Haemorrhage as a test of the function of the cardiovascular system in rabbits of different ages

Low-potassium University of Wisconsin solution for cardioplegia: improved protection of the isolated ischemic neonatal rabbit heart

Recovery of cardiac function and high-energy phosphates following ischemia and reperfusion were determined for hearts perfused with low potassium University of Wisconsin solution, high potassium University of Wisconsin solution, St Thomas' solution, or subjected to hypothermia alone. Isolated hearts were arrested for either 3 h at 15 degrees C or 6 h at 20 degrees C (n = 7 for each group) with one of the four solutions and then reperfused. Aortic flow after ischemic arrest at 20 degrees C was 40.3 +/- 13.3%, 79.3 +/- 10.0%, 64.3 +/- 11.9% and 43.9 +/- 15.9% of control values for high potassium University of Wisconsin solution, low potassium University of Wisconsin solution, St Thomas' solution and hypothermia alone, respectively. Similar results were observed in hearts subjected to ischemic arrest at 15 degrees C. Myocardial adenosine triphosphate and creatine phosphate after reperfusion tended to be higher in the low potassium University of Wisconsin solution group. It is concluded that low potassium University of Wisconsin solution may provide reliable cardioplegia during surgery that requires prolonged cardiac arrest in neonates and infants.

Intestinal permeability to small- and large-molecular-weight substances in the newborn rabbit

BACKGROUND/PURPOSE

The authors have previously reported the occurrence of spontaneous bacterial translocation (BT) and its resolution with age in the newborn rabbit. They have also reported a close correlation between small bowel bacterial colonization (BC-SB) and BT at 1 week of age, suggesting that the presence of luminal bacteria and their production of endotoxins may increase the intestinal permeability. The aim of this study was to evaluate intestinal permeability to small and large molecules in the newborn rabbit and to correlate it with BT.

MATERIALS AND METHODS

New Zealand White rabbits (n = 96) 1, 7, 14, 21, and over 120 days (adult) of age were given either C14-labeled ethylene diamine tetraacetic acid (EDTA) (MW 290) or C14-Dextran (MW 70,000) via an orogastric tube at 1 mCi per 100 g of body weight. Five hours later, blood, urine, liver, and intestine were collected, and scintillation counting was performed after solubilization. In a separate series of rabbits (n = 136), the incidence of BT, BC-SB, and small intestinal surface area (SA) were measured.

RESULTS

Total permeability to Dextran decreased with age and was significantly reduced at 14 days of age. In contrast, total permeability to EDTA increased and was maximal in 7- to 14-day-old rabbits and began to decrease at 21 days of age. The incidence of BC-SB rapidly increased at 7 days of age and reached 100% at 14 days of age. The incidence of BT peaked at 7 days of life (30%) and then decreased with age. SA increased rapidly in the first 3 weeks and SA growth rate of 21-day-old rabbits was almost 1,400% compared with 1-day-old rabbits.

CONCLUSIONS

This study has shown an age-related reduction of intestinal permeability to large (Dextran) and small (EDTA) molecular weight particles. However, intestinal permeability to EDTA had a different pattern than Dextran, suggesting that there may be different mechanisms of intestinal permeability to different size molecules. Intestinal permeability to EDTA closely correlated with bacterial colonization and bacterial translocation, suggesting that changes in the intestinal bacterial environment may affect the intestinal permeability, possibly by activating the immune system secondary to increases in endotoxins and bacteria.

Regulation of extracellular fluid volume in neonates

This review summarises mechanisms of control of extracellular fluid volume in the neonatal period. 'Normal' body fluid distribution and methods of its measurement are discussed as well as regulatory mechanisms with particular emphasis on hormonal and renal aspects.

