Reversal by methysergide of inhibition of insulin secretion by prostaglandin E in the dog.

These studies were designed to examine whether interrelationships exist between serotonin and prostaglandin E (PGE) during regulation of insulin secretion in dogs in vivo. In our studies serotonin was found to inhibit insulin responses to intravenous glucose. This inhibition was not reversed by complete adrenergic blockade provided through combined phentolamine and propranolol pretreatment. This property of serotonin is similar to that of PGE which also inhibits glucose-induced insulin secretion in vivo independently of adrenergic activity. To investigate whether these effects of serotonin and PGE are related, studies with methysergide (a serotonin antagonist) and indomethacin (a PGE synthesis inhibitor) were performed. Methysergide reversed the effects of both PGE and serotonin. In contrast, indomethacin did not diminish the inhibitory effect of serotonin upon insulin secretion. It is hypothesized that endogenous serotonin may play a role in the inhibitory effect of PGE upon insulin secretion in dogs in vivo.     

Plasma norepinephrine concentrations during resuscitation in the dog

The objective of this study was to evaluate whether the adrenal glands contribute to the increase in plasma norepinephrine concentrations during cardiopulmonary resuscitation, by releasing norepinephrine and/or by secreting epinephrine that facilitates the release of norepinephrine from sympathetic nerve endings via stimulation of presynaptic beta receptors. The experiments were performed in adrenalectomized and in sham-operated dogs. In adrenalectomized dogs the increase in plasma norepinephrine concentrations during cardiopulmonary arrest and basic life support (BLS) was markedly smaller than in sham-operated dogs. Intravenous infusion of physiologic doses of epinephrine during BLS in adrenalectomized animals did not influence the plasma norepinephrine concentrations. These data indicate that, as suggested by others, the marked increase in plasma norepinephrine concentrations during BLS in dogs is mainly of adrenomedullary origin. They also suggest that presynaptic facilitation of norepinephrine release by epinephrine is not important, but further experiments using higher doses of epinephrine are necessary.     

Indomethacin and sodium retention in the rat: role of inhibition of prostaglandin E2 synthesis.

1. To further explore the Na(+)-retaining effect of indomethacin along the whole length of the nephron, the Na(+)-K(+)-ATPase activity of isolated tubules from indomethacin-pretreated rats was compared with that of tubules isolated from intact rats and exposed directly to prostaglandin E2. 2. Indomethacin increased Na(+)-K(+)-ATPase activity in the proximal convoluted tubule (+24%, P < 0.001 versus control), proximal straight tubule (+75%, P < 0.001 versus control), medullary thick ascending limb (+68%, P < 0.001 versus control), cortical thick ascending limb (+7%, not significant) and cortical collecting duct (+18%, P < 0.025 versus control). In contrast, in the distal convoluted tubule indomethacin decreased Na(+)-K(+)-ATPase activity by -42% (P < 0.001 versus control). 3. Indomethacin also strongly increased Na(+)-K(+)-ATPase activity in the cortical collecting duct of adrenalectomized rats. 4. In isolated tubules from control rats, prostaglandin E2 reduced Na(+)-K(+)-ATPase activity in the proximal convoluted tubule (-33%, P < 0.05), proximal straight tubule (-60%, P < 0.001), medullary thick ascending limb (-43%, P < 0.001), cortical thick ascending limb (-25%, P < 0.001) and cortical collecting duct (-45%, P < 0.001) and in the distal convoluted tubule, prostaglandin E2 increased Na(+)-K(+)-ATPase activity (+32%, P < 0.05). 5. That these changes in Na(+)-K(+)-ATPase activity in indomethacin-pretreated rats and prostaglandin E2-treated controls are similar in magnitude but occur in opposite directions suggests that the response to indomethacin is mediated by inhibition of prostaglandin E2 synthesis in the nephron. In the cortical collecting duct the effect of indomethacin is aldosterone-independent.     

