Dietary sodium restriction and pressor responsiveness to tyramine in spontaneously hypertensive rats

OBJECTIVE

To determine in vivo whether in young spontaneously hypertensive rats (SHR) dietary sodium restriction decreases adrenergic transmitter release from the sympathetic nerve terminal.

DESIGN

Dietary sodium restriction was initiated in young and mature SHR and Wistar-Kyoto (WKY) rats, and subsequently changes in pressor responsiveness to norepinephrine and to the indirectly acting sympathomimetic tyramine were determined in relation to their effects upon plasma catecholamines.

RESULTS

In young SHR sodium restriction for 3-6 weeks prevented the development of hypertension, whereas in mature SHR sodium restriction did not affect blood pressure. Sodium restriction caused modest decreases in pressor responsiveness to the exogenous alpha-agonist, not different in young and mature SHR compared with WKY rats. In contrast, sodium restriction markedly inhibited pressor-responses to tyramine in young SHR and WKY rats, but not at all in mature rats. Tyramine increased plasma norepinephrine 5-10-fold. However, sodium restriction did not affect this response. The pressor response to tyramine was related to increases in total peripheral resistance, with minimal changes in cardiac output, and could be blocked by alpha 1-receptor blockade in rats on either control or low-sodium diets.

CONCLUSIONS

These results show that sodium restriction causes only a small decrease in the pressor response to norepinephrine, but a more marked inhibition of the pressor response to tyramine in young SHR and WKY rats without affecting the plasma norepinephrine response to tyramine. These results suggest that dietary sodium can indeed affect presynaptic functions in vivo, but that plasma norepinephrine responses to tyramine may not reflect changes in arterial norepinephrine release, or that sodium restriction affects a co-transmitter rather than norepinephrine release per se.

Dietary sodium and the antihypertensive effect of nifedipine in spontaneously hypertensive rats

To assess the interaction of dietary sodium and the antihypertensive response to a calcium antagonist, spontaneously hypertensive rats (SHR) were given a diet of regular or increased (120 v 342 mumol Na+/g food) sodium intake from 4 until 16 weeks of age. Nifedipine was added at 10 weeks of age. This level of sodium intake did not enhance the development of hypertension in SHR. In rats with the regular sodium intake, nifedipine caused only a minor decrease in blood pressure. In contrast, with increased sodium intake nifedipine caused a marked antihypertensive response, preventing the rise in blood pressure occurring between 10 and 16 weeks of age. This enhanced response was associated with a diminished blood pressure fall from ganglionic blockade. These results indicate that modest increases in sodium intake enhance the blood pressure response to a calcium antagonist possibly by potentiating its inhibitory effects on sympathetic activity.

A method for rapid screening of recombinant proteins for recognition by T lymphocytes

A simple, cost-effective method is described that allows rapid screening of recombinant protein sequences for their ability to stimulate T cells. Individual microcultures of E. coli each expressing a gene product or peptide sequence fused to protein A are grown in 96-well plates. Following lysis of the bacteria, the fusion peptide is readily captured with immobilized immunoglobulin in tissue culture wells. No further purification is required. T lymphocytes plus appropriate antigen-presenting cells are added directly to the wells and assayed for proliferation. The DNA in bacteria from wells stimulating T cell proliferation is then sequenced. The technique allows rapid mapping of T cell epitopes by facilitating screening of truncation mutants without extensive purification. Described here is a further application of the technique to study monosubstituted analogues of a known T cell epitope.

Role of the deep mesencephalic nucleus in the antinociception induced by stimulation of the anterior pretectal nucleus in rats

The present study showed that the inhibitory effect on the tail-flick reflex (TF) of stimulating the deep mesencephalic nucleus (DpMe) was very similar to that produced by stimulation of the anterior pretectal nucleus (APtN). An electrolytic lesion of the ipsilateral DpMe greatly reduced the inhibitory effect of APtN stimulation on the TF. Furthermore, activating the neuronal cell bodies in DpMe but not the fibers of passage by microinjection of L-glutamate into this area was also shown to elicit inhibition of TF. On the other hand, inhibiting the neuronal cells in DpMe by microinjection of gamma-aminobutyric acid produced a marked reduction in the APtN-induced inhibition of TF, which was comparable to that produced by DpMe lesions. It is suggested that the APtN-induced antinociception is, at least in part, mediated via a relay through the DpMe.