Development of the rabbit retina. I. Size of eye and retina, and postnatal cell proliferation

Measures of rabbit eyes and retinal wholemounts were used to evaluate the development of retinal area and shape. The retina is shown to have a horizontal axis about a third longer than the vertical axis just before birth, and to adopt an almost symmetrical shape during postnatal development to adulthood. In general, retinal thickness is shown to decrease after birth, but differently in particular retinal regions: the reduction is marked in the periphery, and less pronounced in the visual streak. As an exception, the myelinated region--after it becomes really myelinated, from 9 days p.p.--even increases in thickness. In all regions of the retina, the absolute and relative thickness of the nuclear layers decreases, whereas the relative thickness of plexiform and fibrous layers increases. Proliferation of cells within the rabbit retina was studied during the first three postnatal weeks. 3H-thymidine incorporation was used to demonstrate DNA synthesis autoradiographically in histological sections as well as in enzymatically isolated retinal cells. A first proliferation phase occurs in the neuroblastic cell layer and ceases shortly after birth in the retinal center, but lasts for about one week in the retinal periphery. We found, however, a few 3H-thymidine-labeled cells as late as in the third postnatal week. These late-labeled cells were found within the nerve fiber layer and in the inner plexiform layer. The latter cells were shown to express antigens detected by antibodies directed to the intermediate-sized filament protein vimentin, which are known to label Müller cells and neuroepithelial stem cells. This was confirmed in our preparation of enzymatically isolated cells; all cells with autoradiographically labeled nuclei revealed a characteristic elongated morphology typical for Müller radial glia (and also for early neuroepithelial stem cells). 3H-thymidine-labeled cells in the nerve fiber layer were most probably astrocytic. In analogy to the brain, we conclude that the mammalian retina undergoes a series of proliferation phases: first an early phase producing both neurons and glial cells, and then a late phase producing glial cells, e.g., in the nerve fiber layer. Most probably, the late phase within the inner nuclear layer is glial as well, i.e., consists of dividing Müller cells; it cannot be excluded, however, that there may remain some mitotically active stem cells.

The effects of frusemide, saralasin and hypotension on fetal plasma renin activity and on fetal renal function

1. In eleven chronically catheterized fetal sheep aged 124-142 days, hypotension caused by infusion of sodium nitroprusside (1.6-3.3 mg/h) and competitive antagonism of angiotensin II by saralasin (3.3 mg/h) both caused a fall in fetal urine flow (P less than 0.02 and P less than 0.05, respectively), and in sodium excretion (P less than 0.05 and P less than 0.01) because they both caused a fall in glomerular filtration rate (G.F.R., P less than 0.02 and P less than 0.01). Neither hypotension nor saralasin had any significant effect on fractional sodium reabsorption. Saralasin only caused a significant fall in systolic pressure (P = 0.05) while infusion of sodium nitroprusside caused a fall in both systolic and diastolic pressure (P less than 0.005 and P less than 0.02). 2. Frusemide (6 mg I.V) caused a marked natriuresis and diuresis (F = 24.9, P less than 0.005 and F = 30.5, P less than 0.005). This effect was maximal within 30 min. There was no change in fetal G.F.R. and there was a significant decrease in the fraction of the filtered sodium load that was reabsorbed (F = 10.44, P less than 0.0025). Fetal mean plasma renin activity (p.r.a.) rose progressively throughout (F = 9.3, P less than 0.005). When frusemide was given to fetal sheep which were hypotensive because they were infused with sodium nitroprusside, it still caused a diuresis (F = 5.73, P less than 0.025) and the fraction of the filtered sodium load that was reabsorbed decreased (F = 4.06, P less than 0.05) to a similar extent to that seen in animals given frusemide alone. On the other hand, frusemide was ineffective as a diuretic i.e. it had no effect on fractional sodium reabsorption, when given to fetal sheep which were infused with saralasin. 3. Injection of frusemide was associated with a significant rise in the diastolic pressures of hypotensive fetuses (P less than 0.05). Furthermore, when the infusion of saralasin was terminated 1.5 h after frusemide injection, blood pressure rose significantly (F = 11.19, P less than 0.0005 for systolic pressure and F = 7.15, P less than 0.005 for diastolic pressure) and p.r.a. fell (F = 4.78, P less than 0.025). 4. It is concluded that the fetal renin-angiotensin system can play a significant role in regulation of fetal blood pressure.(ABSTRACT TRUNCATED AT 400 WORDS)