The effect of sodium intake on rabbit renal prostaglandin E2 and thromboxane A2: dissociation from the renin-angiotensin system

The effects of chronic alterations in dietary sodium intake on urinary prostaglandin (PG) E2 and thromboxane (TX) B2 was investigated in the rabbit. Sodium restriction, over a 15-day period, reduced daily urinary PGE2 and TXB2 in concordance with urinary flow (V) and sodium excretion (UNa+V), but increased plasma renin activity (PRA) and plasma aldosterone concentration (PAC). Sodium repletion, on the other hand, increased urinary PGE2 and TXB2 in proportion to the rise in V, but reduced PRA and PAC. During both sodium diets PGE2 and TXB2 correlated positively with V and negatively with PRA. It is concluded that chronic sodium intake produces opposite changes in the renal prostaglandin and the renin-angiotensin systems.     

Prostacyclin and prostaglandin E2 effects on adrenergic transmission in the kidney of anesthetized dog

The effect of prostaglandin E2 (PGE2) and prostacyclin (PGI2) on the renal venous output of the adrenergic transmitter and on the renal vasoconstriction produced by sympathetic nerve stimulation and by norepinephrine was studied in pentobarbital-anesthetized dog. Renal arterial infusion of either PGE2 or PGI2 (4 ng kg-1 min-1) increased blood flow to the kidney and inhibited the vasoconstrictor response elicited by renal nerve stimulation (1-8 Hz) and by renal arterial injections of norepinephrine (0.006-0.5 microgram). However, during infusion of either PGE2 or PGI2, the renal venous output of norepinephrine caused by nerve stimulation was not altered. In contrast, substance P (2 ng kg-1 min-1), which also increased the blood flow to the kidney, did not affect the renal vasoconstrictor response elicited by either adrenergic stimulus. These results suggest that PGE2 and PGI2 reduce the vasoconstrictor effect of sympathetic nerve stimulation in the canine kidney by acting on the vascular smooth muscle.     

Uterine prostaglandin E secretion and uterine blood flow in the pregnant rabbit.

Studies were performed in pregnant rabbits to assess the effect of inhibition of prostaglandin synthesis on uterine blood flow. Cardiac output and uteroplacental blood flow (UPBF) were measured using radiolabeled microspheres. Prostaglandin E (PGE) concentration was measured by radioimmunoassay in the uterine vein and peripheral artery of the pregnant nephrectomized rabbit. Either meclofenamate or indomethacin 2 mg/kg were utilized to inhibit prostaglandin synthesis. Systemic arterial pressure increased from 86 mm Hg to 98 mm Hg (P less than0.0001) after prostaglandin inhibition. Cardiac output was unchanged after the inhibition of prostaglandin synthesis, 326 ml/min to 7.8 ml/min. Uterine vein PGE concentration was extremely high, 172.4 ng/ml, with concomitant peripheral arterial PGE 2.1 NG/ML. Intravenous administration of either meclofenamate or indomethacin reduced uterine vein PGE to 23 ng/ml (P less than 0.01) and arterial PGE to 1.0 ng/ml (P less than 0.05). Male and nonpregnant female rabbits had lower arterial PGE, 0.37 ng/ml (P less 0.05). Studies in non-nephrectomized pregnant animals demonstrated that uteroplacental secretion of PGE was greater than five times renal secretion. These studies demonstrate that the rabbit uteroplacental unit is a rich source of PGE and suggest that production of the vasoactive lipid may have a key role in regulating UPBF during pregnancy.     

The effect of altered sodium balance upon renal vascular reactivity to angiotensin II and norepinephrine in the dog. Mechanism of variation in angiotensin responses.

