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.

Effect of intracerebroventricular captopril on vasopressin and blood pressure in spontaneously hypertensive rats

In conscious, unrestrained spontaneously hypertensive rats (SHR), mean arterial blood pressure (MAP) increased from a pretreatment value of 150 +/- 4 to 179 +/- 7mm Hg within 10 min (p less than 0.01) following an intracerebroventricular (i.c.v.) injection of captopril (2 mg/kg body weight), and the plasma vasopressin concentration was increased eightfold (p less than 0.01). MAP than fell to 131 +/- 5 mm Hg at 120 minutes (p less than 0.01), and plasma vasopressin concentration returned to pretreatment levels. The initial increase in MAP was due in large part to increased plasma vasopressin levels since this increase was reduced 50% by pre-treatment with a specific antagonist of the pressor action of vasopressin. The reduction in MAP at 120 minutes in captopril-treated rats may been nonspecific, since a similar effect was observed in SHR given an i.c.v. injection of a control solution. In (Wistar-Kyoto) WKY rats, i.c.v. captopril was without a statistically significant effect on MAP, but the plasma vasopressin concentration increased three-fold (p less than 0.01). These findings may reflect an increased sensitivity of the control system for vasopressin release in the SHR.

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.

The role of central adrenoreceptors in the control of vasopressin release and blood pressure

In order to investigate the role of central noradrenergic neurons in the control of vasopressin (ADH) release and cardiovascular regulation, norepinephrine (1.4 microgram/kg), clonidine (0.1 microgram/kg), and isoproterenol (1.4 microgram/kg were infused into the lateral cerebral ventricle of the anesthetized dog. The drugs were given over a 20-min period, dissolved in 0.9% saline at a volume rate of 10 microliter/min. Both norepinephrine and clonidine markedly reduced ADH release and lowered arterial blood pressure and heart rate. Isoproterenol had no effect on ADH release and produced a slight reduction in arterial pressure and a large increase in heart rate. Pretreatment with phenoxybenzamine (100 microgram/kg, iv) completely blocked the effects of norepinephrine on blood pressure and heart rate but only partially (about 50%) inhibited the norepinephrine effect on ADH release. Intravenous isoproterenol lowered blood pressure and increased ADH release and heart rate. In none of the experiments could changes in ADH release be attributed to changes in plasma osmolality or plasma sodium and potassium concentrations. It is concluded that, in the anesthetized dog, intraventricular norepinephrine and clonidine decreased ADH release, blood pressure, and heart rate by stimulating alpha-adrenoreceptors. The increased release of ADH after peripheral administration of isoproterenol was presumably due to the reduction in blood pressure and decreased baroreceptor inhibition of ADH release.

Vasopressin secretion in the New Zealand genetically hypertensive rat

A study was undertaken to evaluate the role of vasopressin in the pathogenesis of hypertension in New Zealand genetically hypertensive (NZGH) rats. During the course of development of hypertension in NZGH rats from 4 to 11 weeks of age, the 24 h urinary excretion of vasopressin did not differ from that of the New Zealand normotensive control rats (NZNR). Furthermore, at the conclusion of the study (rats 13 to 14 weeks old), the plasma vasopressin concentrations in NZGH and NZNR rats were not significantly different. Although there was no evidence for a difference in secretion of vasopressin from the neurohypophysis in the NZGH rats, there was a substantially increased pressor responsiveness to vasopressin in these rats. This was not specific since NZGH rats also had an increased pressor responsiveness to angiotensin II. The importance of increased pressor responsiveness to vasopressin in the hypertensive process in the NZGH rat requires further study.