Chronic arterial vasodilation, central hemodynamics, and cardiac hypertrophy in spontaneously hypertensive rats

To determine the possible contribution of cardiac volume overload in the failure of arterial vasodilators to induce regression of left ventricular hypertrophy (LVH), we evaluated in spontaneously hypertensive rats (SHRs) the changes in central hemodynamics caused by treatment with hydralazine or minoxidil. Cardiac output measured by the thermodilution technique and filling pressures were measured in conscious, freely moving rats. Increases in cardiac output were observed after 1 day of treatment, and persisted during chronic treatment; ganglionic blockade did not affect this increase. However, the LV end-diastolic pressure and right atrial pressure of SHRs were not increased by hydralazine or minoxidil. Minoxidil increased the LV weight, and decreased the LV wall thickness to LV internal radius ratio, whereas hydralazine did not change these parameters. We conclude that in SHRs changes in filling pressures do not represent the primary stimulus for the persistence or progression of cardiac hypertrophy during chronic arterial vasodilation.

Electrohydraulic-clamshell heart with energy converter inside the compliance reservoir

Two new ideas on the electrohydraulic actuation of blood pumps have been combined. The first idea was to put the energy converters that propel the hydraulic fluid inside the compliance reservoir instead of having them separate. Compactness of the device and better cooling of the energy converter by the surrounding fluid are two major advantages of this approach. Secondly, we put the pumping membrane inside a clamshell that fits over a soft ventricle (1). The ventricle can be implanted first, after which the shell is slid over it. These two ideas have resulted in devices described in this paper. Preliminary in vitro and in vivo data are presented.

Dietary sodium restriction and the development of two-kidney, one-clip hypertension in young versus adult rats

To determine the effects of moderate versus severe dietary sodium restriction on the development of 2-kidney, 1-clip (2K,1C) hypertension, young male Wistar rats were placed on diets containing 9, 26, or 101 (control) mumol sodium/g food. Three days later, a solid silver clip (i.d. 0.20 mm) was placed on the left renal artery and diets were continued up to 6 weeks. Adult rats received a 0.25-mm clip. In young clipped rats receiving the 101 mumol/g diet, blood pressure (BP), plasma renin activity (PRA), and BP response to captopril were increased as early as 1 week after clipping and increased further over time. Moderate sodium restriction (26 mumol sodium/g) led to only a slight delay in the development of hypertension; the levels of BP and PRA, the BP response to captopril, and the extent of cardiac hypertrophy achieved by 6 weeks were not different between the 2K, 1C rats receiving 26 or 101 mumol sodium/g. Sodium restriction to 9 mumol/g decreased rate of growth and completely prevented the rise in BP and in left ventricular weight. At 3 and 6 weeks the severely sodium-restricted rats had significantly higher PRA levels than the 2K, 1C control group. However, the BP response to captopril was attenuated relative to the other hypertensive groups. In adult rats, this level of sodium restriction had a small, but significant effect on body weight, but still prevented the increase in BP and in left ventricular weight. In conclusion, dietary sodium restriction can prevent the development of 2K,1C hypertension in both young and adult rats, but only if the restriction is severe. This effect may relate to a marked reduction in the pressor effectiveness of the renin-angiotensin system by low sodium intake per se or by associated metabolic or other changes.

