Joan T. Crofton

Central Adrenergic Control of Vasopressin Release

The role of central adrenergic receptors in the control of vasopressin release was studied in the conscious rat. Norepinephrine (10 micrograms) and the alpha-1 agonist, phenylephrine (50 micrograms), administered intracerebroventricularly resulted in significant increases in the plasma vasopressin concentration and blood pressure. The alpha-2 agonist, BHT 933 (50 micrograms) and the beta agonist, isoproterenol (10 micrograms) both caused a significant decrease in the plasma vasopressin concentration with only small changes in blood pressure. The central administration of the alpha-1 antagonist corynanthine (20 micrograms) had no effect on the plasma vasopressin concentration; however, increases in plasma vasopressin levels were observed when either the alpha-2 antagonist yohimbine (20 micrograms) or the beta antagonist propranolol (20 micrograms) were given. It is concluded that central noradrenergic pathways may play an important role in the control of vasopressin release and that this control may involve alpha-1, alpha-2 and beta adrenoreceptors.

Effects of 17β-estradiol on sympathetic activity and pressor response to phenylephrine in ovariectomized rats

The effects of 17beta-estradiol (E2) on sympathetic activity were examined in conscious unrestrained ovariectomized rats, instrumented under methohexital anesthesia to record mean arterial pressure (MABP), heart rate (HR), renal nerve activity (RNA), and splanchnic nerve activity (SNA) 1 day before the experiment. Injection of E2 (150 micrograms/kg iv) caused reductions (P < 0.01) in RNA (29 +/- 6%), SNA (25 +/- 2%), and HR (26 +/- 5 beats/min) within 20 min, but MABP remained unchanged. Ninety minutes after intravenous injection of E2 or vehicle, intravenous infusion of phenylephrine (PE; 6.2 micrograms . min-1 . kg-1) induced similar increases in MABP and decreases in HR, RNA, and SNA in both groups. By contrast, in rats chronically treated with E2, the pressor response to PE was smaller (P < 0.01; 22 +/- 5 mmHg) than in vehicle-treated rats (40 +/- 4 mmHg). The changes in HR, RNA, and SNA were similar in both groups, but the ratios of changes in HR and SNA to MABP, an index of baroreflex sensitivity, were greater in the E2-treated rats. These findings suggest that E2 can act centrally to modulate sympathetic function and thereby participate in cardiovascular regulation.The effects of 17β-estradiol (E2) on sympathetic activity were examined in conscious unrestrained ovariectomized rats, instrumented under methohexital anesthesia to record mean arterial pressure (MABP), heart rate (HR), renal nerve activity (RNA), and splanchnic nerve activity (SNA) 1 day before the experiment. Injection of E2 (150 μg/kg iv) caused reductions ( P < 0.01) in RNA (29 ± 6%), SNA (25 ± 2%), and HR (26 ± 5 beats/min) within 20 min, but MABP remained unchanged. Ninety minutes after intravenous injection of E2 or vehicle, intravenous infusion of phenylephrine (PE; 6.2 μg ⋅ min-1 ⋅ kg-1) induced similar increases in MABP and decreases in HR, RNA, and SNA in both groups. By contrast, in rats chronically treated with E2, the pressor response to PE was smaller ( P < 0.01; 22 ± 5 mmHg) than in vehicle-treated rats (40 ± 4 mmHg). The changes in HR, RNA, and SNA were similar in both groups, but the ratios of changes in HR and SNA to MABP, an index of baroreflex sensitivity, were greater in the E2-treated rats. These findings suggest that E2 can act centrally to modulate sympathetic function and thereby participate in cardiovascular regulation.

Changes in vasopressin concentration in plasma and cerebrospinal fluid in response to hemorrhage in anesthetized dogs.

In the anesthetized dog, the concentrations of vasopressin (ADH) in plasma and cerebrospinal fluid (CSF) were similar under basal conditions, and there was a highly significant positive correlation between them (r = 0.71, p less than 0.01). Although hemorrhage was capable of increasing the ADH concentration in both plasma and CSF, the threshold for the increase in plasma ADH was much lower than for the increase in the concentration of ADH in CSF. In addition, the magnitude of the increase in the concentration of ADH in plasma was considerably greater than that in CSF at a comparable degree of hemorrhage. Our results suggest that ADH released into CSF during hemorrhage may have a different origin from that released into blood.

