Spontaneous rhythmic contractile behaviour of aortic ring segments isolated from pressure loaded regions of the vasculature.

STUDY OBJECTIVE: The aim was to study contractile responses of segments of rat arteries taken from pressure loaded and pressure protected regions and to examine the role of endothelial derived factors on the spontaneous activity of pressure loaded ring segments. DESIGN: Rats were subjected to complete aortic coarctation between the origins of the renal arteries and allowed to recover for 8 d. After 8 d arterial pressures were measured in awake animals from the pressure loaded and pressure protected regions simultaneously. Ring segments (2-3 mm) were taken from the two regions and mounted in a tissue bath for isometric force measurements. Similar studies were conducted in sham animals and in animals in which the kidney distal to the coarctation had been removed. EXPERIMENTAL MATERIAL: Female Sprague-Dawley rats, weight 200-250 g, were used. MEASUREMENTS AND MAIN RESULTS: Eight days after coarctation mean aortic pressure proximal to the occlusion was 168(SEM 1.29) mm Hg (n = 104) while distally it was 38(2.42) (n = 40). Of the rings tested 96% showed spontaneous rhythmic activity, having a mean frequency of 3.94(0.17) cycles.min-1. Spontaneous activity was not present in the pressure protected segments taken from the same animals. Rats with the distal kidney removed (n = 25) failed to become hypertensive and similarly prepared ring segments failed to show spontaneous rhythmic activity. Prior removal of the endothelial layer had no effect on the spontaneous contractile responses in pressure loaded segments. Histological examination showed that the media to lumen ratio was increased in coarcted rats in both pressure loaded and pressure protected regions compared to similar regions in sham operated animals. CONCLUSIONS: Pressure loaded arterial segments show spontaneous contractile activity when compared to sham segments by mechanisms not dependent on endothelial derived factors. The increase in pressure proximal to the occlusion is dependent on the renin-angiotensin system, since pressure was not increased when the distal kidney was removed. We hypothesise that the chronic pressure load induces fundamental changes in membrane permeability which leads to spontaneous contractile activity.

Isolation and purification of a vascular hyperreactivity factor from rabbit kidney cortex.

A compound capable of amplifying the threshold pressor response to norepinephrine (NE) was obtained from rabbit kidney cortex. This compound was purified and characterized using a series of techniques including gel filtration, ion exchange chromatography, preparative electrofocusing, HPLC, FAB mass spectrometry (FAB-MS), and Fourier transform infrared spectrometry. From this, an acid/heat stable (6N HC1, 160 degrees C, 24 hours), low molecular weight (ca 147) compound with a strong (+) charge density (Pi greater than 10) was identified. When injected into assay rats (i.v.), this compound amplified the pressor response to fixed doses of NE. Taken together, this compound exhibits nearly identical characteristics (i.e. acid/heat stability, structure, charge and biologic activity to the naturally occurring polyamine spermidine (SPD-145.6 daltons). Moreover, bolus injections of SPD (10 micrograms, i.v.) amplified the pressor response to NE over a range of doses from 5-25 ng.

Inhibition of renin release following left circumflex bradykinin injection in dogs.

We examined the renin secretory response to bradykinin (BK) injection into the left circumflex coronary artery (LCx) in dogs. Studies were conducted in anesthetized, carotid sinus denervated dogs which had been maintained on a low sodium diet. A 25 ga needle was inserted into the LCx for injection of BK (0.15 micrograms/kg). The rate of renin secretion (RS) was obtained during a 30 min control period, at 5 min after a non-hypotensive hemorrhage (10 ml/kg), at 1, 3 and 5 min after BK injection and at 15 min after the reinfusion of withdrawn blood. Four series of studies were conducted. Series I: BK injection into the LCx, Series II: saline injection into the LCx (sham), Series III: intravenous injection of BK, and Series IV: BK injection into the LCx in dogs with prior renal denervation. RS was suppressed by 80% (P less than 0.05) 5 min after injection of BK into the LCx. Saline injection (sham) into the LCx or intravenous BK administration did not inhibit RS. Furthermore, suppression of RS was not present in dogs with prior renal denervation. These results indicate that BK injection into the LCx causes a prompt reduction in the rate of RS and that this response is reflexively mediated by the renal nerves.

Statistical criteria for using short-term measurements as an index of 24-hour mean arterial pressure in unanesthetized unrestrained dogs.