Inactivity of acrylonitrile epoxide to modify a Ha-ras DNA in a non-focus transfection-transformation assay

The transforming potential of acrylonitrile epoxide (ANO) was tested in a modified NIH3T3 transfection-transformation assay. This involves a new ras construct obtained by ligating a human c-Ha-ras-1 proto-oncogene to the pSV2neo mammalian vector. The new plasmid was allowed to react with ANO or an established carcinogen in vitro, and the modified ras DNA transfected into NIH 3T3 cells. The transfectants are subjected to triple selections: G418 (neomycin) resistance, low serum growth, and limit dilutions. The end points are scored by cell growth kinetics and monolayer saturation density. In using this protocol, the EJ tumor ras plasmid was the positive control, and anti-benzo[a]pyrene-7,8- dihydrodiol-9,10-epoxide (anti-BPDE) and N-methyl-N-nitrosourea were found to be positive in yielding transformants. Although ANO-modified ras gave rise to two G418R clones, both were scored negative due to their normal growth rate and monolayer density similar to the negative controls. Southern blot analysis of anti-BPDE transformant DNA revealed a fragment of 411 bp, indicating a ras mutation at codon 11 or 12. However, both the ANO clones showed the wild-type band of 355 bp by the same method.

Concept of a soft, compressible artificial ventricle under evaluation

This study was designed to compare the relative merits of soft and rigid artificial ventricles. A cascade mock circulation was used to measure cardiac output under different circumstances. The data show that these soft air driven ventricles show a Starling's-like response over a wider range of filling pressures than identical, but rigid, ventricles. Compression of soft ventricles by high intrathoracic pressures was simulated in vitro. Air pressures up to +20 mm Hg did not seriously affect soft ventricles. Cardiac tamponade was simulated by compressing the ventricle in a closed fluid compartment. Tamponade became severe when volume reduction of the ventricle rose to 60 ml. Hemolysis caused by soft and rigid ventricles was tested in a blood bag set-up and was 48-82% higher in the rigid ventricle, depending on the driving conditions. Possibly, this could be explained by the authors' finding that rigid ventricles showed a 20% higher intraventricular dP/dtmax value than soft ventricles. Soft ventricles were implanted in three calves as a total artificial heart (TAH). Implantation without quick connectors was easy because the surgeon could easily fold and compress the ventricles. No physiological complications of softness were observed. Blood damage in the animals was low (less than 5 mg/dl). The authors conclude that soft ventricles show distinct surgical and functional advantages over rigid ventricles.

Development of a new inflow valve for a 20cc semisoft ventricle: preliminary results

We remodeled and tested our semisoft 20cc ventricle and made a new bileaflet flap inflow valve. Housings, bases, outflow valve, and a newly designed diaphragm were all made by vacuum forming and put together by radiofrequency welding or glue. In vitro, the ventricle produced a cardiac output of 2.5 to 3.0 L/min and showed reliable durability results. Hematological testing showed no important thrombogenicity of the new valve. Cardiac output was higher than expected for the volume of the ventricle, perhaps because of stretching or flow through. Animal experiments with the left ventricular assist device (LVAD) version was done at Ohio State University. Earlier in Utah, we did 20 cc total artificial heart (TAH) implantations and LVAD experiments in lambs and recently in calves with the 60cc TAH version. A soft ventricle is easy to implant and low in production costs.

Soft artificial ventricles for infants and adults, with or without a clamshell

The quick connect system and mechanical disk valves used in total artificial hearts (TAH) are sources of thrombogenesis and blood damage. Our soft TAH, which has no quick connectors, can be squeezed and bent, making it easily implantable, and blood damage is reduced by the use of trileaflet and biflap polyurethane valves. The soft ventricles were made by vacuum forming, after which the pieces were welded together by radiofrequency heat sealing. A rapid clamshell can be pushed and slipped over the soft heart to prevent deformation of the ventricle. Three calves have had the 60 cc soft TAH implanted, both with and without a clamshell. The cardiac outputs were as high as 7 L/min, without a vacuum applied during diastole. Two lambs received the 20 cc TAH (as an acute experiment); it fit and functioned well. One healthy lamb received a 20 cc left ventricular assist device (LVAD) with a pulsating artificial atrium as a survival experiment. The lamb survived for 8 days, after which the device was removed and the lamb returned to the meadow. Thrombosis in the TAH was minimal, and the plasma free hemoglobin values in all the TAH and LVAD experiments were usually lower than 5 mg/dl and often lower than 2 mg/dl.