Effect of norepinephrine on plasma vasopressin concentration and renal water metabolism

In the anesthetized, normally hydrated dog, the i. v. infusion of norepinephrine (NE; 0.5 microgram/kg/min) resulted in an increased blood pressure and a marked reduction in the plasma vasopressin (ADH) concentration (2.6 +/- 0.2 to 1.1 +/- 0.2 muU/ml). Urine flow and osmolar clearance, and urine osmolality fell. However, the negative free water clearance (TcH2O) increased, despite the reduction in plasma ADH levels. Thus, the NE-induced diuresis appeared to be due largely to the increased solute excretion, but the reduction in plasma ADH levels may also have been a factor. These data show that change in free water clearance is not a satisfactory index of change in the plasma ADH levels may also have been a factor. These data show that change in free water clearance is not a satisfactory index of change in the plasma ADH concentration when there are acute changes in renal hemodynamics and solute excretion. The norepinephrine-induced reduction in ADH secretion appeared to be due largely to increased activity of the arterial baroreceptors, but a central action cannot be ruled out.

Pressor responsiveness to vasopressin in the rat with DOC-salt hypertension

To further characterize the role of vasopressin in DOC-salt hypertension, four groups of unilaterally nephrectomized rats were studied: control rats given no further treatment, rats treated with DOC and given 1% saline to drink, or rats treated with only DIC or 1% saline had similar pressor responses to exogenous vasopressin and angiotensin II. Within the DOC-salt group, two populations of rats were identified: one with normal pressor responsiveness to vasopressin, and one with markedly enhanced pressor responsiveness to vasopressin. Incidence of enhanced responsiveness increased with duration of treatment. Urinary excretion of vasopressin was elevated in the 1% saline and DOC-salt groups after 1 week of treatment, and in the DOC group after 4 weeks. However, the plasma vasopressin concentration was elevated only in the rats treated with both DOC and saline. It is suggested that vasopressin is essential for the expansion of blood volume in the early stages of DOC-salt hypertension, and functions as a direct pressor agent only in the later stages.

Effects of lumen volume transit time and pressure on loop of Henle function

Experiments on Henle's loops were designed to demonstrate the relationships of absorption to distal pressure, transit time, and luminal diameter. Loops of superficial nephrons in hydropenic rats, isolated from the rest of the nephron by oil or solid paraffin blocks, were microperfused at 13.6--20 nl/min. Two samples of fluid were collected from the early distal tubule--one with suction in order to lower distal pressure and reduce luminal volume, the other without suction so that the lumen was distended. Transit times were 30 +/- 2 s without and 19 +/- 2 s with suction. Proximal tubule pressure and perfusion rate were not altered by collection with suction. Absolute absorption, however, descreased from 10.6 +/- 0.4 to 8.4 +/- 0.4 nl/min (P less than 0.001). When salt transport was inhibited by 10(-4) M furosemide in the perfusate, water absorption was 7.8 +/- 0.7 nl/min without suction and 6.1 +/- 0.8 nl/min with suction (P less than 0.01). Computer simulation of Henle's loop shows that these observations cannot be explained by changes in transit time, hydrostatic pressure, or unstirred layers. The observations are simulated when radial fluxes depend on wall thickness and surface area in the descending thin limb.

Computer simulation of flow-dependent absorption in microperfused short Henle's loop of rats

With computer simulation we examined the extent to which current theories and experimental data explain function of single microperfused superficial Henle's loops in rats. In the model standard phenomenological equations describe transport; two sets of transport parameters labeled rat and rabbit were taken from published experiments; Michaelis-Menten kinetics in the ascending thick limb were adjusted arbitrarily; tubular radius is either constant or depends on luminal pressure with compliance based on experimental observations; the interstitium is an infinite sink with salt and urea concentrations constant in the cortex and exponentially increasing in the outer medulla; concentrations resemble those found in hydropenic or saline diuretic rats. The following predictions were obtained. The model with rabbit parameters does not recirculate urea and will not operate with high medullary urea concentrations; with rat parameters too much urea recirculates an the results of perfusion with equilibrium solution are not reproduced. Using a compromise between rat and rabbit parameters, the model reproduces water absorption, salt reabsorption, and urea recirculation as observed in vivo in rat loops perfused at 5-40 nl/min. It also simulates perfusion with saline, equilibrium solution, saline plus furosemide, and 300 mM mannitol. When the model includes a short early distal segment, effluent salt concentration reaches a minimum at a 15 nl/min perfusion rate as observed in vivo; however, concentration at the macula densa is a monotonically increasing function of flow. When permeation rate is a function of wall surface area and thickness a better fit to experimental results is produced. However, the effect is small: water absorption alters by 4% or less and effluent salt concentration is reduced by up to 10% at low perfusion rates. Comparison of rigid and compliant loops shows no relationship between transit time per se and reabsorption.