This study assessed the statistical validity of short time-interval measurements as estimators of true 24 hour mean arterial pressure in unanesthetized, unrestrained dogs. 24 hour intra-arterial pressure recordings were obtained using a stable FM telemetry system. The 24 hour pressure measurements approximated a normal distribution whose variance was inversely related to the selected averaging interval. Given the variance of a normal distribution one can calculate the 95% confidence interval for any single random measurement. Conversely the number of random samples necessary to be within a prescribed confidence interval can be determined. In this study, the 95% confidence interval for a single, random 30 minute arterial pressure average was calculated to be 11.2 mmHg. Only 4.8 +/- 1.4% of 480 individual 30 minute arterial pressure measurements fell beyond this confidence interval. These outlying values were distributed throughout the 24 hour period. The data suggest that randomly chosen short time-interval measurements may be a valid index of true 24 hour mean pressure if the average variance of a population is known and confidence intervals are defined.

Atrial natriuretic factor in the freshwater turtle Pseudemys scripta: a partial characterization.

The presence of natriuretic and vasorelaxant materials in the atria and ventricles of a Chelonian reptile, the freshwater turtle Pseudemys scripta, was verified and the active substance partially characterized. Crude atrial and ventricular extracts were acutely natriuretic and diuretic when administered to anesthetized rats increasing sodium excretion 23.5 +/- 7.9 and 5.11 +/- 18 microM Na/10 min/mg extract, respectively. Although atrial extracts were relatively more natriuretic than ventricular extracts, the total natriuretic content of ventricular extracts was approximately twofold greater. Moreover, partially purified atrial extracts were found to relax precontracted isolated rat aortic ring segments and exhibited immunoreactivity to mammalian ANP antisera. Also, the natriuretic and immunoreactive substance was purified further by size exclusion chromatography and a molecular weight of 3-5 kDa was determined. Using the bovine adrenal glomerulosa ANP receptor, this partially purified natriuretic substance displayed an apparent binding affinity similar to that of mammalian ANP. Finally, in order to demonstrate that this reptile is physiologically responsive to ANP, synthetic rat ANP (10 micrograms/kg) was shown to decrease arterial pressure in conscious turtles from a control value of 37.2 +/- 5.2 to 30.3 +/- 2.7 mm Hg. These data demonstrate that unlike other non-Chelonian reptiles, both the atria and the ventricles of this Chelonian reptile synthesize and store substantial levels of biologically active ANP-like materials, and further, that ANP is a highly conserved and primitive cardiac hormone.

Partition of PGE between renal venous plasma and urine during renal ischemia.

Radioimmunoassay was used to study the effects of renal ischemia on the distribution of PGE-like material between renal venous plasma and urine in anesthetized dogs. Renal venous and urinary concentrations of these substances were equal during control, ischemia and recovery periods. This relationship obtained despite significant increases in the concentration of PGE of both compartments during the ischemic insult. The renal secretion rates of PGE, calculated as the product of renal plasma flow and renal venous concentrations, was reduced during ischemia while urinary excretion, was unchanged. The evidence suggests that the increased PGE concentrations observed in both compartments during renal ischemia are primarily due to a dilutional factor rather than an increased synthesis. Furthermore, the data suggest that the net secretion of renal PG's per unit time may, in fact, be reduced during renal ischemia.

Renin release in turtles: effects of volume depletion and furosemide administration.

To gain insight into the phylogenetic history of mechanisms controlling renin release, we conducted studies in the freshwater turtle Pseudemys scripta. Maneuvers known to stimulate renin release in mammals were evoked in the turtle, and the response was compared with that in mammals. Cumulative hemorrhage (30% blood vol) in anesthetized turtles failed to increase renin even though arterial pressure was reduced to 50% of control. An even more severe hemorrhage (60% blood loss) or hypotension induced by nitroprusside infusion in unanesthetized turtles also failed to evoke an enhanced level of renin. However, under identical experimental conditions, a 15% blood loss in rats increased renin at least fourfold (P less than 0.01). In other studies 48 h of furosemide administration in awake turtles increased renin more than threefold (P less than 0.05) and were accompanied by concomitant reductions in plasma sodium and potassium (P less than 0.05). The general conclusions drawn from these studies is that renin secretion in this primitive vertebrate is similar to that in mammals with respect to renal tubular and electrolyte mechanisms, but unlike all mammals tested these turtles do not possess an intrarenal baroreceptor component in renin control.

Role of calcium in the interaction of alpha and beta adrenoceptor-mediated renin release in isolated, constant pressure perfused rabbit kidneys.

