Preoptic recess lesions reduce right atrial pressure responses to volume expansion

Systemic angiotensin II and volume expansion release norepinephrine in the preoptic recess

The periventricular tissue surrounding the anteroventral portion of the third cerebral ventricle (AV3V) of the preoptic recess is critical for several responses evoked by central and peripheral angiotensin II (ang II), and the natriuresis produced by extracellular fluid volume expansion. In addition, several other studies have suggested that the pressor and natriuretic responses evoked by these stimuli are regulated by activation of noradrenergic systems in AV3V tissue. The purpose of the present experiments was to directly determine if i.v. infusion of ang II and volume expansion increase norepinephrine release in the AV3V area using in vivo microdialysis. Male, Sprague-Dawley rats were implanted with microdialysis probes in the AV3V and were tested conscious and unrestrained. Dialysate was collected before, during, and following i.v. administration of ang II or phenylephrine which raised blood pressure 40-50 mmHg, or volume expansion with isotonic saline. Dialysate norepinephrine concentrations from these rats were compared to dialysate norepinephrine content from animals which received no treatment. Both i.v. infusion of ang II and isotonic volume expansion significantly increased dialysate norepinephrine concentration, while infusion of phenylephrine did not alter norepinephrine release in AV3V tissue. These data demonstrate that peripheral ang II and volume expansion selectively increase norepinephrine release in the AV3V region, and are consistent with the conclusion that activation of noradrenergic systems in the AV3V region contribute to cardiovascular and drinking responses evoked by peripheral ang II and to natriuresis following volume expansion.

Central control of cardiac baroreflex responses during peripheral hyperosmolality

Acute increases in peripheral osmolality evoke a pressor response and baroreflex-mediated bradycardia. These experiments were designed to determine if the fall in heart rate during peripheral sodium loading is 1) equivalent to bradycardia accompanying phenylephrine (PE) infusion, 2) mediated by the parasympathetic (PSNS) or sympathetic (SNS) nervous system, and 3) controlled by the median preoptic nucleus (MnPO). Male rats received an intravenous infusion of isotonic saline, hypertonic saline (2.5 M NaCl), or PE for 30 min. Blood pressure increased equivalently in the hypertonic NaCl and PE groups. However, heart rate fell more in animals infused with PE. Furthermore, pretreatment with methylatropine to block the PSNS had no effect on bradycardia, whereas blocking SNS influences on cardiac function significantly attenuated the fall in heart rate during peripheral hyperosmolality. Finally, kainic acid administration in the MnPO before testing increased bradycardia observed during hypertonic saline loading. Taken together, these data suggest that acute peripheral hyperosmolality acts at the MnPO to reduce cardiac SNS withdrawal during the pressor response that reduces the associated baroreflex bradycardia.

Osmolality: a physiological long-term regulator of lumbar sympathetic nerve activity and arterial pressure

Acute infusion of hypertonic fluid increases mean arterial pressure (MAP) in part by elevating nonrenal sympathetic activity. However, it is not known whether chronic, physiological increases in osmolality also increase sympathetic activity. To test this hypothesis, MAP, heart rate (HR), and lumbar sympathetic nerve activity (LSNA) were measured in conscious, 48-h water-deprived rats (WD) during a progressive reduction in osmolality produced by a 2-h systemic infusion (0.12 ml/min) of 5% dextrose in water (5DW). Water deprivation significantly increased osmolality (308 +/- 2 vs. 290 +/- 2 mosmol/kgH2O, P < 0.001), HR (453 +/- 7 vs. 421 +/- 10 beats/min, P < 0.05), and LSNA (63.5 +/- 1.8 vs. 51.9 +/- 3.8% baroreflex maximum, P < 0.01). Two hours of 5DW infusion reduced osmolality (-15 +/- 5 mosmol/kgH2O), LSNA (-23 +/- 3% baseline), and MAP (-10 +/- 1 mmHg). To evaluate the role of vasopressin in these changes, rats were pretreated with a V1-vasopressin receptor antagonist. The antagonist lowered MAP (-5 +/- 1 mmHg) and elevated HR (32 +/- 7 beats/min) and LSNA (11 +/- 3% baseline) in WD (P < 0. 05), but not in water-replete, rats. 5DW infusion had a similar cumulative effect on all variables in V1-blocked WD rats, but had no effect in water-replete rats. Infusion of the same volume of normal saline in WD rats did not change osmolality, LSNA or MAP. Together these data indicate that, in dehydrated rats, vasopressin supports MAP and suppresses LSNA and HR and that physiological changes in osmolality directly influence sympathetic activity and blood pressure independently of changes in vasopressin and blood volume.