Experimental model for pharmacokinetic studies during continuous peritoneal dialysis in the rabbit

An experimental model permitting continuous peritoneal dialysis in rabbits, very close to continuous ambulatory peritoneal dialysis (CAPD) as performed in humans, is described. Animals were carefully monitored before and during dialysis for plasma, urine, and dialysate biochemical parameters, and electrocardiogram, body temperature, weight, and white cell count in dialysate. Dialysis was performed successfully for 21 days without failure. Difficulties in setting up the final model are reported. The suitability of the model for pharmacokinetic and pharmacodynamic studies was borne out by administering atenolol (i.v. before CADP, i.v. after one week of CAPD, and i.p. one week later) and analyzing the findings.

A comparison of the effects of salbutamol, etilefrine and dextran during hypotension and low cardiac output states in rabbit

The effects of salbutamol (25 and 50 micrograms/kg), etilefrine (50 and 200 micrograms/kg) and dextran (8 ml/kg) on cardiovascular function have been studied in the rabbit. The three drugs raised the resting cardiac output (with salbutamol producing delta max 52%, P less than 0.001, n = 5) and right heart filling pressure (RHFP, delta max from dextran 3.5 cmH2O; P less than 0.001, n = 6) and lowered total peripheral resistance (TPR, delta max from salbutamol 46%, P less than 0.001, n = 5). However, TPR rose with the 200 micrograms/kg etilefrine (P less than 0.05, n = 5). Pulse pressure rose with salbutamol (P less than 0.001, n = 6) and etilefrine (P less than 0.05, n = 6). Etilefrine raised resting BP (delta max, P less than 0.001, n = 6); salbutamol lowered resting BP (delta max, P less than 0.001, n = 6) while dextran (n = 6) had little effect on resting BP. The actions of salbutamol are mediated mainly through beta 2-adrenoceptors although the drug also has some minor beta 1-adrenoceptor action. With etilefrine, the increase in cardiac output and the reduction in TPR are mediated through beta-adrenoceptors while the increases in RHFP, blood pressure and TPR are a direct action on the alpha-adrenoceptors. However, during haemorrhage the fall in diastolic pressure produced by salbutamol was considerably reduced while the reduction in mean BP and systolic pressure (which sometimes rose) was abolished. Dextran raised BP during hypotension produced by either sympathectomy or haemorrhage but not during normotension. The reflex recovery in RHFP and the reflex tachycardia were slightly attenuated during lower body negative pressure (LBNP) after salbutamol or dextran. The reflex recovery in blood pressure was complete and the pressure sometimes exceeded the resting level by up to 10 mmHg during LBNP after salbutamol, etilefrine (50 micrograms/kg only) and dextran. The reduced TPR (presumably due to vasodilation) and the increases in cardiac output and RHFP at an adequately maintained blood pressure produced by suitable doses of the three drugs may be useful in the management of circulatory shock and related states.

Development of the vascular bed in the rabbit ear: scanning electron microscopy of vascular corrosion casts