The mechanism whereby the vasoconstrictor response to angiotensin II (AII) is influenced by sodium balance or disease is unclear. To explore this question, the renal vascular responses (RVR) to intrarenal injections of subpressor doses of AII and norepinephrine were studied in dogs with an electromagnetic flowmeter. Acute and chronic sodium depletion increased plasma renin activity (PRA) and blunted the RVR to AII, while acute sodium repletion and chronic sodium excess plus desoxycorticosterone acetate decreased PRA and enhanced the RVR to AII. The magnitude of the RVR to AII was inversely related to PRA. The RVR to norepinephrine was unaffected by sodium balance and was not related to PRA. Inhibition of the conversion of angiotensin I to AII by SQ 20,881 during sodium depletion lowered mean arterial blood pressure (MABP), increased renal blood flow (RBF), and enhanced the RVR to AII but not to norepinephrine. Administration of bradykinin to chronically sodium-depleted dogs also lowered the MABP and increased RBF but had no effect on the RVR to AII. SQ 20,881 had no effect on MABP, RBF, or the RVR to AII in the dogs with chronic sodium excess and desoxycorticosterone acetate. Administration of indomethacin to chronically sodium-depleted dogs lowered RBF but did not influence the RVR to AII. The results indicate that the RVR to AII is selectively influenced by sodium balance and that the magnitude of the response is inversely related to the availability of endogenous AII. The data did not suggest that the variations in the RVR to AII were because of direct effects of sodium on vascular contraction, changes in the number of vascular AII receptors, or the renal prostaglandins. The results are consistent with the hypothesis that the vasoconstrictor effect of AII in the renal vasculature is primarily dependent upon the degree to which the AII vascular receptors are occupied by endogenous hormone.     

Sites of action of sodium depletion on aldosterone biosynthesis in the dog

Secretion of cortisol, corticosterone, and aldosterone was measured in vivo in normal and sodium-depleted hypophysectomized dogs. Biogenesis of steroids was then measured in vitro with outer slices of the adrenals of the same dogs. In some studies, metyrapone or puromycin was added.In vivo, sodium depletion stimulated the production of cortisol, corticosterone, and aldosterone. In vitro, tissues from sodium-depleted animals released more aldosterone, but less corticosterone than those from sodium-replete controls.The results are interpreted to indicate that (a) biosynthesis of aldosterone is regulated at at least two sites in the biosynthetic pathway. The final conversion, that of corticosterone to aldosterone, is stimulated by sodium depletion. This effect persists for at least 3 hr while slices from sodium-depleted dogs are incubated in vitro. Stimulation at this site is thus relatively stable in vitro; its activation by sodium depletion is not inhibited by puromycin in the dog. Stimulation at this site can explain, at least in part, the increased effectiveness of adrenocorticotropin (ACTH) on aldosterone biogenesis during sodium depletion.(b) the earlier site at which sodium depletion stimulates the secretion of aldosterone is "above" the position of desoxycorticosterone in the pathway; it is probably at the conversion of cholesterol to pregnenolone. Stimulation at this site is quickly lost during incubation of adrenal slices. It is thus relatively unstable in vitro; its activation by sodium depletion is inhibited by puromycin in the dog.     

Autoregulatory vasodilation enhances renal prostaglandin E2 and associated renin release during arachidonic acid infusion in dogs

To examine whether autoregulatory dilation of preglomerular vessels enhances prostaglandin (PG)E2 and renin release during arachidonic acid infusion, the ureter was occluded or the renal artery constricted in anesthetized dogs. Intrarenal arachidonic acid infusion (40 micrograms X kg-1 X min-1) increased PGE2 release by 41 +/- 17 pmol/min at control pressures and by 149 +/- 60 pmol/min during ureteral occlusion. Arachidonic acid infusion (160 micrograms X kg-1 X min-1) increased PGE2 release by 149 +/- 60 pmol/min at control pressures, by 505 +/- 211 pmol/min during ureteral occlusion and by 581 +/- 201 pmol/min during renal arterial constriction. Thus, PGE2 release during arachidonic acid infusion was trebled by autoregulatory dilation. Arachidonic acid infusion (160 micrograms X kg-1 X min-1) raised renin release by 6 +/- 2 micrograms of angiotensin I per min at control pressures, by 25 +/- 9 micrograms of angiotensin I per min during renal arterial constriction and during ureteral occlusion by 16 +/- 4 micrograms of angiotensin I per min, which was not significantly higher than induced by the lower rate of infusion. Arachidonic acid infusion (160 micrograms X kg-1 X min-1) raised renal blood flow by 54 +/- 5% at control pressures but exerted no vasoactive effect during ureteral occlusion and renal arterial constriction. We conclude that autoregulatory dilation enhances the stimulatory effects of arachidonic acid on renal PG synthesis. Both increased intrarenal PG concentration and autoregulatory dilation may contribute to enhancement of renin release. The stimulatory effects of arachidonic acid on PG synthesis and renin release are independent of the vasoactive effects of arachidonic acid.     

