The survival of young rats in nitrogen

Extracellular potassium concentration in juvenile and adult rat brain cortex during anoxia

The extracellular K+ concentration, ([K]e), in the brain cortex of rats at different ages was measured by means of K+-sensitive microelectrodes. [K]e was between 3 and 5 mM at all ages. Following nitrogen inhalation there was an increase in [K]e which exhibited a sigmoid pattern in every age group: Firstly, there was a slow rate of rise which was followed by a sudden, steep increase where the [K]e rose to about 70 mM. During the subsequent 5-10 min a plateau value of about 90 mM was reached. The prominent difference between the age groups was the time until the steep rise began. The time to the steep increase in [K]e and the [K]e at the start of the steep was inversely related to the age of the animal. A close relation was found between the time to the start of the steep increase and the time to the last gasp during nitrogen breathing (r = 0.98). It is suggested that the different resistence to anoxia in young and adult animals is related to differences in the ability to keep near normal potassium gradients across the cells in the brain.

Positive inotropic action of insulin on piglet heart

This study was designed to investigate changes in cardiac performance during hypoglycemia produced by the administration of insulin in the newborn piglet. With heart rate, aortic pressure, and aortic flow held constant, the treated group demonstrated a pronounced positive inotropic response manifested by an increase of dP/dt max to 138% of control values. Central nervous system function and beta adrenergic activity were excluded from the preparation by ligation of the brachiocephalic vessels and administration of practolol. For reasons discussed, it is unlikely that the findings can be ascribed to glucagon contamination. Therefore, the increase in contractility presumably resulted from a direct effect of insulin upon the myocardium. Clinical and laboratory data suggest that the resistance of the neonate to hypoxia is modified by glycogen stores. Insulin is known to increase glycogen synthesis, and this effect might be expected to augment myocardial resistance to hypoxia. Under the conditions of these experiments, however, pretreatment with insulin had no demonstrable influence on the rate of deterioration of cardiac function during hypoxia. The mechanism of cardiac stimulation by insulin is unknown but may involve calcium fluxes.

Protective role of increased myocardial glycogen stores in cardiac anoxia in the rat

To determine whether increased glycogen stores might protect the heart against anoxia, experiments were performed in the isolated perfused rat heart. Marked differences in cardiac glycogen were produced by comparing hearts from rats previously treated with reserpine with hearts from control rats. Lesser differences in cardiac glycogen were produced in hearts by perfusing them for 15 minutes without glucose (0 mM glucose) or with 20 mM glucose. Both groups were then studied during a 5-minute anoxic cycle with 5 mM glucose as the exogenous substrate. Hearts from the reserpine-treated rats had higher left ventricular pressures, maximal rate of left ventricular pressure rise, and lactate output after 2 minutes of anoxia than the hearts from control rats. Similar but less marked mechanical differences were observed between 0 mM glucose and 20 mM glucose hearts. The mechanical differences during anoxia between the two groups were not abolished by simultaneous L-norepinephrine administration. Hearts with greater initial glycogen stores had higher glycogenolytic rates, and proportionately more lactate was produced from glycogen than from glucose. Thus, anaerobic ATP production per mole of hexose was greater in hearts with higher glycogen stores. Calculated ATP production was also greater in hearts from the reserpine-treated rats than in those from control animals.

These studies demonstrate that both marked and minor elevations in cardiac glycogen are associated with greater glycolytic reserve and improved mechanical resistance to anoxia. This appears to be mainly due to enhanced glycogenolysis and anaerobic ATP production.

Aspects of applied physiology of neonatal circulation

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Oxygen consumption during pouch development of the macropod marsupial Setonix brachyurus

1. Measurements of O(2) consumption at 9 or 10 temperatures in the 20-40 degrees C ambient temperature range were made on joeys with ages selected to cover the 180-day period of pouch occupancy.2. The rate of O(2) consumption of joeys younger than 100 days increased directly with ambient temperature.3. After 100 days of age the O(2) consumption rate at low temperatures rose and at about 140 days of age a constant rate was maintained over the full ambient temperature range.4. Heat transfer from joey to mother commenced after 100 days of age.5. At 150-180 days of age the rate of O(2) consumption at 20 degrees C was approximately 12 times greater than at ages less than 100 days. A thermal neutral zone was established in the range 32-36 degrees C by joeys older than 150 days.6. At the usual pouch temperature of 36.5 degrees C, O(2) consumption per unit wet body weight rose from 12 ml./kg.min at birth to 17 ml./kg.min at the end of pouch life. On a unit dry body weight basis it fell from 120 to 56 ml./kg.min. This decline parallels the decrease in growth rate.

Tissue lactic dehydrogenase isozymes. Developmental patterns in the neonatal rat

The isozyme forms of lactic dehydrogenase (LDH) were studied in excised rat tissues up to 45 days after birth. The predominant subtype at birth was the muscle form, reflecting anaerobic glycolysis. Development was characterized by an increase in the relative ratio of heart (H) to muscle (M) in all tissues studied; however, the major form in liver and muscle was quite clearly the M type. Emergence of the typical adult LDH distribution would appear to be complete in 2–3 weeks after birth, but the adult levels of total LDH activity were not attained during the 45 postnatal days of this study.

Hypothermia, Asphyxia, and Cardiac Glycogen in Guinea Pigs

Cardiac glycogen was not affected by cooling guinea pigs for short periods. In normothermic animals it was reduced 75 percent or more at the time of death from asphyxia. Quickly cooled animals asphyxiated until the time of death of warm controls showed no significant losses of cardiac glycogen; animals cooled while breathing 10 percent oxygen plus 5 percent carbon dioxide showed slight reductions. Therefore, hypothermia spares cardiac glycogen during asphyxia, but there are factors other than cardiac glycogen which influence survival of asphyxiated animals.