The effect of melatonin on steroidogenesis by the human ovary in vitro.

Melatonin stimulated steroidogenesis in two compartments of the human ovary in vitro. In a corpus luteum of the menstrual cycle, melatonin increased progesterone synthesis in a dose related manner. Both serotonin and N-acetyl serotonin had no effect on progesterone synthesis. In the ovarian stroma, melatonin stimulated the incorporation of acetate-1-14-C into androstenedione. Binding of radioactive hCG by the corpus luteum was unaffected by melatonin. No specific binding of radioactive melatonin of low specific activity could be detected in homogenates of a human corpus luteum. These observations suggest that melatonin may directly modulate steroidogenesis in the human ovary.

Rat atrial natriuretic peptides inhibit oxygen consumption by rat kidney.

The inhibitory effect of high and low molecular weight native and synthetic rat atrial peptides on oxygen consumption in isolated rat kidney mitochondria and slices was measured. Oxygen consumption by mitochondria was measured in the presence of succinate and/or adenosine diphosphate, furosemide, and low and high molecular weight native and synthetic rat atrial peptides. After the addition of succinate, adenosine diphosphate limiting respiration (State 4) increased in the presence of low, but not high, molecular weight native rat atrial peptides. Furosemide caused a significant decrease in State 4 respiration (p less than 0.001). Angiotensin II and arginine vasopressin did not alter State 4 respiration. The rate of oxygen consumption after the addition of saturating adenosine diphosphate in the presence of saturating succinate (State 3 respiration) was reduced by low and high molecular weight native rat atrial peptides. Furosemide completely blocked oxygen consumption after the addition of adenosine diphosphate. Oxygen consumption was unchanged by trypsin treated (natriuretically inactive) low molecular weight rat atrial peptides and ventricular protein extracts of high and low molecular weight native rat atrial peptides. Synthetic and low molecular weight native rat atrial peptides had similar effects on mitochondrial oxygen consumption. Low molecular weight native and synthetic rat atrial peptides decreased the adenosine diphosphate to oxygen ratio, and these peptides, as well as furosemide, also induced mitochondrial swelling; none of the other rat atrial peptide combinations nor angiotensin II produced this effect. In kidney slices, basal oxygen consumption (without substrates) was stimulated by succinate.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat platelets activate high molecular weight atrial natriuretic peptides in vitro.

Evidence suggests that the more biologically active low molecular weight forms (less than 10,000) of rat atrial natriuretic peptides are proteolytically derived from a less active precursor of higher molecular weight. Conversion and activation could occur within the myocyte as well as during circulation. The present study found that in vitro rat blood and platelets were capable of converting the high molecular weight atrial natriuretic peptides (greater than 10,000) to low molecular weight atrial natriuretic peptides within minutes and that enhanced biological activity attended the conversion. Rat high molecular weight peptides were partially purified by gel filtration, lyophilized, and reconstituted in Krebs-Ringer bicarbonate buffer. One milliliter of fresh rat blood was incubated with the high molecular weight peptides at 37 degrees C for 2 minutes. After centrifugation, the supernatant was fractionated on Sephadex G-75. Natriuretic activity was determined by bioassay in anesthetized rats. In contrast to the results following incubation of high molecular weight peptides in Krebs-Ringer bicarbonate buffer alone, which showed that 95% of the natriuretic activity remained in the high molecular weight peptide region, the natriuretic activity of the blood-treated high molecular weight peptides eluted almost exclusively in the low molecular weight peptide region, which indicates conversion. Blood was separated into plasma, erythrocytes, lymphocytes, and platelets. Conversion of high to low molecular weight peptides occurred only after incubation with platelets. Compared with control high molecular weight peptides, rat platelet-treated high molecular weight peptides had significantly greater activity in relaxing histamine-contracted rabbit aortic smooth muscle (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Antiadrenergic therapy: special aspects in hypertension in the elderly.

The effect of antiadrenergic treatment with methyldopa was studied in 17 patients with established essential hypertension who were subdivided with respect to age in a group younger (n = 10; mean age, 47 +/- 2.4 (SEM) years; and a group older than 60 years of age (n = 7, mean age, 67 +/- 2.8 SEM). The fall in arterial pressure was associated with a significant (p less than 0.05) decrease in cardiac output and heart rate in patients over 60 years of age and no change in total peripheral resistance, whereas a (nonsignificant) fall in resistance occurred in younger patients. In both age groups, a significant (p less than 0.05 and less than 0.01, respectively) decrease in plasma norepinephrine levels was observed, whereas epinephrine and dopamine showed no changes. Pre- and posttreatment values of mean arterial pressure correlated directly with plasma norepinephrine values (r = 0.35 p less than 0.05). Regardless of whether cardiac output was reduced or remained unchanged, renal blood flow, plasma and total blood volume did not change in either group with antiadrenergic treatment. Further, reflexive cardiac changes (responses to isometric exercise and upright tilt) remained qualitatively unchanged. It is concluded that antiadrenergic treatment with methyldopa lowers arterial pressure additionally by decreasing circulating norepinephrine levels. The antihypertensive effect is associated with a fall in peripheral resistance in the younger and a decrease in cardiac output in the older patients, and does not compromise renal blood flow or cardiac reflexive responses.