New polyurethane valves in new soft artificial hearts

This article describes new bistable valves, and introduces a new soft heart that is easy to implant. Earlier, five polyurethane (PU) valves were implanted in the mitral position in sheep. All five survived for 1 year or more, and the valves, although calcified, were intact. Since the opening resistance was somewhat high, valves that are bistable were developed, which means they may be open or closed. These valves have lower opening resistance, and regurgitation is similar to that of mechanical valves. Eight calves have been implanted with a new, soft total artificial heart (TAH). Seven had bistable leaflet valves; the eighth had mechanical (Bicer) valves in the inflow position. Four of the calves were sacrificed after 22 to 43 days. At autopsy, the number of thromboemboli found, particularly in the kidneys, was low compared with previous experiments. None of these animals received anticoagulants other than the heparin given during heart/lung bypass.

The role of aluminum in the pathogenesis of anemia in an outpatient hemodialysis population

Anemia is a well-defined complication of aluminum overload in chronic dialysis patients which may be present before other manifestations of aluminum toxicity are obvious. Causes of anemia in chronic renal failure are multiple, and at the present time there is no marker for aluminum-induced anemia. Deferoxamine (DFO) treatment can correct aluminum-related anemia and microcytosis, but may be associated with side effects. Because of the possible role of aluminum in red blood cells in causing the anemia associated with aluminum overload, we attempted to test red blood cell (RBC) aluminum as a marker for aluminum-associated anemia and to assess the prevalence of aluminum-associated anemia in an outpatient dialysis population. Both random plasma aluminum and RBC aluminum correlated well with the increase in plasma aluminum seen following DFO challenge. However, RBC aluminum was affected less by changes in oral aluminum intake than plasma aluminum. There were strong correlations of RBC and plasma aluminum to corpuscular volume (MCV) in our patients. Moreover, patients within the highest quartile of RBC aluminum had a lower mean MCV (82.1 +/- 1.7 vs 89.6 +/- 1.7, p less than .01) and hematocrit (HCT) (24.3 +/- 4 vs 28.2 +/- 1.5, p less than .05) than those within the lowest quartile. These data suggest that aluminum toxicity is an important cause of microcytic anemia in outpatient hemodialysis patients. Prospective long-term studies are needed to further define the usefulness of RBC aluminum in diagnosing and following hemodialysis patients with aluminum-induced anemia.

Corticofugal influences of S1 cortex on ventrobasal thalamic neurons in the awake rat

Corticofugal influences on the responses of 39 ventrobasal (VB) thalamic neurons to repetitive stimuli were studied in awake rats by focally suppressing the evoked and spontaneous electrocortical activity of the primary (S1) somatosensory cortex with magnesium or lidocaine plus magnesium. Suppression of the S1 cortex reduced the number of spikes discharged by 19 (66%) of 29 VB units in response to each of 25 electrical stimuli delivered to the medial lemniscus; 9 units were unaffected and 1 showed an increased response to 1-10 Hz stimuli. The responses of 6 (38%) of 16 VB units to electrical somatic stimuli were also reduced following S1 cortical suppression; 9 units were unaffected and 1 showed an increased response to 20-40 Hz stimuli. A comparison of the reduced responsiveness of 5 units studied during medial lemniscal and somatic stimulation did not reveal any additional response attenuation attributable to subthalamic corticofugal influences. We conclude that, in the awake rat, somatosensory transmission to VB thalamic neurons is primarily facilitated by S1 corticothalamic neurons.