The development of the vascular bed in the rabbit ear was investigated using vascular corrosion casts from animals of various ages. Examination of the casts revealed that the arrangement of the major auricular arteries and veins was determined before birth and was maintained during postnatal growth of the ear. Furthermore, the number of arteries branching off the central ear artery and the lateral arteries did not increase with increasing ear length. Scanning electron microscopic examination of lateral segments of adult ear casts revealed many anastomoses between marginal arteries and veins. These arteriovenous anastomoses (AVAs) occurred singly, in pairs, or in clusters of three to six. Their size and shape were variable, even in the same cast. The central segment of many AVA casts showed surface impressions of endothelial cell nuclei which were different from the impressions on adjacent arteries and veins. Arteriovenous anastomoses were also detected in ear casts from animals as young as 8 days. The density of AVAs in lateral ear segments ranged from 95-165 cm-2 in 8- to 11-day-old rabbits to 80-115 cm-2 in adults. However, estimates of the total number of AVAs in the lateral ear margin indicated that AVAs continued to be formed at a steady rate during growth of the ear. During the early neonatal period the cutaneous capillary plexuses developed prominent tufts projecting toward the skin surface, which were apparently associated with developing hair follicles. These capillary tufts were not seen in casts from fetal or adult rabbits.

Developmental aspects of the renal responses to hemorrhage during converting-enzyme inhibition in fetal lambs

The role of the renin-angiotensin system in modulating the renal hemodynamic and functional responses to reductions of fetoplacental blood volume (8.8-35.5%) was studied in two groups of fetal lambs (less than 120 days and greater than 130 days gestation; term 145 days) during infusion of either captopril (experimental fetuses) or dextrose 5% in water (control fetuses). At high hemorrhage levels (level III), renal blood flow decreased and renal vascular resistance increased significantly in both groups of fetuses (less than 120 days and greater than 130 days), either treated or not treated with captopril. However, at low hemorrhage levels (levels I and II), and contrary to what was observed in young fetuses (less than 120 days), near-term fetuses (greater than 130 days) receiving captopril showed neither significant decreases in renal blood flow nor increases in renal vascular resistance, whereas untreated fetuses of the same gestational ages demonstrated significant decreases in renal blood flow and increases in renal vascular resistance. It was found in both less than 120 day and greater than 130 day fetuses that hemorrhage is associated with a decrease in urinary flow rate and free water clearance accompanied by an increase in urine osmolality and sodium reabsorption. It was shown that captopril does not modify this response. The present study also demonstrated that the blood pressure response to hemorrhage was characterized by a similar decrease in less than 120 day fetuses, whether treated or untreated with captopril. On the other hand, blood pressure did not change in control fetuses greater than 130 days, but decreased slightly in captopril-treated fetuses during hemorrhage. Taken together, the present results tend to suggest that the renin-angiotensin system may be an important modulator of the renal hemodynamic response to low level hemorrhage as fetuses approach term, and may be more important in controlling blood pressure in near-term than in young fetuses.

'Structural autoregulation'--the local adaptation of vascular beds to chronic changes in pressure

Blood vessels readily adapt their design in response to sustained functional changes. If pressure (P) increases, the resulting thickening of the walls (w) of the resistance vessels, associated with a reduction in average inner radius (ri), keeps wall tension per unit wall layer (T) constant, because the increased w/ri ratio largely balances the raised pressure (Laplace's law: T = P X ri/w). The opposite occurs when there are sustained reductions in pressure. This locally elicited, mainly precapillary structural adaptation is a long-range equivalent to precapillary functional autoregulation and deserves to be called structural autoregulation. In primary hypertension there is an early 'structural resetting' of the systemic precapillary resistance, due to narrowing of ri and to vascular hyperreactivity ensuing from the increase in w/ri. These structural changes imply an increased resistance to flow at normal levels of vascular smooth muscle activity. Furthermore, even mild functional pressor influences will, if sustained, by a positive feedback interaction with the initially mild vascular hyperreactivity gradually accentuate the structural increase in w/ri. Marked rises in pressure may ensue from this interaction, implying that it is a major causative element in primary hypertension. As the renal preglomerular resistance vessels are similarly structurally autoregulated, this implies an early largely parallel resetting of the important renal 'long-term barostat function'. Further, as the walls of large arteries get thicker and stiffer, this helps to reset the baroreceptors. Finally, as the venous capacitance vessels adapt in a similar way the slight rise in average venous pressure in primary hypertension will reduce venous compliance, which helps to 'centralize' the usually slightly reduced blood volume.