Effects of changes in sodium balance on prostaglandin synthesis and prostaglandin E2 9-ketoreductase activity in the rat kidney

The in vitro synthesis of prostaglandins E2 and F2 alpha by renal cortex, medulla and papilla was measured in normal rats and in rats receiving either a low or a high sodium intake for 14 days. The production of both prostaglandins was unchanged in the cortex. In the medulla, both low and high sodium intakes led to a similar decrease in prostaglandin E2 synthesis in vitro, but prostaglandin F2 alpha synthesis was unchanged. In the papilla, a low sodium intake increased prostaglandin E2 synthesis. The activity of prostaglandin E2 9-ketoreductase, a cytosolic enzyme catalysing the conversion of prostaglandin E2 to prostaglandin F2 alpha, was unchanged in cortical preparations. In medullary slices, prostaglandin E2 9-ketoreductase activity was decreased by both sodium depletion and loading. In the papilla, prostaglandin E2 9-ketoreductase activity was slightly decreased by sodium loading and increased with sodium depletion. These results obtained in the rat are at variance with findings in the rabbit. The role played by prostaglandin E2 9-ketoreductase in the regulation of prostaglandin biosynthesis during changes of sodium balance remains controversial.     

Gastric bicarbonate secretion in humans. Effect of pentagastrin, bethanechol, and 11,16,16-trimethyl prostaglandin E2.

Although the stomach is mainly known for its ability to secrete hydrochloric acid, there is increasing evidence that the gastric mucosa also secretes bicarbonate. A simple method for simultaneous measurement of gastric HCO-3 secretion and H+ secretion was developed from a two-component model of gastric secretion. The method, which is based upon gastric juice volume, H+ concentration, and osmolality, was validated both in vitro and in vivo. In 14 healthy human beings, basal gastric HCO-3 secretion averaged 2.6 mmol/h (range, 0.7-8.7 mmol/h). Basal HCO-3 secretion was approximately 50% of basal H+ secretion and there was a significant correlation between basal HCO-3 and H+ secretion in individual subjects (r = 0.79). HCO-3 was secreted in basal nonparietal secretion at a concentration of approximately 90 mmol/liter. Intravenous pentagastrin infusion markedly stimulated H+ secretion but did not increase HCO-3 secretion. During pentagastrin infusion, the cholinergic agonist, bethanechol, significantly augmented H+ secretion (from 20.2 to 24.7 mmol/h) and increased HCO-3 secretion (from 2.2 to 4.2 mmol/h). A prostaglandin E2 analogue significantly reduced H+ secretion and increased HCO-3 secretion during pentagastrin infusion. The reduction in net gastric juice H+ output following prostaglandin E2 was due more to H+ secretory inhibition than to HCO-3 secretory stimulation. We conclude that the healthy human stomach actively secretes HCO-3 and that gastric HCO-3 secretion can be influenced by cholinergic stimulation and by prostaglandin E2.     

Long-term effects of aldosterone on kallikrein, prostaglandin E2 and sodium in rats

To assess the long-term effects of mineralocorticoids on the regulation of the synthesis or release of kallikrein and prostaglandin E2 in the renal kallikrein-kinin-prostaglandin E system, we studied the effects of chronic infusion of aldosterone (50 micrograms/kg/day) on urinary excretion of total and active kallikrein, and prostaglandin E2 for 10 days in conscious rats on regular intakes of sodium and on sodium loading with 1% NaCl as a drinking water. Chronic infusion of aldosterone induced a prompt and transient decrease in the ratio of sodium to potassium and a sustained increase in urinary prostaglandin E2 excretion in rats on regular diets, whereas urinary total and active kallikrein excretion did not increase significantly until the 4th day of aldosterone infusion. In rats loaded with sodium, aldosterone did not induce any changes in urinary total and active kallikrein excretion, whereas it induced similar changes in the ratio of sodium to potassium and urinary prostaglandin E2 excretion to those in rats on regular diets. Thus, the present results suggest that aldosterone might stimulate the synthesis or release of renal prostaglandin E2 independent of sodium balance. Furthermore, it is also suggested that aldosterone might stimulate the synthesis or release of kallikrein, at least partly, via the same pathway as sodium loading does.     