Responsiveness of ventrobasal thalamic neurons after suppression of S1 cortex in the anesthetized rat

Corticofugal influences on the responses of ventrobasal (VB) thalamic neurons to repetitive stimuli were studied in anesthetized rats by suppressing primary somatosensory (S1) electrocortical activity with topically applied lidocaine. Effective concentrations of lidocaine were confined to S1 and immediately adjacent cortex and suppressed evoked S1 responses and corticofugal discharges. Suppression of S1 cortex reduced the average number of spikes discharged by 83 VB neurons in response to each of 25 electrical somatic stimuli delivered at frequencies ranging from 1 to 50 Hz. Of 20 units studied both before and after S1 suppression, 14 (70%) showed a similar reduced response to repetitive stimuli. Cortical suppression produced no consistent changes in spontaneous activity, somatic stimulus threshold, response latency, or size of receptive field. There was no significant difference in the effect of cortical suppression on the responsiveness of 8 VB neurons to repetitive medial lemniscal, as compared to somatic, stimuli. We conclude that, in the anesthetized rat, S1 corticofugal activity facilitates somato-sensory transmission to VB neurons and that this facilitation is mediated, at least in part, by corticothalamic neurons.

9-Hydroxyprostaglandin dehydrogenase from rat kidney. Purification to homogeneity and partial characterization

A 9-hydroxyprostaglandin dehydrogenase has been purified from rat kidney to apparent homogeneity. The molecular weight of the enzyme as determined by gel filtration was 33,000. Electrophoresis in sodium dodecyl sulfate gave a value of 34,000 indicating the absence of subunits. The enzyme catalyzes NAD+-specific irreversible oxidation of 15-keto-13,14-dihydro-prostaglandin F2 alpha as well as of 15-keto-PGF2 alpha and PGF2 alpha although at a lesser degree. It does not catalyze the oxidation of 6-keto-PGF1 alpha. The enzyme was sensitive to sulfhydryl inhibitors and was inhibited by prostaglandins, fatty acids, triiodothyroacetic acid, and indomethacin.

Studies on the thromboxane synthesizing system in human platelet microsomes

The thromboxane synthesizing system from human platelets has been characterized by using arachidonic acid and prostaglandin endoperoxide as indirect and direct substrates. The synthesis of thromboxanes from either substrates catalyzed by the microsomal fraction was monitored by measuring the formation of thromboxane B2 immunoreactivity. Both hemoglobin and phenolic compounds were required for the maximal synthesis of thromboxane B2 from arachidonic acid but not from prostaglandin endoperoxide. Studies on the kinetics of the formation of thromboxane B2 from either substrates indicated that the synthesis of prostaglandin endoperoxide intermediate is the rate-limiting step in the overall production of thromboxanes from arachidonic acid. Effect of the microsomal protein concentrations on the rate of formation of thromboxane B2 showed concave upward relationship suggesting the possible involvement of endogenous stimulator(s) in thromboxane synthetase catalyzed reaction. A variety of compounds including sulfhydryl inhibitors, prostaglandin endoperoxide analog, prostaglandin antagonist, N-0164, and N-substituted imidazoles could directly inhibit the thromboxane synthetase, while nonsteroidal antiinflammatory drugs apparently affected the prostaglandin endoperoxide synthetase which catalyzes the synthesis of the immediate precursor of thromboxanes.

Transformation of arachidonate into 6-oxoprostaglandin F1 alpha, thromboxane B2 and prostaglandin E2 by sheep lung microsomal fraction

In the presence of haemoglobin and isoproterenol, the microsomal fraction of sheep lung catalysed the conversion of arachidonate predominantly into thromboxane B2 and to a lesser extent into 6-oxoprostaglandin F1alpha. Very little prostaglandin E2 and prostaglandin F2alpha were formed. If reduced glutathione was added in combination with haemoglobin and isoproterenol, the synthesis of prostaglandin E2 was favoured over that of thromboxane B2 and 6-oxoprostaglandin F1alpha. The identities of these products were confirmed by t.l.c. and by combined g.l.c.-mass spectrometry. These results indicate that microsomal fraction of sheep lung possesses active prostaglandin synthase, prostacyclin synthase and thromboxane synthase activities.