Elastin and collagen accumulation in rabbit ascending aorta and pulmonary trunk during postnatal growth. Correlation of cellular synthetic response with medial tension

Absolute and relative quantities of elastin, collagen, and DNA in anatomically defined segments of rabbit ascending aorta (AA) and pulmonary trunk (PT) were compared at intervals from birth to 2 months of age. Identical in size, weight, and composition at birth, the vessels maintained similar lengths and diameters at each age but diverged markedly in weight and scleroprotein content after 1 week. By 2 months, 3 times as much elastin and 1.7 times as much collagen had accumulated in the AA as compared to the PT. By contrast, the increase in total DNA content was the same for both segments. Differences in total fibrous protein accumulation, total elastin accumulation, and elastin content relative to DNA paralleled differences in estimated total medial tangential tension. Proportions of elastin and collagen relative to dry weight increased markedly only between 4 and 2 weeks of age and not thereafter despite continuing rapid growth, steadily increasing medial tension, and increasing total scleroprotein content. Thus, medial cells were capable of adapting their quantitative scleroprotein synthetic response to differences in medial tension throughout growth but established a fixed qualitative response within 2 weeks.

Experimental studies on the pathophysiological responses of the newborn to injury

Experimental studies are reviewed which show that there are significant differences with age in both the local and general responses to injury. The results seem to confirm the belief in the striking ability of the newborn to withstand injury and suggest that one important aspect of this resistance is an increased ability to compensate circulatory hypovolaemia.

The compensation of post-traumatic oedema in the rabbit at different ages

1. The compensation of fluid loss from the circulation after an LD(50) period of bilateral hind-limb ischaemia in a thermoneutral environment has been studied in the rabbit from birth until young adulthood.2. The fluid loss from the circulation was calculated from the increase in the great vessel haematocrit value as the red cell volume and F(cells) were unchanged after limb ischaemia. This ;apparent' fluid loss was compared with the ;actual' fluid loss calculated from the increase in water content of the injured limbs.3. The 0- to 1-day-old rabbit was better able to restore fluid, and protein, to the circulation than the older animal and suffered a smaller percentage reduction in arterial blood pressure after an LD(50) period of ischaemia.4. The better compensation of post-traumatic fluid loss during the first 48 hr of life may be explained by the high capillary filtration coefficient and the post-traumatic increase in plasma protein osmotic pressure found at that age.5. The more efficient restoration of the plasma volume by the 0- to 1-day-old rabbit may account for the increased ability to withstand post-traumatic fluid loss at that age.

Resistance to post-traumatic fluid loss at different ages

The ability to withstand post-traumatic fluid loss in a thermoneutral environment has been studied in the rabbit during the first 3 weeks of life.

The LD₅₀ period of bilateral hind-limb ischaemia, calculated by probit analysis, fell from birth to the 10th day of life and then returned to the 1-2 day level by the 22nd day.

The difficulties of defining the acute injury to the animal are discussed and it is concluded that the fluid loss from the circulation expressed as a fraction of the plasma volume or total body water is the most precise. To compare the resistance of different age groups the fluid loss required to kill one half of a group of rabbits (i.e. the fluid loss following an LD₅₀ period of ischaemia) was calculated at each age studied. When the changes in body composition with age had been taken into account the 1-2 day-old rabbit was more resistant to post-traumatic fluid loss in a thermoneutral environment than the older animal. The possible clinical significance of these results is briefly discussed.