Increased urinary excretion of prostaglandin E2 in patients with idiopathic hypercalciuria is a primary phenomenon.

1. Urinary excretion of prostaglandin E2 is increased in patients with idiopathic hypercalciuria, but in order to conclude that hyperprostaglandinuria is a primary phenomenon, it must be demonstrated that high levels of urinary prostaglandin E2 can be dissociated from other factors, such as urine volume and natriuresis, and from the hypercalciuria itself. 2. We studied 10 patients with idiopathic hypercalciuria and 10 control subjects on high and low calcium diets providing daily calcium intakes of 30-35 mmol and 7.5-10 mmol, respectively, and similar sodium intakes. In addition, patients with idiopathic hypercalciuria and control subjects were studied during water restriction and water diuresis. 3. Urinary prostaglandin E2 excretion was more than twice as high in patients with idiopathic hypercalciuria than in control subjects on the low and high calcium diets as well as during water restriction and water diuresis (P less than 0.01). 4. Urinary prostaglandin E2 excretion was not affected by changes in urinary calcium excretion in patients with idiopathic hypercalciuria and in control subjects. Patients with idiopathic hypercalciuria on the low calcium diet and control subjects on the high calcium diet had similar levels of calciuria and natriuresis, yet urinary prostaglandin E2 excretion (mean +/- SEM) was 11.62 +/- 1.71 nmol/day in the patients with idiopathic hypercalciuria and 3.26 +/- 0.48 nmol/day in the control subjects (P = 0.0006). 5. These results indicate that increased urinary prostaglandin E2 excretion is a cardinal characteristic of patients with idiopathic hypercalciuria.     

Suppression by methylene blue of prostaglandin I2 synthesis in isolated dog renal arteries

In dog renal artery strips with intact and damaged endothelium, relaxations induced by angiotensin II possibly mediated via the release of prostaglandin (PG) I2 were abolished or reversed to contractions by treatment with methylene blue and indomethacin. Relaxations elicited by arachidonic acid were also inhibited markedly. PGH2-induced contractions in the arteries with endothelium were potentiated, and relaxations caused by PGH2 in the endothelium-denuded arteries were suppressed by methylene blue, whereas these responses were not influenced by indomethacin. Relaxant responses to PGI2 and beraprost, a stable analog of PGI2, were not reduced by methylene blue. The amount of 6-keto PGF1 alpha in the bathing media released from the renal artery was decreased by treatment with methylene blue and indomethacin, whereas amounts of PGE2 and thromboxane B2 were not influenced by methylene blue but were reduced by indomethacin. It may be concluded that methylene blue interferes with the synthesis and release of PGI2 in the dog renal arteries; therefore, the relaxation mediated by endogenous PGI2 is suppressed. However, methylene blue does not appear to inhibit the synthesis of other cyclooxygenase products.     

The effect of inhibition of prostaglandin synthesis on urinary sodium excretion in the conscious dog.

Studies were performed to determine the effect of decreased endogenous release of renal prostaglandins on urinary sodium excretion. Two structurally dissimilar inhibitors of prostaglandin synthesis were employed, and studies were performed in conscious dogs allowed to recover from prior surgical instrumentation. Either meclofenamate (2 mg/kg) or the competitive prostaglandin inhibitor RO 20-5720 (1 mg/kg) was given to seven unanesthetized dogs undergoing a water diuresis. The administration of either prostaglandin inhibitor did not alter glomerular filtration rate, renal plasma flow, urinary volume, or potassium excretion. Sodium excretion, however, increased from 32 to 130 mueq/min (P less than 0.02). Essentially, the entire increase in sodium excretion was due to an increase in urinary sodium concentration from 7.7 to 28.3 meq/liter (P less than 0.02). On a different day, the same animals were studied before and after administration of the diluent of the prostaglandin inhibitor. No change was noted in sodium excretion or any other parameter. Thus, these findings suggest that prostaglandin inhibition in the conscious dog is associated with a natriuresis without a change in urinary volume or potassium excretion during water diuresis. This may indicate that the natruiresis was due to diminished sodium reabsorption beyond the distal tubule.     

