemically which has been hypothesized to depress ventilation (Kasserra et al., J. Appl. Physiol., 1993). In order to investigate this apparent suppression of ventilation, the ventilatory response to various respiratory challenges (CO2, O2, K+) was studied both before (normosmotic) and after (hyperosmotic) hypertonic sucrose infusion. Increased plasma osmolality caused a significant drop in arterial pH of 0.06 +/- 0.01 units and a 4 Torr increase in PaCO2, yet did not stimulate any significant increase in ventilation despite a significant increase in oxygen consumption. Acute hyperosmolality increased the PaCO2 associated with resting ventilation, and decreased the magnitude of the ventilatory response to a given increase in PaCO2, compared with the response to the same ventilatory challenge in normosmotic animals. Acute hyperosmolality increased the ventilatory response to hypoxia and K+ compared with normosmotic animals. The opposite effect of hyperosmolality on the ventilatory responses to hypercapnia compared with hypoxia suggests that the mechanisms of chemoreception for hypercapnia and hypoxia are different. The depressed ventilatory response curve to increased PaCO2 and decreased arterial pH during hyperosmolality, both alone and during the hypercapnic challenge, suggests that the peripheral chemoreceptor response to pH and CO2 is suppressed. It is hypothesized that the suppression results from the intracellular alkalosis occurring during acute hyperosmolality.

Acid-base changes on transfer between sea- and freshwater in the Chinese crab, Eriocheir sinensis

The time course of changes in hemolymph pH, PCO2 and bicarbonate concentration were followed in euryhaline Chinese crabs, Eriocheir sinensis, after transfer from seawater (SW) to freshwater (FW) and vice versa. In order to correlate these changes with the animal's ionic status, hemolymph osmolarity and Na+ and Cl- concentrations as well as net exchanges of acid-base equivalents with the external water were also measured. Transfer from SW to FW induced a metabolic alkalosis which peaked after 3 days and declined thereafter but remained significant by 29 days. Hemolymph PCO2 was markedly elevated in FW, thus moderating the increase in hemolymph pH. Osmolarity and Na+ and Cl- concentrations reached a new steady state at lowered values by 24-48 h and a significant outflux of acidic equivalents was measured only during the first 2 days. Transfer from FW to SW conversely induced a metabolic acidosis which was almost fully compensated by a marked hypocapnia. It is concluded that external acid-base exchanges coupled to ionic readjustments following a salinity transition cannot solely explain the associated acid-base disturbances, the metabolic component of which must also rely on tissular processes probably linked to cell volume regulation.

Osmoregulatory responses of glucous-winged gulls (Larus glaucescens) to dehydration and hemorrhage

The effects of dehydration and hemorrhage on plasma ionic, osmotic, and antidiuretic hormone (arginine vasotocin) concentrations and of hemorrhage on salt gland secretion and glomerular filtration rate were evaluated in glaucous-winged gulls, Larus glaucescens. Dehydration for 24 h did not affect plasma ionic, osmotic or arginine vasotocin concentrations; 72 h dehydration significantly elevated plasma osmolality, plasma sodium and chloride concentrations, and plasma arginine vasotocin concentration, but did not affect plasma potassium concentration. Constant infusion of 0.8 mol.1-1 NaCl increased plasma arginine vasotocin concentration and produced salt gland secretion in seven gulls; four secreted well, while three secreted less well. Removal of 20% blood volume during saline infusion immediately reduced (P<0.001) salt gland secretion rate in all gulls. After bleeding, good secretors maintained glomerular filtration rate and urine flow rate; the poorer secretors increased glomerular filtration rate and became diuretic. Blood replacement returned salt gland secretion rate to the prebleeding level (P<0.05) without affecting salt gland secretions sodium concentration in gulls which secreted well, but did not restimulate salt gland secretion in gulls which secreted poorly. Reinfusion of blood had no effect on glomerular filtration rate. Bleeding and blood replacement did not affect plasma arginine vasotocin concentration.