Effect of dipivalyl derivatives of catecholamines on cardiovascular function in the conscious dog

Dipivalyl derivatives of epinephrine, norepinephrine and isoproterenol were injected intravenously into conscious dogs while cardiovascular variables were monitored. The dipivalyl compounds produced cardiovascular effects that were comparable to those produced by their respective parent catecholamines except that the responses had a delayed onset and a prolonged duration. The catecholamines were more potent than their derivatives; norepinephrine was 28 times, epinephrine 15 times and isoproterenol 1.7 times as effective as their respective dipivalyl derivatives. Alpha or beta adrenergic responses to the dipivalyl compounds were attenuated or abolished after alpha adrenergic blockade (phenoxybenzamine or phentolamine), or beta adrenergic blockade (propranolol), respectively, or combined alpha and beta blockade. Since other evidence indicates that the dipivalyl derivatives themselves are inactive, our results suggest that these compounds act as prodrugs that exert their cardiovascular effects only after they are biotransformed to catecholamines.

Intracellular buffering of heart and skeletal muscles during the onset of hypercapnia

Changes in the total CO2 content of tissues were determined in order to characterize variations in intracellular acid-base parameters during the onset of hypercapnia. Within two minutes after an increasement in the CO2 tension of the inspired air of rats, there were large increases in the intracellular bicarbonate concentrations of both cardiac and skeletal muscles. Greater changes occurred in the heart, and its intracellular pH remained near normal during the first hour of hypercapnia; whereas there was an intracellular acidosis in skeletal muscle. This greater capacity of the heart to buffer excess CO2 has been linked to an increased movement of bicarbonate ions into and/or hydrogen ions out of cardiac cells during hypercapnia (Lai et al., 1973c). Yet, the buffer capacity of the heart was not compromised by metabolic acidosis during which there was a greatly reduced extracellular bicarbonate ion concentration and a greatly increased extracellular hydrogen ion concentration. The intracellular pH of the cardiac ventricle was stable following the imposition of a noncarbonic acid load on normocapnic rats.

Intracellular and extracellular acid-base changes in hemorrhagic shock

Each of 21 dogs was bled until mean arterial blood pressure fell to 50 torr; this hemorrhagic shock state was then maintained for two hours. During hemorrhagic shock, the blood lactate concentration increased sixfold. The severe metabolic acidosis in arterial blood was partially compensated by a decreased PCO2 caused by increased ventilation. However, in mixed venous blood, the metabolic acidosis was combined with a respiratory acidosis. This hypercapnia in venous blood was indicative of the increased PCO2 in tissues poorly perfused following hemorrhage. The increase in the PCO2 of the femoral venous blood was greater than that in mixed venous blood, suggesting that some tissue beds were better perfused than those of the hind limb during shock. The intracellular lactate concentration of hind limb skeletal muscle was greatly increased in the shock state, and tissue PCO2 rose. Intracellular pH of skeletal muscle was only slightly decreased and bicarbonate concentration was unchanged during this combined metabolic and respiratory acidosis. This capacity of skeletal muscle to maintain a high HCO-3 concentration in intracellular fluid during metabolic acidosis may be an enhanced response of the mechanism responsible for maintaining (HCO-3)i normally at a level approximately ten times that which would be expected if HCO-3 were distributed passively.

Anesthetic hazards for patients who have undergone bilateral thoracolumbar sympathectomy

Abrupt and serious circulatory changes may take place during and after operations under general anesthesia in patients who have had extensive thoracolumbar sympathectomy performed previously. These changes should be anticipated and treated promptly and vigorously, if prevention is not possible. Experiences with 23 such cases are presented.