Brown adipose tissue metabolism in vivo and serum insulin concentrations in rabbits soon after birth

1. In rabbits kept unfed for 4 or 24 or 48 hr after delivery by Caesarean section at term, noradrenaline infusion (I.V. for 30 min) caused a similar increase in oxygen consumption but the increase in serum free fatty acid concentration was greatest in rabbits kept unfed for 48 hr.2. The brown adipose tissue of anaesthetized rabbits under 3 hr old took glucose from the circulation but did not release fatty acids. In similar rabbits noradrenaline infusion stimulated the tissue to generate heat, but there was no release of fatty acids even though the rate of triglyceride hydrolysis was high (as judged by the rate of glycerol release).3. In rabbits kept unfed for 48 hr from birth in a warm environment, brown adipose tissue released small amounts of fatty acids but continued to take glucose from the circulation. Heat production in response to noradrenaline infusion was accompanied by an increased release of fatty acids. The fat content of the brown adipose tissue did not fall with starvation.4. The mean serum insulin concentration of rabbits at birth was 54 muu./ml. compared to 23 muu./ml. in the mother. In new-born rabbits kept unfed for 48 hr the insulin concentration had fallen to 14 muu./ml.5. It is concluded (i) that at birth brown adipose tissue has the capacity to generate heat but the tissue is slow to release its stores of fat in response to starvation, (ii) that brown adipose tissue has a high rate of glucose uptake even during starvation and (iii) that the high circulating concentration of insulin may be responsible for the tissue's slow adaptation to the demands of starvation.

Changes in the blood volume of the rabbit with age

1. The changes in blood haemoglobin concentration, great vessel haematocrit value, red cell volume, plasma volume and total blood volume have been studied in rabbits from birth to 4 months of life.2. The plasma volumes were measured with [(131)I]human serum albumin and/or rabbit serum [(14)C]albumin. Red cell volumes were measured with [(51)Cr]-rabbit red blood cells. Blood volumes were also calculated from carbon clearance curves.3. The haemoglobin concentration and the great vessel haematocrit value fell during the first 11 days of life. The great vessel haematocrit value was then not significantly different from the value at 4 months; whereas the haemoglobin concentration fell further until 22 days, when it started to rise towards the concentration at 4 months.4. The true plasma volume could not be accurately measured during the first 48 hr of life by the injection of labelled albumin since it rapidly equilibrated with an extravascular albumin space. As the ratio of the whole body haematocrit to the great vessel haematocrit (F(cells)) did not appear to change with age the plasma volume at this time was best obtained by calculation from the red volume and corrected great vessel haematocrit.5. The blood volume fell significantly during the first 4 months of life due to significant falls in the red cell and plasma volumes.

Local and general responses to injury in the newborn rabbit

Some physiological responses to experimental bilateral hind limb ischaemia have been studied and compared in neonatal and young rabbits.

The fluid loss into the injured limbs after removal of the tourniquets has been expressed as a percentage of the plasma volume and of the total body water at the different ages.

The reactivity of skin blood vessels to a variety of known mediators of an inflammatory response has been investigated using labelling techniques.

The kidneys and arterial pressure in immature and adult rabbits

1. The effect of stepwise haemorrhage on arterial pressure in adult and immature (9-15 days old) rabbits lightly anaesthetized with sodium pentobarbitone is described.2. Bethanidine (1 or 3 mg/kg) lowered initial arterial pressure but did not impair the maintenance of arterial pressure in immature or adult rabbits during stepwise haemorrhage.3. Arterial pressure was only slightly lower in nephrectomized than in intact adult rabbits; in some immature rabbits nephrectomy caused a substantial fall of resting arterial pressure. This fall was largest in animals of low body weight and low haematocrit. Nephrectomy reduced the ability of both immature and adult rabbits to maintain arterial pressure during haemorrhage.4. Arterial pressure fell more precipitately on stepwise haemorrhage in adult rabbits in which the carotid sinus and depressor nerves had been cut than in intact rabbits. No such difference was seen in immature rabbits despite the fact that initial arterial pressure was higher in denervated animals at all ages.5. The responses to stepwise haemorrhage were compared in dummy operated and nephrectomized immature and adult rabbits with carotid sinus and depressor nerves cut to minimize changes of sympathetic tone. Under these conditions the presence or absence of kidneys made no significant difference in the response to stepwise bleeding in adult rabbits. In immature rabbits maintenance of arterial pressure was greatly impaired in the absence of the kidneys.6. The results suggest that a pressor mechanism of renal origin may be relatively more important in the maintenance of arterial pressure in the face of haemorrhage in immature than in adult rabbits.7. Resting arterial pressure in small and anaemic immature rabbits may partly depend on the presence of the kidneys. At birth arterial pressure is higher in rabbits of higher haematocrit but this relationship reverses in the second week of life when the haematocrit level is falling.