Does the renin-angiotensin-aldosterone system modify cardiac structure and function in essential hypertension?

To determine the impact of the renin-angiotensin-aldosterone system on left ventricular function and structure, 36 untreated patients with essential hypertension (WHO class I and II) were examined. Posterior wall thickness, relative wall thickness, and left ventricular mass were determined by M-mode echocardiography. Plasma renin activity, aldosterone, angiotensin I, and angiotensin II levels were measured by radioimmunoassay. Plasma renin activity was related to 24-hour urinary sodium excretion. Of all the endocrine parameters, only the angiotensin II level correlated with posterior wall thickness (r = 0.50, p less than 0.05) and relative wall thickness (r = 0.46, p less than 0.05). This relationship was confirmed by stepwise multiple regression analysis taking arterial pressure, obesity, and sodium excretion into account (p less than 0.05). Plasma renin activity but not the angiotensin II level correlated positively with the ejection fraction (r = 0.42, p less than 0.05) and velocity of circumferential fiber shortening (r = 0.57, p less than 0.01). Thus, angiotensin II emerged as a determinant of left ventricular structural adaptation in essential hypertension.

Hemodynamic and humoral effects of intravenous dilevalol in patients with moderate hypertension.

The acute hemodynamic and humoral responses to intravenous dilevalol (10 to 390 mg) were evaluated in 10 patients with moderate hypertension. Dilevalol, in doses of 30 mg or more, decreased arterial pressure (p less than 0.0001) through a decrease in total peripheral resistance (p less than 0.0001) associated with an increase in stroke volume and cardiac output (p less than 0.0001). Heart rate increased moderately at doses above 190 mg. Plasma norepinephrine levels increased (p less than 0.05), but epinephrine levels remained unchanged. Plasma renin activity and level of atrial natriuretic peptide decreased (p less than 0.01 and p less than 0.01, respectively). The hypotensive and humoral changes persisted 3 hours after the last dose. Dilevalol modified the pattern of hemodynamic response to isometric stress, slightly enhancing the increases in peripheral resistance and blunting increases in cardiac output and heart rate. The response in arterial pressure during administration of dilevalol remained similar to that seen in the pretreatment phase. The results show that dilevalol, when given intravenously in a dose of 30 to 90 mg, reduces arterial pressure by reducing total peripheral resistance without acceleration in heart rate. On the basis of these hemodynamic effects, dilevalol should be further evaluated for treatment of hypertensive emergencies.

Plasma beta-endorphin immunoreactivity in dogs during anesthesia, surgery, Escherichia coli sepsis, and naloxone therapy.

To improve understanding of the role of endorphins in septic shock, we examined the effects of anesthesia, splenectomy, live Escherichia coli infusion, and treatment with naloxone, respectively, on plasma beta-endorphin immunoreactivity (beta-EI) and plasma cortisol in dogs. Baseline levels of plasma beta-EI and cortisol were established in awake dogs. Pentobarbital anesthesia alone did not affect plasma beta-EI, but splenectomy was followed by a significant (P less than 0.001) rise in both plasma beta-EI and cortisol. Infusion of saline over a 3-hour period following splenectomy induced no further increase in plasma beta-EI, but infusion of live E. coli in splenectomized dogs caused a further rise in plasma beta-EI (P less than 0.02). Following induction of septic shock in a separate group of splenectomized animals, treatment with naloxone (3 mg/kg bolus and 2 mg/kg/hr infusion) did not alter the rise in plasma beta-EI. These results confirm release of beta-endorphin during septic shock and further implicate the hypothalamic-pituitary-adrenal axis in its pathophysiology. Based on the finding that naloxone did not affect the dynamics of plasma beta-EI, mechanisms are postulated to explain the therapeutic value of this drug in septic shock.

The specificity of angiotensin II receptor binding in rat brain.