Vasopressin in Rats With Genetic and Streptozocin-Induced Diabetes

Rats were administered streptozocin (STZ; 50 or 75 mg/kg i.v., tail vein) or vehicle. Approximately 2 wk later, venous and arterial catheters were implanted for subsequent (24 h later) vasopressin, electrolyte, and hemodynamic measurements. STZ-induced diabetic (STZ-D) rats demonstrated a dose-dependent increase in the plasma glucose concentration, plasma osmolality, and plasma vasopressin concentration. Mean arterial blood pressure (MABP) was unchanged, but heart rate was reduced. Diabetes-prone BB rats, maintained on or withdrawn from insulin treatment for 24–48 h, and diabetes-resistant rats were instrumented and studied as above. Spontaneous-diabetes–prone rats demonstrated increases in plasma glucose concentration and plasma osmolality similar to STZ-D rats but had significantly greater plasma vasopressin concentrations. The significant decrease in MABP observed in these animals probably contributed to the enhanced vasopressin response. We conclude that both osmotic and cardiovascular parameters play important roles in vasopressin secretion in diabetic rats.

Effect of Intravenous and Intracerebroventricular Infusion of Hypertonic Solutions on Plasma and Cerebrospinal Fluid Vasopressin Concentrations

In the anesthetized dog, intravenous infusion of 2.5 M saline (40 microliters/kg . min) increased plasma and cerebrospinal fluid (CSF) osmolality and the plasma vasopressin (ADH) concentration, but did not increase the CSF ADH concentration. The increase in the plasma ADH concentration coincided with the increase in plasma osmolality, but preceded the increase in CSF osmolality. Intracerebroventricular infusion of hypertonic artificial CSF (2,000 mosm/kg . H2O, 10 microliters/min) increased CSF osmolality and plasma CSF ADH concentrations; plasma osmolality did not increase. Thus, receptors which sense changes in plasma osmolality appear to be outside the blood-brain barrier; different receptors may sense changes in CSF osmolality.

The Handling of Immunoreactive Vasopressin by the Isolated Perfused Rat Kidney

Using the isolated rat kidney perfused with an artificial medium containing glucose as the sole fuel, we studied the renal handling of immunoreactive arginine vasopressin (AVP) and determined the effect of various factors on the ability of the kidney to remove AVP. In control kidneys perfused with AVP at concentrations below 116 muU/ml, the organ clearance of AVP (OC(AVP)) was 1,145+/-47 (SE) mul/min, whereas glomerular filtration rate (GFR) averaged 515+/-37 mul/min. Filtration could thus account for up to 45% of the OC(AVP), the balance presumably being cleared from the peritubular circulation. Of the AVP filtered, only 38% could be recovered in the urine (urinary clearance AVP averaged 205+/-12 mul/min) suggesting that the balance was taken up by the tubular epithelium and degraded. Fractional excretion of filtered AVP rose significantly in the presence of anoxia and cold (10 degrees C) to 49 and 59%, respectively, but was not affected by ouabain or high levels of AVP (458+/-58 muU/ml). The OC(AVP) was not significantly altered by the absence of glucose in the perfusate, anoxia, or ureteral ligation, maneuvers that were associated with significant reductions in GFR. In these and the control experiments, there was a significant inverse correlation between GFR and peritubular clearance emphasizing the importance of the latter (r = -0.749; P < 0.001). Cold, ouabain, and high concentrations of AVP reduced the clearance of AVP by the kidneys. On the basis of these studies we conclude that the kidney clears AVP from the circulation via two pathways, glomerular clearance and peritubular clearance. This exposes both the luminal and contraluminal surfaces of the tubular cells to the hormone, allowing these cells to remove AVP from the filtrate and the peritubular compartment. Noteworthy is the observation that under several conditions when GFR falls reducing the glomerular clearance of AVP, peritubular clearance increases and the total clearance of AVP by the kidney remains constant.

Interrelationship between central bradykinin and vasopressin in conscious rats

Intracerebroventricular administration of bradykinin (1, 5 and 20 micrograms) into conscious rats resulted in significant dose-dependent increases in the plasma vasopressin concentration, mean arterial blood pressure and heart rate. Peripheral blockade of the pressor action of vasopressin with a vasopressin pressor antagonist (10 micrograms/kg, i.v.) did not cause an attenuation but rather a potentiation and prolongation of the pressor effects of central bradykinin (20 micrograms). Central administration of the vasopressin antagonist (150 ng) caused no peripheral blockade of the pressor effects of exogenous i.v. vasopressin but almost abolished the bradykinin-induced tachycardia, with little effect on the pressor effects of central bradykinin (20 micrograms). The results indicate that centrally administered bradykinin stimulates vasopressin release into the plasma and that central vasopressin may modulate the cardiovascular actions of central bradykinin.