Increased norepinephrine levels during catheterization in patients with spinal cord injury.

The hypothesis for this study was that catecholamine levels increase during urinary catheterization in human patients with spinal cord injury. Catecholamine levels, blood pressure, and pulse were measured prospectively in 40 subjects at baseline and during urinary catheterization. Results showed a significant increase in norepinephrine levels from baseline 245 +/- 240 pg (standard deviation (SD)) to 314 +/- 311 pg (SD) during catheterization (P = 0.018, Wilcoxon's). Results also showed a nonsignificant increase in epinephrine levels from baseline (56 +/- 70 pg, SD) to catheterization (84 +/- 125 pg, SD; P = 0.35, Wilcoxon's). Systolic blood pressure increased from 114 to 124 mm Hg (P = 0.004, paired t test). Diastolic blood pressure increased from 75 to 78 mm Hg (P = 0.11, paired t test). There was no significant change in diastolic blood pressure or pulse (P = 0.11 and P = 0.29, respectively, paired t test). In conclusion, norepinephrine levels increased during catheterization in patients with spinal cord injury. Knowledge of catecholamine levels in this process may assist in determining both pathophysiology and potential pharmacologic treatment options in future studies.     

Inhibition of sodium transport by prostaglandin E2 across the isolated, perfused rabbit collecting tubule.

This study was designed to examine whether prostaglandin E2 can directly affect sodium transport across isolated perfused rabbit renal collecting tubules. Changes in transepithelial potential and isotopic sodium fluxes in response to peritubular prostaglandin E2 were measured. In addition, changes in transepithelial potential of the outer medullary collecting tubule in response to prostaglandin E2 were also measured. With few exceptions, all rabbits received 5 mg/day desoxycorticosterone acetate for 4-11 days before experimentation. The results of the experiments show that: (a) prostaglandin E2 inhibits the negative transepithelial potential in the cortical collecting tubule as well as the outer medullary collecting tubule; (b) prostaglandin E2 inhibits net sodium transport out of the lumen by inhibiting efflux while backflux is unaffected; (c) prostaglandin E2 produces this inhibition within 15 min, and the effects are dose dependent and reversible. These results suggest that prostaglandin E2 may modulate sodium transport in vivo and may contribute to the final regulation of sodium excretion.     

Time-course and relationship of the early changes in renal sodium excretion and aldosterone secretion during dietary sodium restriction in normal man

1. The fall in renal sodium excretion after dietary sodium restriction is prompt and reproducible. The importance of increased aldosterone secretion during the early phase (within 48 h) of this response is unclear. Using two indirect measures of aldosterone secretion (in urine and saliva), we have tried to relate changes in excretion and concentration of this hormone to renal sodium excretion during the abrupt transition from a normal (approximately 150 mmol/day) or high (260 mmol/day) to a low (5-25 mmol/day) sodium intake in 11 and seven male volunteers, respectively. 2. All subjects showed reduced renal sodium excretion within 36 h of dietary restriction, but the times at which increases in renal aldosterone excretion, saliva aldosterone concentration and plasma renin activity became statistically significant varied widely (8-72 h, 2.5- greater than 62.5 h and less than 4- greater than 38 h for renal aldosterone secretion, saliva aldosterone concentration and plasma renin activity, respectively). Circadian fluctuations in saliva aldosterone concentration were apparent and increased in amplitude during sodium restriction. 3. Urine flow rate tended to increase on the first day of sodium restriction and this reached statistical significance in the group initially on a high sodium intake (64.0 +/- 8.8 to 84.3 +/- 11.2 ml/h, P less than 0.01); although the pattern of urine flow did correlate with plasma arginine vasopressin concentration (r = -0.49, P less than 0.01), there was no significant decrease in mean plasma arginine vasopressin concentration [1.15 (0.92-1.44) to 0.90 (0.72-1.12) pmol/l, P = 0.08; geometric mean and 95% confidence limits].(ABSTRACT TRUNCATED AT 250 WORDS)