125I-angiotensin II (125I-AII) binding was examined in the hypothalamic-thalamic-septal-midbrain (HTSM) region of HLA-Wistar rats in the presence of CNS-active agents. Angiotensin I, II, and III and saralasin competed for 125 I-AII binding, whereas structurally unrelated peptides such as arginine and lysine vasopressin, oxytocin, LHRH, TRH, bradykinin, and substance P did not. In contrast, ACTH and neurotensin exhibited a weak, dose-dependent competition for 125 I-AII binding. The relative potencies of AII, AI, neurotensin and ACTH were 100:1:0.1:0.05, respectively. Neurotensin and ACTH competition was not additive with AII suggesting interaction at shared binding sites. Most importantly, a wide variety of other CNS active agents such as methyldopa, naloxone, catecholamines, clondidine, and reserpine, failed to inhibit 125 I-AII binding, thus further defining the specificity of the CNS AII receptor.

Angiotensin II receptors are reduced in the CNS of the young Brattleboro rat.

In the rat, angiotensin II (AII), following specific interaction with sensitive central nervous system (CNS) receptors promotes release of vasopressin (ADH). We have examined the integrity of this chain of events by comparing the concentration, Bmax, and dissociation constant, Kd, in the CNS of Brattleboro rats (BB), a strain incapable of synthesizing ADH, with Long Evans (LE) control rats that can synthesize ADH. AII binding properties in the hypothalamic-thalamic-septal-midbrain (HTSM) area from young and old BB and LE, as well as systolic blood pressure, were determined. There was a reduction in the HTSM-AII receptor concentration of young BB when compared with young LE rats. Young BB had lower pressure than age and sex matched LE controls. Neither Bmax nor pressure was significantly different between older BB and LE. A decline in HTSM-AII receptor concentration with age observed with LE is consistent with observations in SHR and WKY rats. Parallel Scatchard plots obtained indicated the presence of a single class of CNS AII receptors. These data suggest that ADH synthesis and AII receptor concentration are partially interdependent and that the CNS AII-ADH system is redundant in the maintenance of blood pressure.

Cardio-renal effects of endothelin-3 in the rat.

Endothelin-3 (ET-3), a recently described vasoconstricting peptide was infused in Inactin anesthetized rats at 0, 40, 170 or 340 ng/kg/min for 45 minutes (n = 8 in each group). ET-3 infusion increased mean arterial pressure at all infusion rates by increasing total peripheral resistance. Cardiac output (delta CO) was significantly decreased at the two highest ET-3 infusion doses. The decrease in cardiac output was associated with a decrease in central venous pressure and stroke volume and an increase in hematocrit. ET-3 infusion at 40 ng/kg/min increased sodium excretion (delta UNaV) by 0.14 +/- 0.08 microEq/min (p less than 0.05 compared to vehicle infusion) without affecting the glomerular filtration rate (GFR). At higher infusion rates ET-3 markedly decreased the GFR, urine flow and urinary potassium excretion. ET-3 infusion significantly increased circulating levels of ANF. The present study demonstrates that ET-3 increases blood pressure by increasing total peripheral resistance but decreases cardiac output. Further, ET-3 has natriuretic effects at low doses but markedly attenuates renal function at high doses.

Dehydration attenuates the acute natriuretic response to rat atriopeptin III (rAPIII).

The acute natriuretic response to atrial peptides (AP) is highly variable in anesthetized rats, and some rats are unresponsive. To determine if this response to AP was affected by dehydration, we measured hematocrit, plasma volume, and natriuresis (delta UNaV) after intravenous injection of 3 micrograms/kg of rat atriopeptin III (rAPIII) in anesthetized rats deprived of water for 0, 12, 20, 29, 44, and 68 hours. Data were compared with those from rats receiving 1.5 mg/kg furosemide (FU) after 0 and 68 hours without water. There were 10- and 3-fold decreases in delta UNaV following rAPIII and FU injection after 20 and 68 hours without water, respectively. Hematocrit increased and plasma and total blood volumes decreased after 12 hours of dehydration. Plasma volumes and delta UNaV were correlated (r = 0.64, p less than 0.05; r = 0.75, p less than 0.001) in the combined groups receiving rAPIII (n = 30) and FU (n = 10), respectively. These results demonstrate that a relatively short period of water deprivation (WD) and the resulting hemoconcentration in rats decreased their acute natriuretic response to diuretics. Thus, differences in water intake may account for some of the large variation in delta UNaV after exogenous administration of rAPIII.

Quinaprilat increases total body vascular compliance in rats with myocardial infarction.