The effect of haemorrhage on haemoglobin concentration, blood volume and arterial pressure in kittens and cats

1. The arterial haemoglobin concentration in kittens less than 24 hr old was inversely related to body weight. There was about twice as much haemoglobin/unit body weight at birth as in adult cats. Haemoglobin concentrations were minimal at 3-6 weeks of age.2. In animals lightly anaesthetized with sodium pentobarbitone, arterial pressure rose from 52 mm Hg at birth in kittens to 133 mm Hg in adult cats. Blood volume decreased from 73 ml./kg at birth to 60 ml./kg in adults.3. When kittens less than a fortnight old were subjected to stepwise blood letting, arterial pressure fell proportionately with blood volume; in older kittens and in cats, arterial pressure was less well maintained at similar proportionate reductions of blood volume than in young kittens.4. The responses to haemorrhage of kittens and cats were compared with those of rabbits similarly treated and with those of adult cats anaesthetized with urethane and chloralose reported in the literature.

Acute effects of haemorrhage on the composition of arterial blood in immature and adult rabbits, kittens and cats

1. The fall of haematocrit after stepwise bleeding in rabbits and young kittens was not accompanied by an equivalent fall of plasma nitrogen. The relative failure of plasma nitrogen to decrease after haemorrhage was not attributable to the entry of diffusible nitrogen into the circulation.2. Plasma protein recovery by a combination of stepwise bleeding and viviperfusion was such as to yield values for the whole body/great vessel haematocrit ratio (F(cells)) in the normal range in adult cats but significantly below the normal range in young kittens and rabbits. If viviperfusion was not preceded by stepwise bleeding F(cells) in the normal range was obtained in young kittens and rabbits.3. The results of electrophoretic examination of pre- and post-haemorrhagic plasma were consistent with the entry of fresh protein into the circulation after haemorrhage occurring preferentially in the albumin fraction, but increase of albumin/globulin ratio was small in adult cats.4. It was concluded that the fall of haematocrit after haemorrhage in young kittens and rabbits was mainly due to the entry of protein rich tissue fluid into the circulation. In adult cats, however, the haematocrit did not always fall and any such fall may have been partly due to a shift of plasma from the periphery to the great vessels.5. The bearing of these observations on the validity of calculation of blood volume from red cell mass and haematocrit during stepwise haemorrhage was considered.

The relation of blood volume to body weight and arterial haemoglobin levels in rabbits

1. Relative blood volume (ml./kg) of rabbits calculated from total circulating haemoglobin and blood haemoglobin concentration decreases rapidly during the first few weeks of life and more slowly thereafter.2. In anaemic rabbits over 1 kg body weight, relative blood volume is significantly increased.3. Rabbits under 1 kg body weight have larger blood volumes than would be expected from their anaemia alone, were the relationship deduced in rabbits over 1 kg in body weight applicable. Blood volume was not related to haemoglobin concentration in these immature rabbits.