These experiments were designed to study the role of calcium in the modulation of renin secretion by alpha and beta adrenoceptors. Rabbit kidneys were isolated and single-pass perfused with a modified Ringer's solution. Renal perfusion pressure was precisely controlled by an electronic servocontrol system. Tubular events were minimized by ligation of the ureter before initiating the studies. Under these conditions the predominant factor modifying renin secretion was assumed to originate directly on the juxtaglomerular cells. Isoproterenol infused at 0.1, 0.5, 1.0 and 5.0 nM/min/g of kidney weight increased renin secretion in a dose-dependent manner whereas phenylephrine infused at identical molar doses did not. In addition, phenylephrine (5.0 nM/min/g of kidney weight) blocked the usual response to isoproterenol. Removal of calcium from the perfusing medium had no effect on either the response to isoproterenol or the lack of a response to phenylephrine. On the other hand, when calcium is removed from the perfusate or when D-600, a calcium channel blocker, is added to calcium-containing medium, phenylephrine failed to block the usual response to isoproterenol. We conclude that the suppression of beta adrenoceptor stimulation of renin release by alpha adrenoceptor agonists is calcium dependent by a final mechanism as yet undefined, but probably involving movement of calcium into the juxtaglomerular cells.

The effect of beta adrenergic receptor blockade on the renin response to respiratory acidosis.

The effect of beta adrenergic blockade on the increase in plasma renin activity produced by acute respiratory acidosis was studied in chloralose anesthetized dogs. Sixteen mongrel dogs were given 4%, 8% and 12% CO2 in room air, successively. Propranolol (2 mg/Kg) was given to 8 dogs prior to CO2 inhalation. The other 8 dogs served as the control group. The response of elevated plasma renin activity during 4% and 8% CO2 inhalation was not different between the control and propranolol groups. However, the increase of plasma renin activity in the control group was greater than that of the propranolol treated group during 12% CO2 inhalation. It is suggested that activation of beta adrenergic receptors is not the sole factor in renin control during acute respiratory acidosis, although these receptors do mediate a significant fraction of the renin response to CO2 inhalation.

Alteration of the baroreceptor reflex by an effect of propranolol on the isolated carotid sinus.

The present experiments were designed to measure and characterize direct hemodynamic effects of propranolol on vascularly isolated carotid sinuses. dl-Propranolol, when restricted to the isolated carotid sinuses, inhibited in a dose-dependent manner the reflex increases in heart rate and mean arterial pressure during carotid sinus hypotension. When perfused through isolated sinuses at a concentration of 10 micrograms/ml, the drug totally abolished reflex changes to carotid sinus hypotension. Perfusion of the sinuses with d-propranolol or procaine at the same doses also totally inhibited the response. On the other hand, perfusion of the sinuses with sotalol, an adrenergic blocker without membrane stabilizing effects, did not alter resting levels of arterial pressure or heart rate and did not affect reflex changes during sinus hypotension. These results suggest that propranolol at high doses may affect baroreceptor afferents and inhibit the baroreceptor reflex through membrane-stabilizing and local anesthetic properties rather than through hypothesized beta receptors. It seems unlikely that these pathways contribute to the antihypertensive actions of the drug.

Acute inhibition of renin release during left circumflex coronary occlusion in dogs.

We examined the reflex effect of brief left circumflex coronary artery occlusion (CAO) on renal blood flow (RBF) and renin secretion (RS). Studies were conducted in alpha-chloralose-anesthetized dogs maintained on a salt-free diet for at least 3 days. A snare was placed around the left circumflex artery near its origin. The left renal artery and vein were exposed via flank incision, a flow probe was placed around the artery, and a curved needle was inserted into the vein for collection of renal venous blood. Values of blood pressure (BP), RBF, and RS were obtained for a 30-min control period, 4 min after the completion of a 10-ml/kg hemorrhage, 1 and 5 min after CAO, and 15 min after reinfusion. The CAO consisted of two 1-min occlusions separated by a 1-min interval. The results indicate that CAO reflexively inhibits the RS response to nonhypotensive hemorrhage and prevents renal vasoconstriction (P less than 0.05, n = 7). Both effects were completely abolished after vagotomy (n = 5). Renal denervation also abolished the response (n = 5). No response was observed during identical time controls (n = 5). The response was more pronounced in dogs with carotid sinus denervation, even though CAO resulted in marked reductions in BP (20%) under these conditions. These results demonstrate the presence of a cardiorenal reflex that can be activated by myocardial hypoxia and that acutely suppresses RS. This response is more pronounced in the absence of carotid sinus buffering.

Baroreflex modulation of ultradian oscillations of blood pressure and heart rate in unanesthetized dogs.