To test whether quinaprilat, a new angiotensin converting enzyme inhibitor, has any venous effect, its immediate effects were measured on mean circulatory filling pressure (MCFP), intravascular volume and total body vascular (i.e., venous) compliance in conscious rats with mild congestive heart failure induced by coronary artery ligation. MCFP was determined by inflating a right atrial balloon to arrest the circulation instantly and temporarily. Total body vascular compliance was derived from total circulatory pressure-volume relationships as determined by series measurements of MCFP with different intravascular volume status. In 8 rats with mean infarct size of 26 +/- 4%, 30-minute infusion of quinaprilat (0.1 mg/kg/min) decreased both mean arterial and central venous pressures by 8 mmHg and 0.7 mmHg, respectively (P less than 0.02); heart rate, MCFP, hematocrit and blood volume remained unchanged. Compared with control vehicle infusion, quinaprilat increased the total body vascular compliance (2.09 +/- 0.12 vs 2.69 +/- 0.23 ml/kg/mmHg; P less than 0.05) and decreased extrapolated unstressed circulating volume (34.96 +/- 1.10 vs 28.53 +/- 2.55 ml/kg; P less than 0.02). These data suggest that quinaprilat produces possible venodilation through immediately improved total body vascular compliance thereby reducing cardiac preload in this rat model of chronic heart failure.

Natriuretic response to atrial natriuretic factor enhanced by angiotensin II and vasopressin.

The effect of angiotensin II (ANG II) and arginine vasopressin (AVP) on the natriuretic response to partially purified high molecular weight atrial natriuretic factor (ANF) was examined in anaesthetized rats. ANG II and AVP were infused continuously at equipressor doses. Rats receiving ANG II and AVP showed five- to sevenfold greater natriuretic responses to bolus injections of ANF than controls. A significantly smaller augmentation of the natriuretic response to ANF was produced by equipressor does of norepinephrine and epinephrine, suggesting that the potentiation by ANG II and AVP were not entirely due to increased mean arterial pressure (MAP). Decreased MAP in rats receiving infusions of saralasin and hydralazine did not diminish the natriuretic response to ANF. The results suggest that the ANG II and AVP augmentation of the ANF-induced natriuresis is mediated partly through increased MAP and partly by interacting with the renal action of ANF.

Acute haemodynamic effects of the atrial natriuretic hormone in rats.

Haemodynamic effects of atriopeptin II (AP II) were determined in conscious and anaesthetized rats chronically instrumented with a Doppler flow probe on the ascending aorta. Intravenous injection of AP II (7 micrograms/kg) produced a decrease in mean arterial pressure within 5 min; however, a biphasic change occurred in total peripheral resistance (TPR). At 1 min in the anaesthetized rats, TPR decreased while cardiac output tended to increase, and similar changes occurred in the conscious rats. By 5-8 min the haemodynamic profile had reversed: cardiac output had decreased in both anaesthetized and conscious rats and TPR had increased in anaesthetized rats. Mean circulatory filling pressure (MCFP) was measured during brief circulatory arrest by inflating an intracardiac balloon. Blood volume was measured with 51Cr-erythrocytes and organ blood volume by whole-animal freezing in liquid nitrogen. There were no changes in MCFP in the conscious and the anaesthetized rats, nor in the blood volume or cardiopulmonary blood volume in the anaesthetized rats, at 5-7 min after AP II. Atriopeptin III (10 micrograms/kg intravenously) had no effects on MCFP and the blood volume in the conscious rats at 5 and 15 min after injection. These results suggest multiple mechanisms of action for the acute haemodynamic effects of atrial peptides.

Atrial natriuretic factor concentrations in discrete brain regions, pituitary, cardiac atria and plasma during the estrous cycle in rats.

Peripheral and central atrial natriuretic factor (ANF) concentrations were measured across the rat's estrous cycle. Vaginal smears were obtained from adult Sprague-Dawley rats maintained under controlled illumination (L/D: 14/10, onset 05.00 h). ANF concentrations in plasma, cardiac atria, pituitary and nine microdissected brain regions of females (n = 5-13) were determined by radioimmunoassay during either early proestrus (09.00-11.00 h), late proestrus (17.00-19.00 h), estrus (09.00-11.00 h), early metestrus (09.00-11.00 h) and late metestrus (17.00-19.00 h). Patterns of cyclic ANF immunoreactivity in plasma and atria were inversely related to each other, with plasma levels being significantly elevated during early metestrus when atrial levels were significantly decreased. Statistically significant central fluctuations in ANF levels during the estrous cycle were only found in the hypothalamic periventricular region (hPVA) and in the dorsal raphe (DR). ANF levels declined in both regions after late proestrus. Results indicate a relationship between ANF activity and cyclic patterns of fluid volume regulation and with phasic reproductive hormonal events.