Central Effects of Dopamine and Bromocriptine on Vasopressin Release and Blood Pressure

In order to investigate the role of central dopaminergic receptors in the control of vasopressin release and in cardiovascular regulation, the effects of intracerebroventricular administration of dopamine (DA) and bromocriptine (BC), a specific DA agonist, were compared in the anesthetized dog. The drugs were infused over a 20-min period into a lateral ventricle. DA brought about a transient decrease in mean arterial blood pressure, a slight increase in heart rate toward the end of the experiment, and a suppression of vasopressin release. BC increased heart rate and decreased blood pressure to a greater extent than did DA, and doubled the plasma vasopressin concentration. The increase in vasopressin secretion preceded the fall in blood pressure, ans was, therefore, due to a direct central action of BC. Although in these circumstances it is difficult to determine the role of dopaminergic neurons in the control of vasopressin release, there is some reason to believe that this role may be expressed by the actions of BC under the present experimental conditions.

Role of vasopressin in regulation of renal kinin excretion in Long-Evans and diabetes insipidus rats.

To study the relationship between vasopressin and the renal kallikrein-kinin system we measured the rate of excretion of kinins into the urine of anesthetized rats during conditions of increased and decreased vasopressin level. The excretion of immunoreactive kinins in Brattleboro rats with hereditary diabetes insipidus (DI) (24 +/- 3 pg min-1 kg-1) was lower than in the control Long Evans (LE) rats (182 +/- 22 pg min-1 kg-1; P less than 0.05). The DI rats also exhibited negligible urinary excretion of immunoreactive vasopressin, reduced urine osmolality, and increased urine flow and kininogenase excretion. In LE rats, volume expansion by infusion of 0.45% NaCl-2.5% dextrose to lower vasopressin secretion reduced (P less than 0.05) kinin excretion, vasopressin excretion, and urine osmolality to 41, 26, and 15% of their respective control values, while increasing (P less than 0.05) urine flow and kininogenase excretion. On the other hand, the infusion of 5% NaCl, which promotes vasopressin secretion, increased (P less than 0.05) the urinary excretion of kinins and vasopressin to 165 and 396% of control, while increasing (P less than 0.05) urine flow and kininogenase excretion. Infusion of vasopressin (1.2 mU/h, intravenous) enhanced (P less than 0.05) kinin excretion by two to threefold in DI rats and in LE rats during volume expansion with 0.45% NaCl-2.5% dextrose, while decreasing urine flow and increasing urine osmolality. This study demonstrates that the urinary excretion of immunoreactive kinins varies in relation to the urinary level of vasopressin, irrespective of urine volume and osmolality and of the urinary excretions of sodium and kininogenase. The study suggests a role for vasopressin in promoting the activity of the renal kallikrein-kinin system in the rat.

Hypothalamic Knife Cuts Alter Fluid Regulation, Vasopressin Secretion, and Natriuresis during Water Deprivation

To investigate central neural pathways involved in release of vasopressin and in fluid electrolyte regulation, a retractable wire knife was used to make coronal knife cuts posterior to the organum vasculosum lamina terminalis (OVLT). 4 days following cuts or control surgery, animals were housed in metabolism cages and: (1) deprived of food and water for 48 h; (2) deprived of water only for 48 h; or (3) allowed continuous access to food and water. Water ingestion, food ingestion, urine volume, sodium excretion and urine osmolality were recorded daily. Trunk blood was then collected following decapitation for determination of plasma vasopressin, sodium, and protein concentrations, and osmolality. Animals with knife cuts and ad libitum access to food and water had significantly higher plasma osmolality (310 +/- 2 mosm/kg), and plasma vasopressin concentration (2.02 +/- 0.5 microunits/ml) than controls (306 +/- 1 mosm/kg and 0.60 +/- 0.04 microunits/ml, respectively). When rats were deprived of both food and water, there were no significant differences between the two groups in plasma vasopressin concentration, although plasma osmolality wa higher in animals with cuts. However, rats with knife cuts deprived of water only had significantly higher plasma osmolality (358 +/- 8 mosm/kg), sodium (164 +/- 19 mEq/l) and vasopressin (17.7 +/- 4 microunits/ml), than similarly treated control animals (317 +/- 1 mosm/kg, 145.5 +/- 1.0 mEq/1, 5.5 +/- 3 microunits/ml, respectively). These data indicate that a neural pathway in this brain region is critical for normal fluid and electrolyte balance during ad libitum access to food and water, and during water deprivation.