Nature of species differences in the medial distribution of aortic vasa vasorum in mammals

Thoracic aortic segments of 12 mammalian species were fixed while distended at normal physiological pressures after the vasa vasorum were filled by a perfusion mixture containing gelatin and carbon. Mammals whose aortas had 29 or fewer medial lamellar units had no demonstrable intramural vascular channels; those whose aortas had more than 29 medial lamellar units, had medial vasa. Aortas with medial vasa vasorum always had a subintimal medial zone devoid of vasa vasorum. In growing animals the width of this avascular zone increased with age; in adults, the width increased slightly with increasing species body weight. However, the number of lamellar units in the avascular zone was independent of both age and species and equal to 29.0 ± 2.5. All of the species with intramural aortic vasa vasorum as adults had 29 or more aortic medial lamellar units at birth; none of the species which had 29 or fewer lamellar units at birth had more than 29 lamellar units at maturity. In aortas with medial vasa vasorum, widening of the avascular zone during growth appeared to be due only to the uniform thickening of each of the approximately 29 lamellar units already present at birth; the vascularized outer zone widened both by enlargement of its lamellar units and the addition of new lamellar units. Species differences in medial distribution of vasa vasorum are due primarily to differences in thickness of the vascularized outer zone.

The effects of haemorrhage in the unanaesthetized rabbit

1. The circulatory response following acute loss of 26% of the blood volume was examined in unanaesthetized rabbits. The groups of animals studied were normal rabbits; adrenalectomized rabbits; animals subjected to prolonged treatment with guanethidine in which peripheral adrenergic nerve transmission is blocked, but which can reflexly liberate adrenal medullary hormones; animals subjected to combined adrenalectomy and guanethidine treatment with no functional adrenergic effectors; in each case with or without administration of atropine. The responses of animals with section of the carotid sinus and aortic nerves were also examined.2. The spontaneous rate of replacement of the blood volume after haemorrhage by reabsorption of extravascular fluid was the same in all the above preparations, the blood volume returning to normal 3-4 hr after bleeding.3. The ;passive' effects of haemorrhage were examined in animals without functioning autonomic effectors and include a large fall in right atrial pressure and cardiac output, arterial hypotension, no significant change in total peripheral resistance, and a bradycardia of gradual onset. Reflex autonomic effector activity in normal animals minimizes the fall in atrial pressure, cardiac output and arterial blood pressure, and produces a significant increase in total peripheral resistance and tachycardia. Increased sympathetic nerve activity and secretion of adrenal medullary hormones each play an important and complementary part in the normal circulatory response to haemorrhage of the rabbit. There is also reflex reduction in vagal efferent activity.4. Reflexes from the carotid sinus and aortic arch limit the fall in arterial pressure for the first 4 hr after haemorrhage. These reflexes also account for the tachycardia normally observed after haemorrhage. The baroreceptor reflexes rather than the chemoreceptors appear to be dominant in these responses.5. Twenty-four hours after haemorrhage the haemodynamic pattern is similar in all preparations irrespective of their autonomic effector status: blood volume, right atrial pressures and cardiac outputs are all elevated, and the arterial pressure has virtually recovered, consistent with the development of hypervolaemic anaemia at this time.

A lamellar unit of aortic medial structure and function in mammals

The close association of elastin, collagen, and smooth muscle in the mammalian aortic media results in viscoelastic properties that account for many of its static and dynamic mechanical features. The structural components of the media are precisely oriented in concentric layers, or lamellar units, of fairly uniform composition. A comparative study of the adult thoracic aorta in 10 mammalian species, including 15 canine breeds, showed that the number of lamellar units in the media of adult mammalian aortas is very nearly proportional to aortic radius regardless of species or variations in measured wall thickness. Estimated wall tensions ranged from 7,820 dynes/cm in a 28-g mouse to 203,000 dynes/cm in a 200,000-g sow, but the average tension per lamellar unit of an aortic media was remarkably constant regardless of species, ranging from 1,090 to 3,010 dynes/cm. The findings suggest that the elastin lamella and the contents of its adjacent interlamellar zone represent the unit of structure and function of the mammalian aortic wall.