Telemetered, free-running dogs were studied to determine the role of cardiovascular control systems in modulation of ultradian oscillations of arterial pressure (MAP) and heart rate (HR). Data, aquired (2 Hz) by a stable telemetry system, was stored on a digital computer and analyzed for its harmonic content by a Fast Fourier Transform (FFT) algorithm. Both AP and HR consistently demonstrated rhythms having a period of from 0.6 to 1.0 h. Modulation of these rhythms by arterial pressure control systems was assessed in dogs studied before and carotid sinus baroreceptor denervation, before and after denervation of the aortic arch baroreceptors and before and after a combination of both these procedures. The data indicate the power spectral density (PSD) of MAP, but not HR, is increased (p less than 0.05) after denervation of the carotid sinuses alone, while the primary frequency of the oscillations was unchanged. On the other hand, denervation of the aortic arch baroreceptors alone was without effect on either the frequency or PSD of these oscillations. A combination of both carotid sinus and aortic arch denervation resulted in an increased (p less than 0.05) PSD of MAP oscillations but not in their frequency. These data indicate that the carotid sinuses modulate rhythmic behavior of MAP by buffering the magnitude, but not frequency, of the oscillations. Moreover, since oscillations were present in dogs after denervation of both the carotid sinus and aortic arch baroreceptors, these ultradian oscillations are not a result of a non-linear negative feedback mechanisms arising from these pressure sensitive regions.

Autonomic modulation of ultradian blood pressure and heart rate oscillations in dogs.

Autonomic receptor modulation of ultradian oscillations of blood pressure and heart rate was studied in telemetered free-running dogs. Data, analyzed for their harmonic content by fast Fourier transform (FFT) methods, indicated that ultradian and circadian oscillations of 22.9 +/- 2.5 and 10.5 +/- 0.9 (SD) mmHg, respectively, were present. The average principal frequency for the ultradian oscillations in 12 dogs was 0.760 +/- 0.11 cycles/h for arterial pressure and 0.808 +/- 0.10 for heart rate. Atropine had no effect on periodicity of either arterial pressure or heart rate. Metoprolol, a beta 1-antagonist, or hexamethonium, a ganglionic blocker, significantly reduced the power of both arterial pressure and heart rate (P less than 0.05), whereas the primary frequencies of both were unchanged. Prazosin, an alpha 1-blocker, sharply reduced arterial pressure power (P less than 0.05) and increased the power of heart rate (P less than 0.05), demonstrating that it is possible to uncouple arterial pressure oscillations from influences of heart rate. We conclude that the sympathetic limb of the autonomic nervous system is primarily responsible for these oscillations and that vagal influences on the heart partially dampen these rhythms.

The effects of respiratory acidosis on plasma renin activity in the dog.

The effects of arterial PCO2 on plasma activity was studied in chloralose anesthetized dogs undergoing carefully controlled respiratory acidosis. Plasma renin activity, measured by radioimmunoassay, was enhanced (P less than 0.001) during inhalation of CO2 despite insignificant changes in renal blood flow and arterial pressure. Although underlying mechanism remained to be investigated, activation of sympathetic nervous system and intrarenal effects of CO2 seemed to be involved in this enchancement.

Characterization of the renin-angiotensin system in the turtle Pseudemys scripta.

Studies were conducted in freshwater turtles Pseudemys scripta to define some characteristics of the renin-angiotensin system in this reptile. Dialyzed acid-treated kidney extract (1 g tissue per ml water) produced a prolonged pressor response in unanesthetized turtles, which was eliminated by boiling the extract or by pretreating the turtle with [Sar1, Ile8]angiotensin II. A rat pressor assay was employed because turtle angiotensin (ANG) was bound poorly by the anti-[Asp1, Ile5, His9]ANG I used in our radioimmunoassay. Kidney extract incubated with homologous plasma (pH 5.5 and 25 degrees C) produced a time-dependent pressor response in rats. The pressor activity of the product was eliminated by dialysis or by pretreating the rats with [Sar1, Ile8]ANG II. The pressor response in anesthetized turtles to ANG I was significantly reduced by captopril, whereas the ANG II response remained unchanged, thus demonstrating the presence of ANG-converting enzyme activity in these animals. We determined the velocity of turtle ANG formation at various dilutions of enzyme (kidney extract) or substrate (plasma). Turtle kidney extract incubated with homologous plasma displayed typical Michaelis-Menten kinetics. Finally we conducted experiments to determine whether a portion of turtle plasma renin exists in an inactive form. Trypsinization caused a slight increase in plasma renin activity (PRA), whereas acidification to pH 3.3 yielded a fourfold increase in PRA.